Network Working Group U. Blumenthal
Request for Comments: 2274 IBM T. J. Watson Research
Obsoletes: 2264 B. Wijnen
Category: Standards Track IBM T. J. Watson Research
January 1998
User-based Security Model (USM) for version 3 of the
Simple Network Management Protocol (SNMPv3)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
IANA Note
Due to a clerical error in the assignment of the snmpModules in this
memo, this RFC provides the corrected number assignment for this
protocol. This memo obsoletes RFC 2264.
Abstract
This document describes the User-based Security Model (USM) for SNMP
version 3 for use in the SNMP architecture [RFC2271]. It defines the
Elements of Procedure for providing SNMP message level security.
This document also includes a MIB for remotely monitoring/managing
the configuration parameters for this Security Model.
Table of Contents
1. Introduction 3
1.1. Threats 4
1.2. Goals and Constraints 5
1.3. Security Services 6
1.4. Module Organization 7
1.4.1. Timeliness Module 7
1.4.2. Authentication Protocol 8
1.4.3. Privacy Protocol 8
1.5. Protection against Message Replay, Delay and Redirection 8
1.5.1. Authoritative SNMP engine 8
Blumenthal & Wijnen Standards Track [Page 1]
RFC 2274 USM for SNMPv3 January 1998 1.5.2. Mechanisms 9 1.6. Abstract Service Interfaces. 10 1.6.1. User-based Security Model Primitives for Authentication 11 1.6.2. User-based Security Model Primitives for Privacy 11 2. Elements of the Model 12 2.1. User-based Security Model Users 12 2.2. Replay Protection 13 2.2.1. msgAuthoritativeEngineID 13 2.2.2. msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime 14 2.2.3. Time Window 15 2.3. Time Synchronization 15 2.4. SNMP Messages Using this Security Model 16 2.5. Services provided by the User-based Security Model 17 2.5.1. Services for Generating an Outgoing SNMP Message 17 2.5.2. Services for Processing an Incoming SNMP Message 19 2.6. Key Localization Algorithm. 21 3. Elements of Procedure 21 3.1. Generating an Outgoing SNMP Message 22 3.2. Processing an Incoming SNMP Message 25 4. Discovery 30 5. Definitions 31 6. HMAC-MD5-96 Authentication Protocol 45 6.1. Mechanisms 45 6.1.1. Digest Authentication Mechanism 46 6.2. Elements of the Digest Authentication Protocol 46 6.2.1. Users 46 6.2.2. msgAuthoritativeEngineID 47 6.2.3. SNMP Messages Using this Authentication Protocol 47 6.2.4. Services provided by the HMAC-MD5-96 Authentication Module 47 6.2.4.1. Services for Generating an Outgoing SNMP Message 47 6.2.4.2. Services for Processing an Incoming SNMP Message 48 6.3. Elements of Procedure 49 6.3.1. Processing an Outgoing Message 49 6.3.2. Processing an Incoming Message 50 7. HMAC-SHA-96 Authentication Protocol 51 7.1. Mechanisms 51 7.1.1. Digest Authentication Mechanism 51 7.2. Elements of the HMAC-SHA-96 Authentication Protocol 52 7.2.1. Users 52 7.2.2. msgAuthoritativeEngineID 52 7.2.3. SNMP Messages Using this Authentication Protocol 53 7.2.4. Services provided by the HMAC-SHA-96 Authentication Module 53 7.2.4.1. Services for Generating an Outgoing SNMP Message 53 7.2.4.2. Services for Processing an Incoming SNMP Message 54 7.3. Elements of Procedure 54 7.3.1. Processing an Outgoing Message 55 7.3.2. Processing an Incoming Message 55 8. CBC-DES Symmetric Encryption Protocol 56 Blumenthal & Wijnen Standards Track [Page 2]
RFC 2274 USM for SNMPv3 January 1998 8.1. Mechanisms 56 8.1.1. Symmetric Encryption Protocol 57 8.1.1.1. DES key and Initialization Vector. 57 8.1.1.2. Data Encryption. 58 8.1.1.3. Data Decryption 59 8.2. Elements of the DES Privacy Protocol 59 8.2.1. Users 59 8.2.2. msgAuthoritativeEngineID 59 8.2.3. SNMP Messages Using this Privacy Protocol 60 8.2.4. Services provided by the DES Privacy Module 60 8.2.4.1. Services for Encrypting Outgoing Data 60 8.2.4.2. Services for Decrypting Incoming Data 61 8.3. Elements of Procedure. 61 8.3.1. Processing an Outgoing Message 61 8.3.2. Processing an Incoming Message 62 9. Intellectual Property 62 10. Acknowledgements 63 11. Security Considerations 64 11.1. Recommended Practices 64 11.2. Defining Users 66 11.3. Conformance 67 12. References 67 13. Editors' Addresses 69 A.1. SNMP engine Installation Parameters 70 A.2. Password to Key Algorithm 71 A.2.1. Password to Key Sample Code for MD5 71 A.2.2. Password to Key Sample Code for SHA 72 A.3. Password to Key Sample Results 73 A.3.1. Password to Key Sample Results using MD5 73 A.3.2. Password to Key Sample Results using SHA 74 A.4. Sample encoding of msgSecurityParameters 74 B. Full Copyright Statement 76 1. Introduction The Architecture for describing Internet Management Frameworks [RFC2271] describes that an SNMP engine is composed of: 1) a Dispatcher 2) a Message Processing Subsystem, 3) a Security Subsystem, and 4) an Access Control Subsystem. Applications make use of the services of these subsystems. It is important to understand the SNMP architecture and the terminology of the architecture to understand where the Security Model described in this document fits into the architecture and Blumenthal & Wijnen Standards Track [Page 3]
RFC 2274 USM for SNMPv3 January 1998 interacts with other subsystems within the architecture. The reader is expected to have read and understood the description of the SNMP architecture, as defined in [RFC2271]. This memo [RFC2274] describes the User-based Security Model as it is used within the SNMP Architecture. The main idea is that we use the traditional concept of a user (identified by a userName) with which to associate security information. This memo describes the use of HMAC-MD5-96 and HMAC-SHA-96 as the authentication protocols and the use of CBC-DES as the privacy protocol. The User-based Security Model however allows for other such protocols to be used instead of or concurrent with these protocols. Therefore, the description of HMAC-MD5-96, HMAC-SHA-96 and CBC-DES are in separate sections to reflect their self-contained nature and to indicate that they can be replaced or supplemented in the future. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 1.1. Threats Several of the classical threats to network protocols are applicable to the network management problem and therefore would be applicable to any SNMP Security Model. Other threats are not applicable to the network management problem. This section discusses principal threats, secondary threats, and threats which are of lesser importance. The principal threats against which this SNMP Security Model should provide protection are: - Modification of Information The modification threat is the danger that some unauthorized entity may alter in-transit SNMP messages generated on behalf of an authorized user in such a way as to effect unauthorized management operations, including falsifying the value of an object. - Masquerade The masquerade threat is the danger that management operations not authorized for some user may be attempted by assuming the identity of another user that has the appropriate authorizations. Two secondary threats are also identified. The Security Model defined in this memo provides limited protection against: Blumenthal & Wijnen Standards Track [Page 4]
RFC 2274 USM for SNMPv3 January 1998 - Disclosure The disclosure threat is the danger of eavesdropping on the exchanges between managed agents and a management station. Protecting against this threat may be required as a matter of local policy. - Message Stream Modification The SNMP protocol is typically based upon a connection-less transport service which may operate over any sub-network service. The re-ordering, delay or replay of messages can and does occur through the natural operation of many such sub-network services. The message stream modification threat is the danger that messages may be maliciously re-ordered, delayed or replayed to an extent which is greater than can occur through the natural operation of a sub-network service, in order to effect unauthorized management operations. There are at least two threats that an SNMP Security Model need not protect against. The security protocols defined in this memo do not provide protection against: - Denial of Service This SNMP Security Model does not attempt to address the broad range of attacks by which service on behalf of authorized users is denied. Indeed, such denial-of-service attacks are in many cases indistinguishable from the type of network failures with which any viable network management protocol must cope as a matter of course. - Traffic Analysis This SNMP Security Model does not attempt to address traffic analysis attacks. Indeed, many traffic patterns are predictable - devices may be managed on a regular basis by a relatively small number of management applications - and therefore there is no significant advantage afforded by protecting against traffic analysis. 1.2. Goals and Constraints Based on the foregoing account of threats in the SNMP network management environment, the goals of this SNMP Security Model are as follows. 1) Provide for verification that each received SNMP message has not been modified during its transmission through the network. 2) Provide for verification of the identity of the user on whose behalf a received SNMP message claims to have been generated. Blumenthal & Wijnen Standards Track [Page 5]
RFC 2274 USM for SNMPv3 January 1998 3) Provide for detection of received SNMP messages, which request or contain management information, whose time of generation was not recent. 4) Provide, when necessary, that the contents of each received SNMP message are protected from disclosure. In addition to the principal goal of supporting secure network management, the design of this SNMP Security Model is also influenced by the following constraints: 1) When the requirements of effective management in times of network stress are inconsistent with those of security, the design should prefer the former. 2) Neither the security protocol nor its underlying security mechanisms should depend upon the ready availability of other network services (e.g., Network Time Protocol (NTP) or key management protocols). 3) A security mechanism should entail no changes to the basic SNMP network management philosophy. 1.3. Security Services The security services necessary to support the goals of this SNMP Security Model are as follows: - Data Integrity is the provision of the property that data has not been altered or destroyed in an unauthorized manner, nor have data sequences been altered to an extent greater than can occur non-maliciously. - Data Origin Authentication is the provision of the property that the claimed identity of the user on whose behalf received data was originated is corroborated. - Data Confidentiality is the provision of the property that information is not made available or disclosed to unauthorized individuals, entities, or processes. - Message timeliness and limited replay protection is the provision of the property that a message whose generation time is outside of a specified time window is not accepted. Note that message reordering is not dealt with and can occur in normal conditions too. Blumenthal & Wijnen Standards Track [Page 6]
RFC 2274 USM for SNMPv3 January 1998 For the protocols specified in this memo, it is not possible to assure the specific originator of a received SNMP message; rather, it is the user on whose behalf the message was originated that is authenticated. For these protocols, it not possible to obtain data integrity without data origin authentication, nor is it possible to obtain data origin authentication without data integrity. Further, there is no provision for data confidentiality without both data integrity and data origin authentication. The security protocols used in this memo are considered acceptably secure at the time of writing. However, the procedures allow for new authentication and privacy methods to be specified at a future time if the need arises. 1.4. Module Organization The security protocols defined in this memo are split in three different modules and each has its specific responsibilities such that together they realize the goals and security services described above: - The authentication module MUST provide for: - Data Integrity, - Data Origin Authentication - The timeliness module MUST provide for: - Protection against message delay or replay (to an extent greater than can occur through normal operation) The privacy module MUST provide for - Protection against disclosure of the message payload. The timeliness module is fixed for the User-based Security Model while there is provision for multiple authentication and/or privacy modules, each of which implements a specific authentication or privacy protocol respectively. 1.4.1. Timeliness Module Section 3 (Elements of Procedure) uses the timeliness values in an SNMP message to do timeliness checking. The timeliness check is only performed if authentication is applied to the message. Since the Blumenthal & Wijnen Standards Track [Page 7]
RFC 2274 USM for SNMPv3 January 1998 complete message is checked for integrity, we can assume that the timeliness values in a message that passes the authentication module are trustworthy. 1.4.2. Authentication Protocol Section 6 describes the HMAC-MD5-96 authentication protocol which is the first authentication protocol that MUST be supported with the User-based Security Model. Section 7 describes the HMAC-SHA-96 authentication protocol which is another authentication protocol that SHOULD be supported with the User-based Security Model. In the future additional or replacement authentication protocols may be defined as new needs arise. The User-based Security Model prescribes that, if authentication is used, then the complete message is checked for integrity in the authentication module. For a message to be authenticated, it needs to pass authentication check by the authentication module and the timeliness check which is a fixed part of this User-based Security model. 1.4.3. Privacy Protocol Section 8 describes the CBC-DES Symmetric Encryption Protocol which is the first privacy protocol to be used with the User-based Security Model. In the future additional or replacement privacy protocols may be defined as new needs arise. The User-based Security Model prescribes that the scopedPDU is protected from disclosure when a message is sent with privacy. The User-based Security Model also prescribes that a message needs to be authenticated if privacy is in use. 1.5. Protection against Message Replay, Delay and Redirection 1.5.1. Authoritative SNMP engine In order to protect against message replay, delay and redirection, one of the SNMP engines involved in each communication is designated to be the authoritative SNMP engine. When an SNMP message contains a payload which expects a response (for example a Get, GetNext, GetBulk, Set or Inform PDU), then the receiver of such messages is authoritative. When an SNMP message contains a payload which does not expect a response (for example an SNMPv2-Trap, Response or Report PDU), then the sender of such a message is authoritative. Blumenthal & Wijnen Standards Track [Page 8]
RFC 2274 USM for SNMPv3 January 1998 1.5.2. Mechanisms The following mechanisms are used: 1) To protect against the threat of message delay or replay (to an extent greater than can occur through normal operation), a set of timeliness indicators (for the authoritative SNMP engine) are included in each message generated. An SNMP engine evaluates the timeliness indicators to determine if a received message is recent. An SNMP engine may evaluate the timeliness indicators to ensure that a received message is at least as recent as the last message it received from the same source. A non-authoritative SNMP engine uses received authentic messages to advance its notion of the timeliness indicators at the remote authoritative source. An SNMP engine MUST also use a mechanism to match incoming Responses to outstanding Requests and it MUST drop any Responses that do not match an outstanding request. For example, a msgID can be inserted in every message to cater for this functionality. These mechanisms provide for the detection of authenticated messages whose time of generation was not recent. This protection against the threat of message delay or replay does not imply nor provide any protection against unauthorized deletion or suppression of messages. Also, an SNMP engine may not be able to detect message reordering if all the messages involved are sent within the Time Window interval. Other mechanisms defined independently of the security protocol can also be used to detect the re-ordering replay, deletion, or suppression of messages containing Set operations (e.g., the MIB variable snmpSetSerialNo [RFC1907]). 2) Verification that a message sent to/from one authoritative SNMP engine cannot be replayed to/as-if-from another authoritative SNMP engine. Included in each message is an identifier unique to the authoritative SNMP engine associated with the sender or intended recipient of the message. A Report, Response or Trap message sent by an authoritative SNMP engine to one non-authoritative SNMP engine can potentially be replayed to another non-authoritative SNMP engine. The latter non-authoritative SNMP engine might (if it knows about the same userName with the same secrets at the authoritative SNMP engine) as a result update its notion of timeliness indicators of the authoritative SNMP engine, but that is not considered a threat. Blumenthal & Wijnen Standards Track [Page 9]
RFC 2274 USM for SNMPv3 January 1998 In this case, A Report or Response message will be discarded by the Message Processing Model, because there should not be an outstanding Request message. A Trap will possibly be accepted. Again, that is not considered a threat, because the communication was authenticated and timely. It is as if the authoritative SNMP engine was configured to start sending Traps to the second SNMP engine, which theoretically can happen without the knowledge of the second SNMP engine anyway. Anyway, the second SNMP engine may not expect to receive this Trap, but is allowed to see the management information contained in it. 3) Detection of messages which were not recently generated. A set of time indicators are included in the message, indicating the time of generation. Messages without recent time indicators are not considered authentic. In addition, an SNMP engine MUST drop any Responses that do not match an outstanding request. This however is the responsibility of the Message Processing Model. This memo allows the same user to be defined on multiple SNMP engines. Each SNMP engine maintains a value, snmpEngineID, which uniquely identifies the SNMP engine. This value is included in each message sent to/from the SNMP engine that is authoritative (see section 1.5.1). On receipt of a message, an authoritative SNMP engine checks the value to ensure that it is the intended recipient, and a non-authoritative SNMP engine uses the value to ensure that the message is processed using the correct state information. Each SNMP engine maintains two values, snmpEngineBoots and snmpEngineTime, which taken together provide an indication of time at that SNMP engine. Both of these values are included in an authenticated message sent to/received from that SNMP engine. On receipt, the values are checked to ensure that the indicated timeliness value is within a Time Window of the current time. The Time Window represents an administrative upper bound on acceptable delivery delay for protocol messages. For an SNMP engine to generate a message which an authoritative SNMP engine will accept as authentic, and to verify that a message received from that authoritative SNMP engine is authentic, such an SNMP engine must first achieve timeliness synchronization with the authoritative SNMP engine. See section 2.3. 1.6. Abstract Service Interfaces. Abstract service interfaces have been defined to describe the conceptual interfaces between the various subsystems within an SNMP entity. Similarly a set of abstract service interfaces have been Blumenthal & Wijnen Standards Track [Page 10]
RFC 2274 USM for SNMPv3 January 1998 defined within the User-based Security Model (USM) to describe the conceptual interfaces between the generic USM services and the self- contained authentication and privacy services. These abstract service interfaces are defined by a set of primitives that define the services provided and the abstract data elements that must be passed when the services are invoked. This section lists the primitives that have been defined for the User-based Security Model. 1.6.1. User-based Security Model Primitives for Authentication The User-based Security Model provides the following internal primitives to pass data back and forth between the Security Model itself and the authentication service: statusInformation = authenticateOutgoingMsg( IN authKey -- secret key for authentication IN wholeMsg -- unauthenticated complete message OUT authenticatedWholeMsg -- complete authenticated message ) statusInformation = authenticateIncomingMsg( IN authKey -- secret key for authentication IN authParameters -- as received on the wire IN wholeMsg -- as received on the wire OUT authenticatedWholeMsg -- complete authenticated message ) 1.6.2. User-based Security Model Primitives for Privacy The User-based Security Model provides the following internal primitives to pass data back and forth between the Security Model itself and the privacy service: statusInformation = encryptData( IN encryptKey -- secret key for encryption IN dataToEncrypt -- data to encrypt (scopedPDU) OUT encryptedData -- encrypted data (encryptedPDU) OUT privParameters -- filled in by service provider ) statusInformation = decryptData( IN decryptKey -- secret key for decrypting IN privParameters -- as received on the wire Blumenthal & Wijnen Standards Track [Page 11]
RFC 2274 USM for SNMPv3 January 1998 IN encryptedData -- encrypted data (encryptedPDU) OUT decryptedData -- decrypted data (scopedPDU) ) 2. Elements of the Model This section contains definitions required to realize the security model defined by this memo. 2.1. User-based Security Model Users Management operations using this Security Model make use of a defined set of user identities. For any user on whose behalf management operations are authorized at a particular SNMP engine, that SNMP engine must have knowledge of that user. An SNMP engine that wishes to communicate with another SNMP engine must also have knowledge of a user known to that engine, including knowledge of the applicable attributes of that user. A user and its attributes are defined as follows: userName A string representing the name of the user. securityName A human-readable string representing the user in a format that is Security Model independent. authProtocol An indication of whether messages sent on behalf of this user can be authenticated, and if so, the type of authentication protocol which is used. Two such protocols are defined in this memo: - the HMAC-MD5-96 authentication protocol. - the HMAC-SHA-96 authentication protocol. authKey If messages sent on behalf of this user can be authenticated, the (private) authentication key for use with the authentication protocol. Note that a user's authentication key will normally be different at different authoritative SNMP engines. The authKey is not accessible via SNMP. The length requirements of the authKey are defined by the authProtocol in use. authKeyChange and authOwnKeyChange The only way to remotely update the authentication key. Does that in a secure manner, so that the update can be completed without the need to employ privacy protection. Blumenthal & Wijnen Standards Track [Page 12]
RFC 2274 USM for SNMPv3 January 1998 privProtocol An indication of whether messages sent on behalf of this user can be protected from disclosure, and if so, the type of privacy protocol which is used. One such protocol is defined in this memo: the CBC-DES Symmetric Encryption Protocol. privKey If messages sent on behalf of this user can be en/decrypted, the (private) privacy key for use with the privacy protocol. Note that a user's privacy key will normally be different at different authoritative SNMP engines. The privKey is not accessible via SNMP. The length requirements of the privKey are defined by the privProtocol in use. privKeyChange and privOwnKeyChange The only way to remotely update the encryption key. Does that in a secure manner, so that the update can be completed without the need to employ privacy protection. 2.2. Replay Protection Each SNMP engine maintains three objects: - snmpEngineID, which (at least within an administrative domain) uniquely and unambiguously identifies an SNMP engine. - snmpEngineBoots, which is a count of the number of times the SNMP engine has re-booted/re-initialized since snmpEngineID was last configured; and, - snmpEngineTime, which is the number of seconds since the snmpEngineBoots counter was last incremented. Each SNMP engine is always authoritative with respect to these objects in its own SNMP entity. It is the responsibility of a non-authoritative SNMP engine to synchronize with the authoritative SNMP engine, as appropriate. An authoritative SNMP engine is required to maintain the values of its snmpEngineID and snmpEngineBoots in non-volatile storage. 2.2.1. msgAuthoritativeEngineID The msgAuthoritativeEngineID value contained in an authenticated message is used to defeat attacks in which messages from one SNMP engine to another SNMP engine are replayed to a different SNMP engine. It represents the snmpEngineID at the authoritative SNMP engine involved in the exchange of the message. Blumenthal & Wijnen Standards Track [Page 13]
RFC 2274 USM for SNMPv3 January 1998 When an authoritative SNMP engine is first installed, it sets its local value of snmpEngineID according to a enterprise-specific algorithm (see the definition of the Textual Convention for SnmpEngineID in the SNMP Architecture document [RFC2271]). 2.2.2. msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime The msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime values contained in an authenticated message are used to defeat attacks in which messages are replayed when they are no longer valid. They represent the snmpEngineBoots and snmpEngineTime values at the authoritative SNMP engine involved in the exchange of the message. Through use of snmpEngineBoots and snmpEngineTime, there is no requirement for an SNMP engine to have a non-volatile clock which ticks (i.e., increases with the passage of time) even when the SNMP engine is powered off. Rather, each time an SNMP engine re-boots, it retrieves, increments, and then stores snmpEngineBoots in non-volatile storage, and resets snmpEngineTime to zero. When an SNMP engine is first installed, it sets its local values of snmpEngineBoots and snmpEngineTime to zero. If snmpEngineTime ever reaches its maximum value (2147483647), then snmpEngineBoots is incremented as if the SNMP engine has re-booted and snmpEngineTime is reset to zero and starts incrementing again. Each time an authoritative SNMP engine re-boots, any SNMP engines holding that authoritative SNMP engine's values of snmpEngineBoots and snmpEngineTime need to re-synchronize prior to sending correctly authenticated messages to that authoritative SNMP engine (see Section 2.3 for (re-)synchronization procedures). Note, however, that the procedures do provide for a notification to be accepted as authentic by a receiving SNMP engine, when sent by an authoritative SNMP engine which has re-booted since the receiving SNMP engine last (re-)synchronized. If an authoritative SNMP engine is ever unable to determine its latest snmpEngineBoots value, then it must set its snmpEngineBoots value to 2147483647. Whenever the local value of snmpEngineBoots has the value 2147483647 it latches at that value and an authenticated message always causes an notInTimeWindow authentication failure. In order to reset an SNMP engine whose snmpEngineBoots value has reached the value 2147483647, manual intervention is required. The engine must be physically visited and re-configured, either Blumenthal & Wijnen Standards Track [Page 14]
RFC 2274 USM for SNMPv3 January 1998 with a new snmpEngineID value, or with new secret values for the authentication and privacy protocols of all users known to that SNMP engine. Note that even if an SNMP engine re-boots once a second that it would still take approximately 68 years before the max value of 2147483647 would be reached. 2.2.3. Time Window The Time Window is a value that specifies the window of time in which a message generated on behalf of any user is valid. This memo specifies that the same value of the Time Window, 150 seconds, is used for all users. 2.3. Time Synchronization Time synchronization, required by a non-authoritative SNMP engine in order to proceed with authentic communications, has occurred when the non-authoritative SNMP engine has obtained a local notion of the authoritative SNMP engine's values of snmpEngineBoots and snmpEngineTime from the authoritative SNMP engine. These values must be (and remain) within the authoritative SNMP engine's Time Window. So the local notion of the authoritative SNMP engine's values must be kept loosely synchronized with the values stored at the authoritative SNMP engine. In addition to keeping a local copy of snmpEngineBoots and snmpEngineTime from the authoritative SNMP engine, a non-authoritative SNMP engine must also keep one local variable, latestReceivedEngineTime. This value records the highest value of snmpEngineTime that was received by the non-authoritative SNMP engine from the authoritative SNMP engine and is used to eliminate the possibility of replaying messages that would prevent the non-authoritative SNMP engine's notion of the snmpEngineTime from advancing. A non-authoritative SNMP engine must keep local notions of these values for each authoritative SNMP engine with which it wishes to communicate. Since each authoritative SNMP engine is uniquely and unambiguously identified by its value of snmpEngineID, the non-authoritative SNMP engine may use this value as a key in order to cache its local notions of these values. Time synchronization occurs as part of the procedures of receiving an SNMP message (Section 3.2, step 7b). As such, no explicit time synchronization procedure is required by a non-authoritative SNMP engine. Note, that whenever the local value of snmpEngineID is changed (e.g., through discovery) or when secure communications are first established with an authoritative SNMP engine, the local Blumenthal & Wijnen Standards Track [Page 15]
RFC 2274 USM for SNMPv3 January 1998 values of snmpEngineBoots and latestReceivedEngineTime should be set to zero. This will cause the time synchronization to occur when the next authentic message is received. 2.4. SNMP Messages Using this Security Model The syntax of an SNMP message using this Security Model adheres to the message format defined in the version-specific Message Processing Model document (for example [RFC2272]). The field msgSecurityParameters in SNMPv3 messages has a data type of OCTET STRING. Its value is the BER serialization of the following ASN.1 sequence: USMSecurityParametersSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN UsmSecurityParameters ::= SEQUENCE { -- global User-based security parameters msgAuthoritativeEngineID OCTET STRING, msgAuthoritativeEngineBoots INTEGER (0..2147483647), msgAuthoritativeEngineTime INTEGER (0..2147483647), msgUserName OCTET STRING (SIZE(1..32)), -- authentication protocol specific parameters msgAuthenticationParameters OCTET STRING, -- privacy protocol specific parameters msgPrivacyParameters OCTET STRING } END The fields of this sequence are: - The msgAuthoritativeEngineID specifies the snmpEngineID of the authoritative SNMP engine involved in the exchange of the message. - The msgAuthoritativeEngineBoots specifies the snmpEngineBoots value at the authoritative SNMP engine involved in the exchange of the message. - The msgAuthoritativeEngineTime specifies the snmpEngineTime value at the authoritative SNMP engine involved in the exchange of the message. - The msgUserName specifies the user (principal) on whose behalf the message is being exchanged. Blumenthal & Wijnen Standards Track [Page 16]
RFC 2274 USM for SNMPv3 January 1998 - The msgAuthenticationParameters are defined by the authentication protocol in use for the message, as defined by the usmUserAuthProtocol column in the user's entry in the usmUserTable. - The msgPrivacyParameters are defined by the privacy protocol in use for the message, as defined by the usmUserPrivProtocol column in the user's entry in the usmUserTable). See appendix A.4 for an example of the BER encoding of field msgSecurityParameters. 2.5. Services provided by the User-based Security Model This section describes the services provided by the User-based Security Model with their inputs and outputs. The services are described as primitives of an abstract service interface and the inputs and outputs are described as abstract data elements as they are passed in these abstract service primitives. 2.5.1. Services for Generating an Outgoing SNMP Message When the Message Processing (MP) Subsystem invokes the User-based Security module to secure an outgoing SNMP message, it must use the appropriate service as provided by the Security module. These two services are provided: 1) A service to generate a Request message. The abstract service primitive is: statusInformation = -- success or errorIndication generateRequestMsg( IN messageProcessingModel -- typically, SNMP version IN globalData -- message header, admin data IN maxMessageSize -- of the sending SNMP entity IN securityModel -- for the outgoing message IN securityEngineID -- authoritative SNMP entity IN securityName -- on behalf of this principal IN securityLevel -- Level of Security requested IN scopedPDU -- message (plaintext) payload OUT securityParameters -- filled in by Security Module OUT wholeMsg -- complete generated message OUT wholeMsgLength -- length of generated message ) 2) A service to generate a Response message. The abstract service primitive is: Blumenthal & Wijnen Standards Track [Page 17]
RFC 2274 USM for SNMPv3 January 1998 statusInformation = -- success or errorIndication generateResponseMsg( IN messageProcessingModel -- typically, SNMP version IN globalData -- message header, admin data IN maxMessageSize -- of the sending SNMP entity IN securityModel -- for the outgoing message IN securityEngineID -- authoritative SNMP entity IN securityName -- on behalf of this principal IN securityLevel -- Level of Security requested IN scopedPDU -- message (plaintext) payload IN securityStateReference -- reference to security state -- information from original -- request OUT securityParameters -- filled in by Security Module OUT wholeMsg -- complete generated message OUT wholeMsgLength -- length of generated message ) The abstract data elements passed as parameters in the abstract service primitives are as follows: statusInformation An indication of whether the encoding and securing of the message was successful. If not it is an indication of the problem. essageProcessingModel The SNMP version number for the message to be generated. This data is not used by the User-based Security module. globalData The message header (i.e., its administrative information). This data is not used by the User-based Security module. maxMessageSize The maximum message size as included in the message. This data is not used by the User-based Security module. securityParameters These are the security parameters. They will be filled in by the User-based Security module. securityModel The securityModel in use. Should be User-based Security Model. This data is not used by the User-based Security module. securityName Together with the snmpEngineID it identifies a row in the usmUserTable that is to be used for securing the message. The securityName has a format that is independent of the Security Model. In case of a response this parameter is ignored and the value from the cache is used. securityLevel The Level of Security from which the User-based Security module determines if the message needs to be protected from disclosure Blumenthal & Wijnen Standards Track [Page 18]
RFC 2274 USM for SNMPv3 January 1998 and if the message needs to be authenticated. In case of a response this parameter is ignored and the value from the cache is used. securityEngineID The snmpEngineID of the authoritative SNMP engine to which a Request message is to be sent. In case of a response it is implied to be the processing SNMP engine's snmpEngineID and so if it is specified, then it is ignored. scopedPDU The message payload. The data is opaque as far as the User-based Security Model is concerned. securityStateReference A handle/reference to cachedSecurityData to be used when securing an outgoing Response message. This is the exact same handle/reference as it was generated by the User-based Security module when processing the incoming Request message to which this is the Response message. wholeMsg The fully encoded and secured message ready for sending on the wire. wholeMsgLength The length of the encoded and secured message (wholeMsg). Upon completion of the process, the User-based Security module returns statusInformation. If the process was successful, the completed message with privacy and authentication applied if such was requested by the specified securityLevel is returned. If the process was not successful, then an errorIndication is returned. 2.5.2. Services for Processing an Incoming SNMP Message When the Message Processing (MP) Subsystem invokes the User-based Security module to verify proper security of an incoming message, it must use the service provided for an incoming message. The abstract service primitive is: statusInformation = -- errorIndication or success -- error counter OID/value if error processIncomingMsg( IN messageProcessingModel -- typically, SNMP version IN maxMessageSize -- of the sending SNMP entity IN securityParameters -- for the received message IN securityModel -- for the received message IN securityLevel -- Level of Security IN wholeMsg -- as received on the wire IN wholeMsgLength -- length as received on the wire OUT securityEngineID -- authoritative SNMP entity OUT securityName -- identification of the principal Blumenthal & Wijnen Standards Track [Page 19]
RFC 2274 USM for SNMPv3 January 1998 OUT scopedPDU, -- message (plaintext) payload OUT maxSizeResponseScopedPDU -- maximum size of the Response PDU OUT securityStateReference -- reference to security state ) -- information, needed for response The abstract data elements passed as parameters in the abstract service primitives are as follows: statusInformation An indication of whether the process was successful or not. If not, then the statusInformation includes the OID and the value of the error counter that was incremented. messageProcessingModel The SNMP version number as received in the message. This data is not used by the User-based Security module. maxMessageSize The maximum message size as included in the message. The User- based Security module uses this value to calculate the maxSizeResponseScopedPDU. securityParameters These are the security parameters as received in the message. securityModel The securityModel in use. Should be the User-based Security Model. This data is not used by the User-based Security module. securityLevel The Level of Security from which the User-based Security module determines if the message needs to be protected from disclosure and if the message needs to be authenticated. wholeMsg The whole message as it was received. wholeMsgLength The length of the message as it was received (wholeMsg). securityEngineID The snmpEngineID that was extracted from the field msgAuthoritativeEngineID and that was used to lookup the secrets in the usmUserTable. securityName The security name representing the user on whose behalf the message was received. The securityName has a format that is independent of the Security Model. scopedPDU The message payload. The data is opaque as far as the User-based Security Model is concerned. maxSizeResponseScopedPDU The maximum size of a scopedPDU to be included in a possible Response message. The User-base Security module calculates Blumenthal & Wijnen Standards Track [Page 20]
RFC 2274 USM for SNMPv3 January 1998 this size based on the mms (as received in the message) and the space required for the message header (including the securityParameters) for such a Response message. securityStateReference A handle/reference to cachedSecurityData to be used when securing an outgoing Response message. When the Message Processing Subsystem calls the User-based Security module to generate a response to this incoming message it must pass this handle/reference. Upon completion of the process, the User-based Security module returns statusInformation and, if the process was successful, the additional data elements for further processing of the message. If the process was not successful, then an errorIndication, possibly with a OID and value pair of an error counter that was incremented. 2.6. Key Localization Algorithm. A localized key is a secret key shared between a user U and one authoritative SNMP engine E. Even though a user may have only one password and therefore one key for the whole network, the actual secrets shared between the user and each authoritative SNMP engine will be different. This is achieved by key localization [Localized- key]. First, if a user uses a password, then the user's password is converted into a key Ku using one of the two algorithms described in Appendices A.2.1 and A.2.2. To convert key Ku into a localized key Kul of user U at the authoritative SNMP engine E, one appends the snmpEngineID of the authoritative SNMP engine to the key Ku and then appends the key Ku to the result, thus enveloping the snmpEngineID within the two copies of user's key Ku. Then one runs a secure hash function (which one depends on the authentication protocol defined for this user U at authoritative SNMP engine E; this document defines two authentication protocols with their associated algorithms based on MD5 and SHA). The output of the hash-function is the localized key Kul for user U at the authoritative SNMP engine E. 3. Elements of Procedure This section describes the security related procedures followed by an SNMP engine when processing SNMP messages according to the User-based Security Model. Blumenthal & Wijnen Standards Track [Page 21]
RFC 2274 USM for SNMPv3 January 1998 3.1. Generating an Outgoing SNMP Message This section describes the procedure followed by an SNMP engine whenever it generates a message containing a management operation (like a request, a response, a notification, or a report) on behalf of a user, with a particular securityLevel. 1) a) If any securityStateReference is passed (Response message), then information concerning the user is extracted from the cachedSecurityData. The securityEngineID and the securityLevel are extracted from the cachedSecurityData. The cachedSecurityData can now be discarded. Otherwise, b) based on the securityName, information concerning the user at the destination snmpEngineID, specified by the securityEngineID, is extracted from the Local Configuration Datastore (LCD, usmUserTable). If information about the user is absent from the LCD, then an error indication (unknownSecurityName) is returned to the calling module. 2) If the securityLevel specifies that the message is to be protected from disclosure, but the user does not support both an authentication and a privacy protocol then the message cannot be sent. An error indication (unsupportedSecurityLevel) is returned to the calling module. 3) If the securityLevel specifies that the message is to be authenticated, but the user does not support an authentication protocol, then the message cannot be sent. An error indication (unsupportedSecurityLevel) is returned to the calling module. 4) a) If the securityLevel specifies that the message is to be protected from disclosure, then the octet sequence representing the serialized scopedPDU is encrypted according to the user's privacy protocol. To do so a call is made to the privacy module that implements the user's privacy protocol according to the abstract primitive: statusInformation = -- success or failure encryptData( IN encryptKey -- user's localized privKey IN dataToEncrypt -- serialized scopedPDU OUT encryptedData -- serialized encryptedPDU OUT privParameters -- serialized privacy parameters ) Blumenthal & Wijnen Standards Track [Page 22]
RFC 2274 USM for SNMPv3 January 1998 statusInformation indicates if the encryption process was successful or not. encryptKey the user's localized private privKey is the secret key that can be used by the encryption algorithm. dataToEncrypt the serialized scopedPDU is the data that to be encrypted. encryptedData the encryptedPDU represents the encrypted scopedPDU, encoded as an OCTET STRING. privParameters the privacy parameters, encoded as an OCTET STRING. If the privacy module returns failure, then the message cannot be sent and an error indication (encryptionError) is returned to the calling module. If the privacy module returns success, then the returned privParameters are put into the msgPrivacyParameters field of the securityParameters and the encryptedPDU serves as the payload of the message being prepared. Otherwise, b) If the securityLevel specifies that the message is not to be protected from disclosure, then the NULL string is encoded as an OCTET STRING and put into the msgPrivacyParameters field of the securityParameters and the plaintext scopedPDU serves as the payload of the message being prepared. 5) The snmpEngineID is encoded as an OCTET STRING into the msgAuthoritativeEngineID field of the securityParameters. Note that an empty (zero length) snmpEngineID is OK for a Request message, because that will cause the remote (authoritative) SNMP engine to return a Report PDU with the proper snmpEngineID included in the msgAuthoritativeEngineID in the securityParameters of that returned Report PDU. 6) a) If the securityLevel specifies that the message is to be authenticated, then the current values of snmpEngineBoots and snmpEngineTime corresponding to the snmpEngineID from the LCD are used. Otherwise, b) If this is a Response message, then the current value of snmpEngineBoots and snmpEngineTime corresponding to the local snmpEngineID from the LCD are used. Blumenthal & Wijnen Standards Track [Page 23]
RFC 2274 USM for SNMPv3 January 1998 Otherwise, c) If this is a Request message, then a zero value is used for both snmpEngineBoots and snmpEngineTime. This zero value gets used if snmpEngineID is empty. The values are encoded as INTEGER respectively into the msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime fields of the securityParameters. 7) The userName is encoded as an OCTET STRING into the msgUserName field of the securityParameters. 8) a) If the securityLevel specifies that the message is to be authenticated, the message is authenticated according to the user's authentication protocol. To do so a call is made to the authentication module that implements the user's authentication protocol according to the abstract service primitive: statusInformation = authenticateOutgoingMsg( IN authKey -- the user's localized authKey IN wholeMsg -- unauthenticated message OUT authenticatedWholeMsg -- authenticated complete message ) statusInformation indicates if authentication was successful or not. authKey the user's localized private authKey is the secret key that can be used by the authentication algorithm. wholeMsg the complete serialized message to be authenticated. authenticatedWholeMsg the same as the input given to the authenticateOutgoingMsg service, but with msgAuthenticationParameters properly filled in. If the authentication module returns failure, then the message cannot be sent and an error indication (authenticationFailure) is returned to the calling module. If the authentication module returns success, then the msgAuthenticationParameters field is put into the securityParameters and the authenticatedWholeMsg represents the serialization of the authenticated message being prepared. Blumenthal & Wijnen Standards Track [Page 24]
RFC 2274 USM for SNMPv3 January 1998 Otherwise, b) If the securityLevel specifies that the message is not to be authenticated then the NULL string is encoded as an OCTET STRING into the msgAuthenticationParameters field of the securityParameters. The wholeMsg is now serialized and then represents the unauthenticated message being prepared. 9) The completed message with its length is returned to the calling module with the statusInformation set to success. 3.2. Processing an Incoming SNMP Message This section describes the procedure followed by an SNMP engine whenever it receives a message containing a management operation on behalf of a user, with a particular securityLevel. To simplify the elements of procedure, the release of state information is not always explicitly specified. As a general rule, if state information is available when a message gets discarded, the state information should also be released. Also, when an error indication with an OID and value for an incremented counter is returned, then the available information (like securityStateReference) must be passed back to the caller so it can generate a Report PDU. 1) If the received securityParameters is not the serialization (according to the conventions of [RFC1906]) of an OCTET STRING formatted according to the UsmSecurityParameters defined in section 2.4, then the snmpInASNParseErrs counter [RFC1907] is incremented, and an error indication (parseError) is returned to the calling module. Note that we return without the OID and value of the incremented counter, because in this case there is not enough information to generate a Report PDU. 2) The values of the security parameter fields are extracted from the securityParameters. The securityEngineID to be returned to the caller is the value of the msgAuthoritativeEngineID field. The cachedSecurityData is prepared and a securityStateReference is prepared to reference this data. Values to be cached are: msgUserName securityEngineID securityLevel 3) If the value of the msgAuthoritativeEngineID field in the securityParameters is unknown then: Blumenthal & Wijnen Standards Track [Page 25]
RFC 2274 USM for SNMPv3 January 1998 a) a non-authoritative SNMP engine that performs discovery may optionally create a new entry in its Local Configuration Datastore (LCD) and continue processing; or b) the usmStatsUnknownEngineIDs counter is incremented, and an error indication (unknownEngineID) together with the OID and value of the incremented counter is returned to the calling module. 4) Information about the value of the msgUserName and msgAuthoritativeEngineID fields is extracted from the Local Configuration Datastore (LCD, usmUserTable). If no information is available for the user, then the usmStatsUnknownUserNames counter is incremented and an error indication (unknownSecurityName) together with the OID and value of the incremented counter is returned to the calling module. 5) If the information about the user indicates that it does not support the securityLevel requested by the caller, then the usmStatsUnsupportedSecLevels counter is incremented and an error indication (unsupportedSecurityLevel) together with the OID and value of the incremented counter is returned to the calling module. 6) If the securityLevel specifies that the message is to be authenticated, then the message is authenticated according to the user's authentication protocol. To do so a call is made to the authentication module that implements the user's authentication protocol according to the abstract service primitive: statusInformation = -- success or failure authenticateIncomingMsg( IN authKey -- the user's localized authKey IN authParameters -- as received on the wire IN wholeMsg -- as received on the wire OUT authenticatedWholeMsg -- checked for authentication ) statusInformation indicates if authentication was successful or not. authKey the user's localized private authKey is the secret key that can be used by the authentication algorithm. wholeMsg the complete serialized message to be authenticated. Blumenthal & Wijnen Standards Track [Page 26]
RFC 2274 USM for SNMPv3 January 1998 authenticatedWholeMsg the same as the input given to the authenticateIncomingMsg service, but after authentication has been checked. If the authentication module returns failure, then the message cannot be trusted, so the usmStatsWrongDigests counter is incremented and an error indication (authenticationFailure) together with the OID and value of the incremented counter is returned to the calling module. If the authentication module returns success, then the message is authentic and can be trusted so processing continues. 7) If the securityLevel indicates an authenticated message, then the local values of snmpEngineBoots and snmpEngineTime corresponding to the value of the msgAuthoritativeEngineID field are extracted from the Local Configuration Datastore. a) If the extracted value of msgAuthoritativeEngineID is the same as the value of snmpEngineID of the processing SNMP engine (meaning this is the authoritative SNMP engine), then if any of the following conditions is true, then the message is considered to be outside of the Time Window: - the local value of snmpEngineBoots is 2147483647; - the value of the msgAuthoritativeEngineBoots field differs from the local value of snmpEngineBoots; or, - the value of the msgAuthoritativeEngineTime field differs from the local notion of snmpEngineTime by more than +/- 150 seconds. If the message is considered to be outside of the Time Window then the usmStatsNotInTimeWindows counter is incremented and an error indication (notInTimeWindow) together with the OID and value of the incremented counter is returned to the calling module. b) If the extracted value of msgAuthoritativeEngineID is not the same as the value snmpEngineID of the processing SNMP engine (meaning this is not the authoritative SNMP engine), then: 1) if at least one of the following conditions is true: - the extracted value of the msgAuthoritativeEngineBoots field is greater than the local notion of the value of snmpEngineBoots; or, Blumenthal & Wijnen Standards Track [Page 27]
RFC 2274 USM for SNMPv3 January 1998 - the extracted value of the msgAuthoritativeEngineBoots field is equal to the local notion of the value of snmpEngineBoots, the extracted value of msgAuthoritativeEngineTime field is greater than the value of latestReceivedEngineTime, then the LCD entry corresponding to the extracted value of the msgAuthoritativeEngineID field is updated, by setting: - the local notion of the value of snmpEngineBoots to the value of the msgAuthoritativeEngineBoots field, - the local notion of the value of snmpEngineTime to the value of the msgAuthoritativeEngineTime field, and - the latestReceivedEngineTime to the value of the value of the msgAuthoritativeEngineTime field. 2) if any of the following conditions is true, then the message is considered to be outside of the Time Window: - the local notion of the value of snmpEngineBoots is 2147483647; - the value of the msgAuthoritativeEngineBoots field is less than the local notion of the value of snmpEngineBoots; or, - the value of the msgAuthoritativeEngineBoots field is equal to the local notion of the value of snmpEngineBoots and the value of the msgAuthoritativeEngineTime field is more than 150 seconds less than the local notion of of the value of snmpEngineTime. If the message is considered to be outside of the Time Window then an error indication (notInTimeWindow) is returned to the calling module; Note that this means that a too old (possibly replayed) message has been detected and is deemed unauthentic. Note that this procedure allows for the value of msgAuthoritativeEngineBoots in the message to be greater than the local notion of the value of snmpEngineBoots to allow for received messages to be accepted as authentic Blumenthal & Wijnen Standards Track [Page 28]
RFC 2274 USM for SNMPv3 January 1998 when received from an authoritative SNMP engine that has re-booted since the receiving SNMP engine last (re-)synchronized. Note that this procedure does not allow for automatic time synchronization if the non-authoritative SNMP engine has a real out-of-sync situation whereby the authoritative SNMP engine is more than 150 seconds behind the non-authoritative SNMP engine. 8) a) If the securityLevel indicates that the message was protected from disclosure, then the OCTET STRING representing the encryptedPDU is decrypted according to the user's privacy protocol to obtain an unencrypted serialized scopedPDU value. To do so a call is made to the privacy module that implements the user's privacy protocol according to the abstract primitive: statusInformation = -- success or failure decryptData( IN decryptKey -- the user's localized privKey IN privParameters -- as received on the wire IN encryptedData -- encryptedPDU as received OUT decryptedData -- serialized decrypted scopedPDU ) statusInformation indicates if the decryption process was successful or not. decryptKey the user's localized private privKey is the secret key that can be used by the decryption algorithm. privParameters the msgPrivacyParameters, encoded as an OCTET STRING. encryptedData the encryptedPDU represents the encrypted scopedPDU, encoded as an OCTET STRING. decryptedData the serialized scopedPDU if decryption is successful. If the privacy module returns failure, then the message can not be processed, so the usmStatsDecryptionErrors counter is incremented and an error indication (decryptionError) together with the OID and value of the incremented counter is returned to the calling module. If the privacy module returns success, then the decrypted scopedPDU is the message payload to be returned to the calling module. Blumenthal & Wijnen Standards Track [Page 29]
RFC 2274 USM for SNMPv3 January 1998 Otherwise, b) The scopedPDU component is assumed to be in plain text and is the message payload to be returned to the calling module. 9) The maxSizeResponseScopedPDU is calculated. This is the maximum size allowed for a scopedPDU for a possible Response message. Provision is made for a message header that allows the same securityLevel as the received Request. 10) The securityName for the user is retrieved from the usmUserTable. 11) The security data is cached as cachedSecurityData, so that a possible response to this message can and will use the same authentication and privacy secrets, the same securityLevel and the same value for msgAuthoritativeEngineID. Information to be saved/cached is as follows: msgUserName, usmUserAuthProtocol, usmUserAuthKey usmUserPrivProtocol, usmUserPrivKey securityEngineID, securityLevel 12) The statusInformation is set to success and a return is made to the calling module passing back the OUT parameters as specified in the processIncomingMsg primitive. 4. Discovery The User-based Security Model requires that a discovery process obtains sufficient information about other SNMP engines in order to communicate with them. Discovery requires an non-authoritative SNMP engine to learn the authoritative SNMP engine's snmpEngineID value before communication may proceed. This may be accomplished by generating a Request message with a securityLevel of noAuthNoPriv, a msgUserName of "initial", a msgAuthoritativeEngineID value of zero length, and the varBindList left empty. The response to this message will be a Report message containing the snmpEngineID of the authoritative SNMP engine as the value of the msgAuthoritativeEngineID field within the msgSecurityParameters field. It contains a Report PDU with the usmStatsUnknownEngineIDs counter in the varBindList. If authenticated communication is required, then the discovery process should also establish time synchronization with the authoritative SNMP engine. This may be accomplished by sending an Blumenthal & Wijnen Standards Track [Page 30]
RFC 2274 USM for SNMPv3 January 1998 authenticated Request message with the value of msgAuthoritativeEngineID set to the newly learned snmpEngineID and with the values of msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime set to zero. The response to this authenticated message will be a Report message containing the up to date values of the authoritative SNMP engine's snmpEngineBoots and snmpEngineTime as the value of the msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime fields respectively. It also contains the usmStatsNotInTimeWindows counter in the varBindList of the Report PDU. The time synchronization then happens automatically as part of the procedures in section 3.2 step 7b. See also section 2.3. 5. Definitions SNMP-USER-BASED-SM-MIB DEFINITIONS ::= BEGIN IMPORTS MODULE-IDENTITY, OBJECT-TYPE, OBJECT-IDENTITY, snmpModules, Counter32 FROM SNMPv2-SMI TEXTUAL-CONVENTION, TestAndIncr, RowStatus, RowPointer, StorageType, AutonomousType FROM SNMPv2-TC MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF SnmpAdminString, SnmpEngineID, snmpAuthProtocols, snmpPrivProtocols FROM SNMP-FRAMEWORK-MIB; snmpUsmMIB MODULE-IDENTITY LAST-UPDATED "9711200000Z" -- 20 Nov 1997, midnight ORGANIZATION "SNMPv3 Working Group" CONTACT-INFO "WG-email: snmpv3@tis.com Subscribe: majordomo@tis.com In msg body: subscribe snmpv3 Chair: Russ Mundy Trusted Information Systems postal: 3060 Washington Rd Glenwood MD 21738 USA email: mundy@tis.com phone: +1-301-854-6889 Co-editor Uri Blumenthal IBM T. J. Watson Research postal: 30 Saw Mill River Pkwy, Hawthorne, NY 10532 USA email: uri@watson.ibm.com Blumenthal & Wijnen Standards Track [Page 31]
RFC 2274 USM for SNMPv3 January 1998 phone: +1-914-784-7964 Co-editor: Bert Wijnen IBM T. J. Watson Research postal: Schagen 33 3461 GL Linschoten Netherlands email: wijnen@vnet.ibm.com phone: +31-348-432-794 " DESCRIPTION "The management information definitions for the SNMP User-based Security Model. " ::= { snmpModules 15 } -- Administrative assignments **************************************** usmMIBObjects OBJECT IDENTIFIER ::= { snmpUsmMIB 1 } usmMIBConformance OBJECT IDENTIFIER ::= { snmpUsmMIB 2 } -- Identification of Authentication and Privacy Protocols ************ usmNoAuthProtocol OBJECT-IDENTITY STATUS current DESCRIPTION "No Authentication Protocol." ::= { snmpAuthProtocols 1 } usmHMACMD5AuthProtocol OBJECT-IDENTITY STATUS current DESCRIPTION "The HMAC-MD5-96 Digest Authentication Protocol." REFERENCE "- H. Krawczyk, M. Bellare, R. Canetti HMAC: Keyed-Hashing for Message Authentication, RFC2104, Feb 1997. - Rivest, R., Message Digest Algorithm MD5, RFC1321. " ::= { snmpAuthProtocols 2 } usmHMACSHAAuthProtocol OBJECT-IDENTITY STATUS current DESCRIPTION "The HMAC-SHA-96 Digest Authentication Protocol." REFERENCE "- H. Krawczyk, M. Bellare, R. Canetti, HMAC: Keyed-Hashing for Message Authentication, RFC2104, Feb 1997. - Secure Hash Algorithm. NIST FIPS 180-1. " ::= { snmpAuthProtocols 3 } Blumenthal & Wijnen Standards Track [Page 32]
RFC 2274 USM for SNMPv3 January 1998 usmNoPrivProtocol OBJECT-IDENTITY STATUS current DESCRIPTION "No Privacy Protocol." ::= { snmpPrivProtocols 1 } usmDESPrivProtocol OBJECT-IDENTITY STATUS current DESCRIPTION "The CBC-DES Symmetric Encryption Protocol." REFERENCE "- Data Encryption Standard, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 46-1. Supersedes FIPS Publication 46, (January, 1977; reaffirmed January, 1988). - Data Encryption Algorithm, American National Standards Institute. ANSI X3.92-1981, (December, 1980). - DES Modes of Operation, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 81, (December, 1980). - Data Encryption Algorithm - Modes of Operation, American National Standards Institute. ANSI X3.106-1983, (May 1983). " ::= { snmpPrivProtocols 2 } -- Textual Conventions *********************************************** KeyChange ::= TEXTUAL-CONVENTION STATUS current DESCRIPTION "Every definition of an object with this syntax must identify a protocol P, a secret key K, and a hash algorithm H that produces output of L octets. The object's value is a manager-generated, partially-random value which, when modified, causes the value of the secret key K, to be modified via a one-way function. The value of an instance of this object is the concatenation of two components: first a 'random' component and then a 'delta' component. Blumenthal & Wijnen Standards Track [Page 33]
RFC 2274 USM for SNMPv3 January 1998 The lengths of the random and delta components are given by the corresponding value of the protocol P; if P requires K to be a fixed length, the length of both the random and delta components is that fixed length; if P allows the length of K to be variable up to a particular maximum length, the length of the random component is that maximum length and the length of the delta component is any length less than or equal to that maximum length. For example, usmHMACMD5AuthProtocol requires K to be a fixed length of 16 octets and L - of 16 octets. usmHMACSHAAuthProtocol requires K to be a fixed length of 20 octets and L - of 20 octets. Other protocols may define other sizes, as deemed appropriate. When a requestor wants to change the old key K to a new key keyNew on a remote entity, the 'random' component is obtained from either a true random generator, or from a pseudorandom generator, and the 'delta' component is computed as follows: - a temporary variable is initialized to the existing value of K; - if the length of the keyNew is greater than L octets, then: - the random component is appended to the value of the temporary variable, and the result is input to the the hash algorithm H to produce a digest value, and the temporary variable is set to this digest value; - the value of the temporary variable is XOR-ed with the first (next) L-octets (16 octets in case of MD5) of the keyNew to produce the first (next) L-octets (16 octets in case of MD5) of the 'delta' component. - the above two steps are repeated until the unused portion of the delta component is L octets or less, - the random component is appended to the value of the temporary variable, and the result is input to the hash algorithm H to produce a digest value; - this digest value, truncated if necessary to be the same length as the unused portion of the keyNew, is XOR-ed with the unused portion of the keyNew to produce the (final portion of the) 'delta' component. For example, using MD5 as the hash algorithm H: iterations = (lenOfDelta - 1)/16; /* integer division */ temp = keyOld; for (i = 0; i < iterations; i++) { temp = MD5 (temp || random); Blumenthal & Wijnen Standards Track [Page 34]
RFC 2274 USM for SNMPv3 January 1998 delta[i*16 .. (i*16)+15] = temp XOR keyNew[i*16 .. (i*16)+15]; } temp = MD5 (temp || random); delta[i*16 .. lenOfDelta-1] = temp XOR keyNew[i*16 .. lenOfDelta-1]; The 'random' and 'delta' components are then concatenated as described above, and the resulting octet string is sent to the receipient as the new value of an instance of this object. At the receiver side, when an instance of this object is set to a new value, then a new value of K is computed as follows: - a temporary variable is initialized to the existing value of K; - if the length of the delta component is greater than L octets, then: - the random component is appended to the value of the temporary variable, and the result is input to the the hash algorithm H to produce a digest value, and the temporary variable is set to this digest value; - the value of the temporary variable is XOR-ed with the first (next) L-octets (16 octets in case of MD5) of the delta component to produce the first (next) L-octets (16 octets in case of MD5) of the new value of K. - the above two steps are repeated until the unused portion of the delta component is L octets or less, - the random component is appended to the value of the temporary variable, and the result is input to the hash algorithm H to produce a digest value; - this digest value, truncated if necessary to be the same length as the unused portion of the delta component, is XOR-ed with the unused portion of the delta component to produce the (final portion of the) new value of K. For example, using MD5 as the hash algorithm H: iterations = (lenOfDelta - 1)/16; /* integer division */ temp = keyOld; for (i = 0; i < iterations; i++) { temp = MD5 (temp || random); keyNew[i*16 .. (i*16)+15] = temp XOR delta[i*16 .. (i*16)+15]; } temp = MD5 (temp || random); Blumenthal & Wijnen Standards Track [Page 35]
RFC 2274 USM for SNMPv3 January 1998 keyNew[i*16 .. lenOfDelta-1] = temp XOR delta[i*16 .. lenOfDelta-1]; The value of an object with this syntax, whenever it is retrieved by the management protocol, is always the zero length string. " SYNTAX OCTET STRING -- Statistics for the User-based Security Model ********************** usmStats OBJECT IDENTIFIER ::= { usmMIBObjects 1 } usmStatsUnsupportedSecLevels OBJECT-TYPE SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION "The total number of packets received by the SNMP engine which were dropped because they requested a securityLevel that was unknown to the SNMP engine or otherwise unavailable. " ::= { usmStats 1 } usmStatsNotInTimeWindows OBJECT-TYPE SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION "The total number of packets received by the SNMP engine which were dropped because they appeared outside of the authoritative SNMP engine's window. " ::= { usmStats 2 } usmStatsUnknownUserNames OBJECT-TYPE SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION "The total number of packets received by the SNMP engine which were dropped because they referenced a user that was not known to the SNMP engine. " ::= { usmStats 3 } usmStatsUnknownEngineIDs OBJECT-TYPE Blumenthal & Wijnen Standards Track [Page 36]
RFC 2274 USM for SNMPv3 January 1998 SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION "The total number of packets received by the SNMP engine which were dropped because they referenced an snmpEngineID that was not known to the SNMP engine. " ::= { usmStats 4 } usmStatsWrongDigests OBJECT-TYPE SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION "The total number of packets received by the SNMP engine which were dropped because they didn't contain the expected digest value. " ::= { usmStats 5 } usmStatsDecryptionErrors OBJECT-TYPE SYNTAX Counter32 MAX-ACCESS read-only STATUS current DESCRIPTION "The total number of packets received by the SNMP engine which were dropped because they could not be decrypted. " ::= { usmStats 6 } -- The usmUser Group ************************************************ usmUser OBJECT IDENTIFIER ::= { usmMIBObjects 2 } usmUserSpinLock OBJECT-TYPE SYNTAX TestAndIncr MAX-ACCESS read-write STATUS current DESCRIPTION "An advisory lock used to allow several cooperating Command Generator Applications to coordinate their use of facilities to alter secrets in the usmUserTable. " ::= { usmUser 1 } -- The table of valid users for the User-based Security Model ******** usmUserTable OBJECT-TYPE SYNTAX SEQUENCE OF UsmUserEntry Blumenthal & Wijnen Standards Track [Page 37]
RFC 2274 USM for SNMPv3 January 1998 MAX-ACCESS not-accessible STATUS current DESCRIPTION "The table of users configured in the SNMP engine's Local Configuration Datastore (LCD)." ::= { usmUser 2 } usmUserEntry OBJECT-TYPE SYNTAX UsmUserEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "A user configured in the SNMP engine's Local Configuration Datastore (LCD) for the User-based Security Model. " INDEX { usmUserEngineID, usmUserName } ::= { usmUserTable 1 } UsmUserEntry ::= SEQUENCE { usmUserEngineID SnmpEngineID, usmUserName SnmpAdminString, usmUserSecurityName SnmpAdminString, usmUserCloneFrom RowPointer, usmUserAuthProtocol AutonomousType, usmUserAuthKeyChange KeyChange, usmUserOwnAuthKeyChange KeyChange, usmUserPrivProtocol AutonomousType, usmUserPrivKeyChange KeyChange, usmUserOwnPrivKeyChange KeyChange, usmUserPublic OCTET STRING, usmUserStorageType StorageType, usmUserStatus RowStatus } usmUserEngineID OBJECT-TYPE SYNTAX SnmpEngineID MAX-ACCESS not-accessible STATUS current DESCRIPTION "An SNMP engine's administratively-unique identifier. In a simple agent, this value is always that agent's own snmpEngineID value. The value can also take the value of the snmpEngineID of a remote SNMP engine with which this user can communicate. Blumenthal & Wijnen Standards Track [Page 38]
RFC 2274 USM for SNMPv3 January 1998 " ::= { usmUserEntry 1 } usmUserName OBJECT-TYPE SYNTAX SnmpAdminString (SIZE(1..32)) MAX-ACCESS not-accessible STATUS current DESCRIPTION "A human readable string representing the name of the user. This is the (User-based Security) Model dependent security ID. " ::= { usmUserEntry 2 } usmUserSecurityName OBJECT-TYPE SYNTAX SnmpAdminString MAX-ACCESS read-only STATUS current DESCRIPTION "A human readable string representing the user in Security Model independent format. The default transformation of the User-based Security Model dependent security ID to the securityName and vice versa is the identity function so that the securityName is the same as the userName. " ::= { usmUserEntry 3 } usmUserCloneFrom OBJECT-TYPE SYNTAX RowPointer MAX-ACCESS read-create STATUS current DESCRIPTION "A pointer to another conceptual row in this usmUserTable. The user in this other conceptual row is called the clone-from user. When a new user is created (i.e., a new conceptual row is instantiated in this table), the privacy and authentication parameters of the new user are cloned from its clone-from user. The first time an instance of this object is set by a management operation (either at or after its instantiation), the cloning process is invoked. Subsequent writes are successful but invoke no action to be taken by the receiver. The cloning process fails with an 'inconsistentName' Blumenthal & Wijnen Standards Track [Page 39]
RFC 2274 USM for SNMPv3 January 1998 error if the conceptual row representing the clone-from user is not in an active state when the cloning process is invoked. Cloning also causes the initial values of the secret authentication key and the secret encryption key of the new user to be set to the same value as the corresponding secret of the clone-from user. When this object is read, the ZeroDotZero OID is returned. " ::= { usmUserEntry 4 } usmUserAuthProtocol OBJECT-TYPE SYNTAX AutonomousType MAX-ACCESS read-create STATUS current DESCRIPTION "An indication of whether messages sent on behalf of this user to/from the SNMP engine identified by usmUserEngineID, can be authenticated, and if so, the type of authentication protocol which is used. An instance of this object is created concurrently with the creation of any other object instance for the same user (i.e., as part of the processing of the set operation which creates the first object instance in the same conceptual row). Once created, the value of an instance of this object can not be changed. If a set operation tries to set a value for an unknown or unsupported protocol, then a wrongValue error must be returned. " DEFVAL { usmHMACMD5AuthProtocol } ::= { usmUserEntry 5 } usmUserAuthKeyChange OBJECT-TYPE SYNTAX KeyChange -- typically (SIZE (0..32)) MAX-ACCESS read-create STATUS current DESCRIPTION "An object, which when modified, causes the secret authentication key used for messages sent on behalf of this user to/from the SNMP engine identified by usmUserEngineID, to be modified via a one-way function. Blumenthal & Wijnen Standards Track [Page 40]
RFC 2274 USM for SNMPv3 January 1998 The associated protocol is the usmUserAuthProtocol. The associated secret key is the user's secret authentication key (authKey). The associated hash algorithm is the algorithm used by the user's usmUserAuthProtocol. When creating a new user, it is an 'inconsistentName' error for a Set operation to refer to this object unless it is previously or concurrently initialized through a set operation on the corresponding value of usmUserCloneFrom. " DEFVAL { ''H } -- the empty string ::= { usmUserEntry 6 } usmUserOwnAuthKeyChange OBJECT-TYPE SYNTAX KeyChange -- typically (SIZE (0..32)) MAX-ACCESS read-create STATUS current DESCRIPTION "Behaves exactly as usmUserAuthKeyChange, with one notable difference: in order for the Set operation to succeed, the usmUserName of the operation requester must match the usmUserName that indexes the row which is targeted by this operation. The idea here is that access to this column can be public, since it will only allow a user to change his own secret authentication key (authKey). " DEFVAL { ''H } -- the empty string ::= { usmUserEntry 7 } usmUserPrivProtocol OBJECT-TYPE SYNTAX AutonomousType MAX-ACCESS read-create STATUS current DESCRIPTION "An indication of whether messages sent on behalf of this user to/from the SNMP engine identified by usmUserEngineID, can be protected from disclosure, and if so, the type of privacy protocol which is used. An instance of this object is created concurrently with the creation of any other object instance for the same user (i.e., as part of the processing of the set operation which creates the first object instance in the same conceptual row). Once created, the value of an instance of this object can not be Blumenthal & Wijnen Standards Track [Page 41]
RFC 2274 USM for SNMPv3 January 1998 changed. If a set operation tries to set a value for an unknown or unsupported protocol, then a wrongValue error must be returned. " DEFVAL { usmNoPrivProtocol } ::= { usmUserEntry 8 } usmUserPrivKeyChange OBJECT-TYPE SYNTAX KeyChange -- typically (SIZE (0..32)) MAX-ACCESS read-create STATUS current DESCRIPTION "An object, which when modified, causes the secret encryption key used for messages sent on behalf of this user to/from the SNMP engine identified by usmUserEngineID, to be modified via a one-way function. The associated protocol is the usmUserPrivProtocol. The associated secret key is the user's secret privacy key (privKey). The associated hash algorithm is the algorithm used by the user's usmUserAuthProtocol. When creating a new user, it is an 'inconsistentName' error for a set operation to refer to this object unless it is previously or concurrently initialized through a set operation on the corresponding value of usmUserCloneFrom. " DEFVAL { ''H } -- the empty string ::= { usmUserEntry 9 } usmUserOwnPrivKeyChange OBJECT-TYPE SYNTAX KeyChange -- typically (SIZE (0..32)) MAX-ACCESS read-create STATUS current DESCRIPTION "Behaves exactly as usmUserPrivKeyChange, with one notable difference: in order for the Set operation to succeed, the usmUserName of the operation requester must match the usmUserName that indexes the row which is targeted by this operation. The idea here is that access to this column can be public, since it will only allow a user to change his own secret privacy key (privKey). " Blumenthal & Wijnen Standards Track [Page 42]
RFC 2274 USM for SNMPv3 January 1998 DEFVAL { ''H } -- the empty string ::= { usmUserEntry 10 } usmUserPublic OBJECT-TYPE SYNTAX OCTET STRING (SIZE(0..32)) MAX-ACCESS read-create STATUS current DESCRIPTION "A publicly-readable value which is written as part of the procedure for changing a user's secret authentication and/or privacy key, and later read to determine whether the change of the secret was effected. " DEFVAL { ''H } -- the empty string ::= { usmUserEntry 11 } usmUserStorageType OBJECT-TYPE SYNTAX StorageType MAX-ACCESS read-create STATUS current DESCRIPTION "The storage type for this conceptual row. Conceptual rows having the value 'permanent' must allow write-access at a minimum to: - usmUserAuthKeyChange, usmUserOwnAuthKeyChange and usmUserPublic for a user who employs authentication, and - usmUserPrivKeyChange, usmUserOwnPrivKeyChange and usmUserPublic for a user who employs privacy. Note that any user who employs authentication or privacy must allow its secret(s) to be updated and thus cannot be 'readOnly'. " DEFVAL { nonVolatile } ::= { usmUserEntry 12 } usmUserStatus OBJECT-TYPE SYNTAX RowStatus MAX-ACCESS read-create STATUS current DESCRIPTION "The status of this conceptual row. Until instances of all corresponding columns are appropriately configured, the value of the corresponding instance of the usmUserStatus column Blumenthal & Wijnen Standards Track [Page 43]
RFC 2274 USM for SNMPv3 January 1998 is 'notReady'. In particular, a newly created row cannot be made active until the corresponding usmUserCloneFrom, usmUserAuthKeyChange, usmUserOwnAuthKeyChange, usmUserPrivKeyChange and usmUserOwnPrivKeyChange have all been set. The RowStatus TC [RFC1903] requires that this DESCRIPTION clause states under which circumstances other objects in this row can be modified: The value of this object has no effect on whether other objects in this conceptual row can be modified. " ::= { usmUserEntry 13 } -- Conformance Information ******************************************* usmMIBCompliances OBJECT IDENTIFIER ::= { usmMIBConformance 1 } usmMIBGroups OBJECT IDENTIFIER ::= { usmMIBConformance 2 } -- Compliance statements usmMIBCompliance MODULE-COMPLIANCE STATUS current DESCRIPTION "The compliance statement for SNMP engines which implement the SNMP-USER-BASED-SM-MIB. " MODULE -- this module MANDATORY-GROUPS { usmMIBBasicGroup } OBJECT usmUserAuthProtocol MIN-ACCESS read-only DESCRIPTION "Write access is not required." OBJECT usmUserPrivProtocol MIN-ACCESS read-only DESCRIPTION "Write access is not required." ::= { usmMIBCompliances 1 } -- Units of compliance usmMIBBasicGroup OBJECT-GROUP OBJECTS { usmStatsUnsupportedSecLevels, usmStatsNotInTimeWindows, Blumenthal & Wijnen Standards Track [Page 44]
RFC 2274 USM for SNMPv3 January 1998 usmStatsUnknownUserNames, usmStatsUnknownEngineIDs, usmStatsWrongDigests, usmStatsDecryptionErrors, usmUserSpinLock, usmUserSecurityName, usmUserCloneFrom, usmUserAuthProtocol, usmUserAuthKeyChange, usmUserOwnAuthKeyChange, usmUserPrivProtocol, usmUserPrivKeyChange, usmUserOwnPrivKeyChange, usmUserPublic, usmUserStorageType, usmUserStatus } STATUS current DESCRIPTION "A collection of objects providing for configuration of an SNMP engine which implements the SNMP User-based Security Model. " ::= { usmMIBGroups 1 } END 6. HMAC-MD5-96 Authentication Protocol This section describes the HMAC-MD5-96 authentication protocol. This authentication protocol is the first defined for the User-based Security Model. It uses MD5 hash-function which is described in [MD5], in HMAC mode described in [RFC2104], truncating the output to 96 bits. This protocol is identified by usmHMACMD5AuthProtocol. Over time, other authentication protocols may be defined either as a replacement of this protocol or in addition to this protocol. 6.1. Mechanisms - In support of data integrity, a message digest algorithm is required. A digest is calculated over an appropriate portion of an SNMP message and included as part of the message sent to the recipient. Blumenthal & Wijnen Standards Track [Page 45]
RFC 2274 USM for SNMPv3 January 1998 - In support of data origin authentication and data integrity, a secret value is prepended to SNMP message prior to computing the digest; the calculated digest is partially inserted into the SNMP message prior to transmission, and the prepended value is not transmitted. The secret value is shared by all SNMP engines authorized to originate messages on behalf of the appropriate user. 6.1.1. Digest Authentication Mechanism The Digest Authentication Mechanism defined in this memo provides for: - verification of the integrity of a received message, i.e., the message received is the message sent. The integrity of the message is protected by computing a digest over an appropriate portion of the message. The digest is computed by the originator of the message, transmitted with the message, and verified by the recipient of the message. - verification of the user on whose behalf the message was generated. A secret value known only to SNMP engines authorized to generate messages on behalf of a user is used in HMAC mode (see [RFC2104]). It also recommends the hash-function output used as Message Authentication Code, to be truncated. This protocol uses the MD5 [MD5] message digest algorithm. A 128-bit MD5 digest is calculated in a special (HMAC) way over the designated portion of an SNMP message and the first 96 bits of this digest is included as part of the message sent to the recipient. The size of the digest carried in a message is 12 octets. The size of the private authentication key (the secret) is 16 octets. For the details see section 6.3. 6.2. Elements of the Digest Authentication Protocol This section contains definitions required to realize the authentication module defined in this section of this memo. 6.2.1. Users Authentication using this authentication protocol makes use of a defined set of userNames. For any user on whose behalf a message must be authenticated at a particular SNMP engine, that SNMP engine must have knowledge of that user. An SNMP engine that wishes to Blumenthal & Wijnen Standards Track [Page 46]
RFC 2274 USM for SNMPv3 January 1998 communicate with another SNMP engine must also have knowledge of a user known to that engine, including knowledge of the applicable attributes of that user. A user and its attributes are defined as follows: <userName> A string representing the name of the user. <authKey> A user's secret key to be used when calculating a digest. It MUST be 16 octets long for MD5. 6.2.2. msgAuthoritativeEngineID The msgAuthoritativeEngineID value contained in an authenticated message specifies the authoritative SNMP engine for that particular message (see the definition of SnmpEngineID in the SNMP Architecture document [RFC2271]). The user's (private) authentication key is normally different at each authoritative SNMP engine and so the snmpEngineID is used to select the proper key for the authentication process. 6.2.3. SNMP Messages Using this Authentication Protocol Messages using this authentication protocol carry a msgAuthenticationParameters field as part of the msgSecurityParameters. For this protocol, the msgAuthenticationParameters field is the serialized OCTET STRING representing the first 12 octets of the HMAC-MD5-96 output done over the wholeMsg. The digest is calculated over the wholeMsg so if a message is authenticated, that also means that all the fields in the message are intact and have not been tampered with. 6.2.4. Services provided by the HMAC-MD5-96 Authentication Module This section describes the inputs and outputs that the HMAC-MD5-96 Authentication module expects and produces when the User-based Security module calls the HMAC-MD5-96 Authentication module for services. 6.2.4.1. Services for Generating an Outgoing SNMP Message The HMAC-MD5-96 authentication protocol assumes that the selection of the authKey is done by the caller and that the caller passes the secret key to be used. Blumenthal & Wijnen Standards Track [Page 47]
RFC 2274 USM for SNMPv3 January 1998 Upon completion the authentication module returns statusInformation and, if the message digest was correctly calculated, the wholeMsg with the digest inserted at the proper place. The abstract service primitive is: statusInformation = -- success or failure authenticateOutgoingMsg( IN authKey -- secret key for authentication IN wholeMsg -- unauthenticated complete message OUT authenticatedWholeMsg -- complete authenticated message ) The abstract data elements are: statusInformation An indication of whether the authentication process was successful. If not it is an indication of the problem. authKey The secret key to be used by the authentication algorithm. The length of this key MUST be 16 octets. wholeMsg The message to be authenticated. authenticatedWholeMsg The authenticated message (including inserted digest) on output. Note, that authParameters field is filled by the authentication module and this field should be already present in the wholeMsg before the Message Authentication Code (MAC) is generated. 6.2.4.2. Services for Processing an Incoming SNMP Message The HMAC-MD5-96 authentication protocol assumes that the selection of the authKey is done by the caller and that the caller passes the secret key to be used. Upon completion the authentication module returns statusInformation and, if the message digest was correctly calculated, the wholeMsg as it was processed. The abstract service primitive is: statusInformation = -- success or failure authenticateIncomingMsg( IN authKey -- secret key for authentication IN authParameters -- as received on the wire IN wholeMsg -- as received on the wire OUT authenticatedWholeMsg -- complete authenticated message ) The abstract data elements are: Blumenthal & Wijnen Standards Track [Page 48]
RFC 2274 USM for SNMPv3 January 1998 statusInformation An indication of whether the authentication process was successful. If not it is an indication of the problem. authKey The secret key to be used by the authentication algorithm. The length of this key MUST be 16 octets. authParameters The authParameters from the incoming message. wholeMsg The message to be authenticated on input and the authenticated message on output. authenticatedWholeMsg The whole message after the authentication check is complete. 6.3. Elements of Procedure This section describes the procedures for the HMAC-MD5-96 authentication protocol. 6.3.1. Processing an Outgoing Message This section describes the procedure followed by an SNMP engine whenever it must authenticate an outgoing message using the usmHMACMD5AuthProtocol. 1) The msgAuthenticationParameters field is set to the serialization, according to the rules in [RFC1906], of an OCTET STRING containing 12 zero octets. 2) From the secret authKey, two keys K1 and K2 are derived: a) extend the authKey to 64 octets by appending 48 zero octets; save it as extendedAuthKey b) obtain IPAD by replicating the octet 0x36 64 times; c) obtain K1 by XORing extendedAuthKey with IPAD; d) obtain OPAD by replicating the octet 0x5C 64 times; e) obtain K2 by XORing extendedAuthKey with OPAD. 4) Prepend K1 to the wholeMsg and calculate MD5 digest over it according to [MD5]. 5) Prepend K2 to the result of the step 4 and calculate MD5 digest over it according to [MD5]. Take the first 12 octets of the final digest - this is Message Authentication Code (MAC). 6) Replace the msgAuthenticationParameters field with MAC obtained in the step 5. Blumenthal & Wijnen Standards Track [Page 49]
RFC 2274 USM for SNMPv3 January 1998 7) The authenticatedWholeMsg is then returned to the caller together with statusInformation indicating success. 6.3.2. Processing an Incoming Message This section describes the procedure followed by an SNMP engine whenever it must authenticate an incoming message using the usmHMACMD5AuthProtocol. 1) If the digest received in the msgAuthenticationParameters field is not 12 octets long, then an failure and an errorIndication (authenticationError) is returned to the calling module. 2) The MAC received in the msgAuthenticationParameters field is saved. 3) The digest in the msgAuthenticationParameters field is replaced by the 12 zero octets. 4) From the secret authKey, two keys K1 and K2 are derived: a) extend the authKey to 64 octets by appending 48 zero octets; save it as extendedAuthKey b) obtain IPAD by replicating the octet 0x36 64 times; c) obtain K1 by XORing extendedAuthKey with IPAD; d) obtain OPAD by replicating the octet 0x5C 64 times; e) obtain K2 by XORing extendedAuthKey with OPAD. 5) The MAC is calculated over the wholeMsg: a) prepend K1 to the wholeMsg and calculate the MD5 digest over it; b) prepend K2 to the result of step 5.a and calculate the MD5 digest over it; c) first 12 octets of the result of step 5.b is the MAC. The msgAuthenticationParameters field is replaced with the MAC value that was saved in step 2. 6) Then the newly calculated MAC is compared with the MAC saved in step 2. If they do not match, then an failure and an errorIndication (authenticationFailure) is returned to the calling module. 7) The authenticatedWholeMsg and statusInformation indicating success are then returned to the caller. Blumenthal & Wijnen Standards Track [Page 50]
RFC 2274 USM for SNMPv3 January 1998 7. HMAC-SHA-96 Authentication Protocol This section describes the HMAC-SHA-96 authentication protocol. This protocol uses the SHA hash-function which is described in [SHA-NIST], in HMAC mode described in [RFC2104], truncating the output to 96 bits. This protocol is identified by usmHMACSHAAuthProtocol. Over time, other authentication protocols may be defined either as a replacement of this protocol or in addition to this protocol. 7.1. Mechanisms - In support of data integrity, a message digest algorithm is required. A digest is calculated over an appropriate portion of an SNMP message and included as part of the message sent to the recipient. - In support of data origin authentication and data integrity, a secret value is prepended to the SNMP message prior to computing the digest; the calculated digest is then partially inserted into the message prior to transmission. The prepended secret is not transmitted. The secret value is shared by all SNMP engines authorized to originate messages on behalf of the appropriate user. 7.1.1. Digest Authentication Mechanism The Digest Authentication Mechanism defined in this memo provides for: - verification of the integrity of a received message, i.e., the the message received is the message sent. The integrity of the message is protected by computing a digest over an appropriate portion of the message. The digest is computed by the originator of the message, transmitted with the message, and verified by the recipient of the message. - verification of the user on whose behalf the message was generated. A secret value known only to SNMP engines authorized to generate messages on behalf of a user is used in HMAC mode (see [RFC2104]). It also recommends the hash-function output used as Message Authentication Code, to be truncated. Blumenthal & Wijnen Standards Track [Page 51]
RFC 2274 USM for SNMPv3 January 1998 This mechanism uses the SHA [SHA-NIST] message digest algorithm. A 160-bit SHA digest is calculated in a special (HMAC) way over the designated portion of an SNMP message and the first 96 bits of this digest is included as part of the message sent to the recipient. The size of the digest carried in a message is 12 octets. The size of the private authentication key (the secret) is 20 octets. For the details see section 7.3. 7.2. Elements of the HMAC-SHA-96 Authentication Protocol This section contains definitions required to realize the authentication module defined in this section of this memo. 7.2.1. Users Authentication using this authentication protocol makes use of a defined set of userNames. For any user on whose behalf a message must be authenticated at a particular SNMP engine, that SNMP engine must have knowledge of that user. An SNMP engine that wishes to communicate with another SNMP engine must also have knowledge of a user known to that engine, including knowledge of the applicable attributes of that user. A user and its attributes are defined as follows: <userName> A string representing the name of the user. <authKey> A user's secret key to be used when calculating a digest. It MUST be 20 octets long for SHA. 7.2.2. msgAuthoritativeEngineID The msgAuthoritativeEngineID value contained in an authenticated message specifies the authoritative SNMP engine for that particular message (see the definition of SnmpEngineID in the SNMP Architecture document [RFC2271]). The user's (private) authentication key is normally different at each authoritative SNMP engine and so the snmpEngineID is used to select the proper key for the authentication process. Blumenthal & Wijnen Standards Track [Page 52]
RFC 2274 USM for SNMPv3 January 1998 7.2.3. SNMP Messages Using this Authentication Protocol Messages using this authentication protocol carry a msgAuthenticationParameters field as part of the msgSecurityParameters. For this protocol, the msgAuthenticationParameters field is the serialized OCTET STRING representing the first 12 octets of HMAC-SHA-96 output done over the wholeMsg. The digest is calculated over the wholeMsg so if a message is authenticated, that also means that all the fields in the message are intact and have not been tampered with. 7.2.4. Services provided by the HMAC-SHA-96 Authentication Module This section describes the inputs and outputs that the HMAC-SHA-96 Authentication module expects and produces when the User-based Security module calls the HMAC-SHA-96 Authentication module for services. 7.2.4.1. Services for Generating an Outgoing SNMP Message HMAC-SHA-96 authentication protocol assumes that the selection of the authKey is done by the caller and that the caller passes the secret key to be used. Upon completion the authentication module returns statusInformation and, if the message digest was correctly calculated, the wholeMsg with the digest inserted at the proper place. The abstract service primitive is: statusInformation = -- success or failure authenticateOutgoingMsg( IN authKey -- secret key for authentication IN wholeMsg -- unauthenticated complete message OUT authenticatedWholeMsg -- complete authenticated message ) The abstract data elements are: statusInformation An indication of whether the authentication process was successful. If not it is an indication of the problem. authKey The secret key to be used by the authentication algorithm. The length of this key MUST be 20 octets. wholeMsg The message to be authenticated. Blumenthal & Wijnen Standards Track [Page 53]
RFC 2274 USM for SNMPv3 January 1998 authenticatedWholeMsg The authenticated message (including inserted digest) on output. Note, that authParameters field is filled by the authentication module and this field should be already present in the wholeMsg before the Message Authentication Code (MAC) is generated. 7.2.4.2. Services for Processing an Incoming SNMP Message HMAC-SHA-96 authentication protocol assumes that the selection of the authKey is done by the caller and that the caller passes the secret key to be used. Upon completion the authentication module returns statusInformation and, if the message digest was correctly calculated, the wholeMsg as it was processed. The abstract service primitive is: statusInformation = -- success or failure authenticateIncomingMsg( IN authKey -- secret key for authentication IN authParameters -- as received on the wire IN wholeMsg -- as received on the wire OUT authenticatedWholeMsg -- complete authenticated message ) The abstract data elements are: statusInformation An indication of whether the authentication process was successful. If not it is an indication of the problem. authKey The secret key to be used by the authentication algorithm. The length of this key MUST be 20 octets. authParameters The authParameters from the incoming message. wholeMsg The message to be authenticated on input and the authenticated message on output. authenticatedWholeMsg The whole message after the authentication check is complete. 7.3. Elements of Procedure This section describes the procedures for the HMAC-SHA-96 authentication protocol. Blumenthal & Wijnen Standards Track [Page 54]
RFC 2274 USM for SNMPv3 January 1998 7.3.1. Processing an Outgoing Message This section describes the procedure followed by an SNMP engine whenever it must authenticate an outgoing message using the usmHMACSHAAuthProtocol. 1) The msgAuthenticationParameters field is set to the serialization, according to the rules in [RFC1906], of an OCTET STRING containing 12 zero octets. 2) From the secret authKey, two keys K1 and K2 are derived: a) extend the authKey to 64 octets by appending 44 zero octets; save it as extendedAuthKey b) obtain IPAD by replicating the octet 0x36 64 times; c) obtain K1 by XORing extendedAuthKey with IPAD; d) obtain OPAD by replicating the octet 0x5C 64 times; e) obtain K2 by XORing extendedAuthKey with OPAD. 3) Prepend K1 to the wholeMsg and calculate the SHA digest over it according to [SHA-NIST]. 4) Prepend K2 to the result of the step 4 and calculate SHA digest over it according to [SHA-NIST]. Take the first 12 octets of the final digest - this is Message Authentication Code (MAC). 5) Replace the msgAuthenticationParameters field with MAC obtained in the step 5. 6) The authenticatedWholeMsg is then returned to the caller together with statusInformation indicating success. 7.3.2. Processing an Incoming Message This section describes the procedure followed by an SNMP engine whenever it must authenticate an incoming message using the usmHMACSHAAuthProtocol. 1) If the digest received in the msgAuthenticationParameters field is not 12 octets long, then an failure and an errorIndication (authenticationError) is returned to the calling module. 2) The MAC received in the msgAuthenticationParameters field is saved. 3) The digest in the msgAuthenticationParameters field is replaced by the 12 zero octets. Blumenthal & Wijnen Standards Track [Page 55]
RFC 2274 USM for SNMPv3 January 1998 4) From the secret authKey, two keys K1 and K2 are derived: a) extend the authKey to 64 octets by appending 44 zero octets; save it as extendedAuthKey b) obtain IPAD by replicating the octet 0x36 64 times; c) obtain K1 by XORing extendedAuthKey with IPAD; d) obtain OPAD by replicating the octet 0x5C 64 times; e) obtain K2 by XORing extendedAuthKey with OPAD. 5) The MAC is calculated over the wholeMsg: a) prepend K1 to the wholeMsg and calculate the SHA digest over it; b) prepend K2 to the result of step 5.a and calculate the SHA digest over it; c) first 12 octets of the result of step 5.b is the MAC. The msgAuthenticationParameters field is replaced with the MAC value that was saved in step 2. 6) The the newly calculated MAC is compared with the MAC saved in step 2. If they do not match, then a failure and an errorIndication (authenticationFailure) are returned to the calling module. 7) The authenticatedWholeMsg and statusInformation indicating success are then returned to the caller. 8. CBC-DES Symmetric Encryption Protocol This section describes the CBC-DES Symmetric Encryption Protocol. This protocol is the first privacy protocol defined for the User- based Security Model. This protocol is identified by usmDESPrivProtocol. Over time, other privacy protocols may be defined either as a replacement of this protocol or in addition to this protocol. 8.1. Mechanisms - In support of data confidentiality, an encryption algorithm is required. An appropriate portion of the message is encrypted prior to being transmitted. The User-based Security Model specifies that the scopedPDU is the portion of the message that needs to be encrypted. Blumenthal & Wijnen Standards Track [Page 56]
RFC 2274 USM for SNMPv3 January 1998 - A secret value in combination with a timeliness value is used to create the en/decryption key and the initialization vector. The secret value is shared by all SNMP engines authorized to originate messages on behalf of the appropriate user. 8.1.1. Symmetric Encryption Protocol The Symmetric Encryption Protocol defined in this memo provides support for data confidentiality. The designated portion of an SNMP message is encrypted and included as part of the message sent to the recipient. Two organizations have published specifications defining the DES: the National Institute of Standards and Technology (NIST) [DES-NIST] and the American National Standards Institute [DES-ANSI]. There is a companion Modes of Operation specification for each definition ([DESO-NIST] and [DESO-ANSI], respectively). The NIST has published three additional documents that implementors may find useful. - There is a document with guidelines for implementing and using the DES, including functional specifications for the DES and its modes of operation [DESG-NIST]. - There is a specification of a validation test suite for the DES [DEST-NIST]. The suite is designed to test all aspects of the DES and is useful for pinpointing specific problems. - There is a specification of a maintenance test for the DES [DESM-NIST]. The test utilizes a minimal amount of data and processing to test all components of the DES. It provides a simple yes-or-no indication of correct operation and is useful to run as part of an initialization step, e.g., when a computer re-boots. 8.1.1.1. DES key and Initialization Vector. The first 8 octets of the 16-octet secret (private privacy key) are used as a DES key. Since DES uses only 56 bits, the Least Significant Bit in each octet is disregarded. The Initialization Vector for encryption is obtained using the following procedure. The last 8 octets of the 16-octet secret (private privacy key) are used as pre-IV. Blumenthal & Wijnen Standards Track [Page 57]
RFC 2274 USM for SNMPv3 January 1998 In order to ensure that the IV for two different packets encrypted by the same key, are not the same (i.e., the IV does not repeat) we need to "salt" the pre-IV with something unique per packet. An 8-octet string is used as the "salt". The concatenation of the generating SNMP engine's 32-bit snmpEngineBoots and a local 32-bit integer, that the encryption engine maintains, is input to the "salt". The 32-bit integer is initialized to an arbitrary value at boot time. The 32-bit snmpEngineBoots is converted to the first 4 octets (Most Significant Byte first) of our "salt". The 32-bit integer is then converted to the last 4 octet (Most Significant Byte first) of our "salt". The resulting "salt" is then XOR-ed with the pre-IV. The 8- octet "salt" is then put into the privParameters field encoded as an OCTET STRING. The "salt" integer is then modified. We recommend that it be incremented by one and wrap when it reaches the maximum value. How exactly the value of the "salt" (and thus of the IV) varies, is an implementation issue, as long as the measures are taken to avoid producing a duplicate IV. The "salt" must be placed in the privParameters field to enable the receiving entity to compute the correct IV and to decrypt the message. 8.1.1.2. Data Encryption. The data to be encrypted is treated as sequence of octets. Its length should be an integral multiple of 8 - and if it is not, the data is padded at the end as necessary. The actual pad value is irrelevant. The data is encrypted in Cipher Block Chaining mode. The plaintext is divided into 64-bit blocks. The plaintext for each block is XOR-ed with the ciphertext of the previous block, the result is encrypted and the output of the encryption is the ciphertext for the block. This procedure is repeated until there are no more plaintext blocks. For the very first block, the Initialization Vector is used instead of the ciphertext of the previous block. Blumenthal & Wijnen Standards Track [Page 58]
RFC 2274 USM for SNMPv3 January 1998 8.1.1.3. Data Decryption Before decryption, the encrypted data length is verified. If the length of the OCTET STRING to be decrypted is not an integral multiple of 8 octets, the decryption process is halted and an appropriate exception noted. When decrypting, the padding is ignored. The first ciphertext block is decrypted, the decryption output is XOR-ed with the Initialization Vector, and the result is the first plaintext block. For each subsequent block, the ciphertext block is decrypted, the decryption output is XOR-ed with the previous ciphertext block and the result is the plaintext block. 8.2. Elements of the DES Privacy Protocol This section contains definitions required to realize the privacy module defined by this memo. 8.2.1. Users Data en/decryption using this Symmetric Encryption Protocol makes use of a defined set of userNames. For any user on whose behalf a message must be en/decrypted at a particular SNMP engine, that SNMP engine must have knowledge of that user. An SNMP engine that wishes to communicate with another SNMP engine must also have knowledge of a user known to that SNMP engine, including knowledge of the applicable attributes of that user. A user and its attributes are defined as follows: <userName> An octet string representing the name of the user. <privKey> A user's secret key to be used as input for the DES key and IV. The length of this key MUST be 16 octets. 8.2.2. msgAuthoritativeEngineID The msgAuthoritativeEngineID value contained in an authenticated message specifies the authoritative SNMP engine for that particular message (see the definition of SnmpEngineID in the SNMP Architecture document [RFC2271]). Blumenthal & Wijnen Standards Track [Page 59]
RFC 2274 USM for SNMPv3 January 1998 The user's (private) privacy key is normally different at each authoritative SNMP engine and so the snmpEngineID is used to select the proper key for the en/decryption process. 8.2.3. SNMP Messages Using this Privacy Protocol Messages using this privacy protocol carry a msgPrivacyParameters field as part of the msgSecurityParameters. For this protocol, the msgPrivacyParameters field is the serialized OCTET STRING representing the "salt" that was used to create the IV. 8.2.4. Services provided by the DES Privacy Module This section describes the inputs and outputs that the DES Privacy module expects and produces when the User-based Security module invokes the DES Privacy module for services. 8.2.4.1. Services for Encrypting Outgoing Data This DES privacy protocol assumes that the selection of the privKey is done by the caller and that the caller passes the secret key to be used. Upon completion the privacy module returns statusInformation and, if the encryption process was successful, the encryptedPDU and the msgPrivacyParameters encoded as an OCTET STRING. The abstract service primitive is: statusInformation = -- success of failure encryptData( IN encryptKey -- secret key for encryption IN dataToEncrypt -- data to encrypt (scopedPDU) OUT encryptedData -- encrypted data (encryptedPDU) OUT privParameters -- filled in by service provider ) The abstract data elements are: statusInformation An indication of the success or failure of the encryption process. In case of failure, it is an indication of the error. encryptKey The secret key to be used by the encryption algorithm. The length of this key MUST be 16 octets. dataToEncrypt The data that must be encrypted. encryptedData The encrypted data upon successful completion. Blumenthal & Wijnen Standards Track [Page 60]
RFC 2274 USM for SNMPv3 January 1998 privParameters The privParameters encoded as an OCTET STRING. 8.2.4.2. Services for Decrypting Incoming Data This DES privacy protocol assumes that the selection of the privKey is done by the caller and that the caller passes the secret key to be used. Upon completion the privacy module returns statusInformation and, if the decryption process was successful, the scopedPDU in plain text. The abstract service primitive is: statusInformation = decryptData( IN decryptKey -- secret key for decryption IN privParameters -- as received on the wire IN encryptedData -- encrypted data (encryptedPDU) OUT decryptedData -- decrypted data (scopedPDU) ) The abstract data elements are: statusInformation An indication whether the data was successfully decrypted and if not an indication of the error. decryptKey The secret key to be used by the decryption algorithm. The length of this key MUST be 16 octets. privParameters The "salt" to be used to calculate the IV. encryptedData The data to be decrypted. decryptedData The decrypted data. 8.3. Elements of Procedure. This section describes the procedures for the DES privacy protocol. 8.3.1. Processing an Outgoing Message This section describes the procedure followed by an SNMP engine whenever it must encrypt part of an outgoing message using the usmDESPrivProtocol. 1) The secret cryptKey is used to construct the DES encryption key, the "salt" and the DES pre-IV (as described in section 8.1.1.1). Blumenthal & Wijnen Standards Track [Page 61]
RFC 2274 USM for SNMPv3 January 1998 2) The privParameters field is set to the serialization according to the rules in [RFC1906] of an OCTET STRING representing the the "salt" string. 3) The scopedPDU is encrypted (as described in section 8.1.1.2) and the encrypted data is serialized according to the rules in [RFC1906] as an OCTET STRING. 4) The serialized OCTET STRING representing the encrypted scopedPDU together with the privParameters and statusInformation indicating success is returned to the calling module. 8.3.2. Processing an Incoming Message This section describes the procedure followed by an SNMP engine whenever it must decrypt part of an incoming message using the usmDESPrivProtocol. 1) If the privParameters field is not an 8-octet OCTET STRING, then an error indication (decryptionError) is returned to the calling module. 2) The "salt" is extracted from the privParameters field. 3) The secret cryptKey and the "salt" are then used to construct the DES decryption key and pre-IV (as described in section 8.1.1.1). 4) The encryptedPDU is then decrypted (as described in section 8.1.1.3). 5) If the encryptedPDU cannot be decrypted, then an error indication (decryptionError) is returned to the calling module. 6) The decrypted scopedPDU and statusInformation indicating success are returned to the calling module. 9. Intellectual Property The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to Blumenthal & Wijnen Standards Track [Page 62]
RFC 2274 USM for SNMPv3 January 1998 obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. 10. Acknowledgements This document is the result of the efforts of the SNMPv3 Working Group. Some special thanks are in order to the following SNMPv3 WG members: Dave Battle (SNMP Research, Inc.) Uri Blumenthal (IBM T.J. Watson Research Center) Jeff Case (SNMP Research, Inc.) John Curran (BBN) T. Max Devlin (Hi-TECH Connections) John Flick (Hewlett Packard) David Harrington (Cabletron Systems Inc.) N.C. Hien (IBM T.J. Watson Research Center) Dave Levi (SNMP Research, Inc.) Louis A Mamakos (UUNET Technologies Inc.) Paul Meyer (Secure Computing Corporation) Keith McCloghrie (Cisco Systems) Russ Mundy (Trusted Information Systems, Inc.) Bob Natale (ACE*COMM Corporation) Mike O'Dell (UUNET Technologies Inc.) Dave Perkins (DeskTalk) Peter Polkinghorne (Brunel University) Randy Presuhn (BMC Software, Inc.) David Reid (SNMP Research, Inc.) Shawn Routhier (Epilogue) Juergen Schoenwaelder (TU Braunschweig) Bob Stewart (Cisco Systems) Bert Wijnen (IBM T.J. Watson Research Center) The document is based on recommendations of the IETF Security and Administrative Framework Evolution for SNMP Advisory Team. Members of that Advisory Team were: David Harrington (Cabletron Systems Inc.) Jeff Johnson (Cisco Systems) David Levi (SNMP Research Inc.) John Linn (Openvision) Blumenthal & Wijnen Standards Track [Page 63]
RFC 2274 USM for SNMPv3 January 1998 Russ Mundy (Trusted Information Systems) chair Shawn Routhier (Epilogue) Glenn Waters (Nortel) Bert Wijnen (IBM T. J. Watson Research Center) As recommended by the Advisory Team and the SNMPv3 Working Group Charter, the design incorporates as much as practical from previous RFCs and drafts. As a result, special thanks are due to the authors of previous designs known as SNMPv2u and SNMPv2*: Jeff Case (SNMP Research, Inc.) David Harrington (Cabletron Systems Inc.) David Levi (SNMP Research, Inc.) Keith McCloghrie (Cisco Systems) Brian O'Keefe (Hewlett Packard) Marshall T. Rose (Dover Beach Consulting) Jon Saperia (BGS Systems Inc.) Steve Waldbusser (International Network Services) Glenn W. Waters (Bell-Northern Research Ltd.) 11. Security Considerations 11.1. Recommended Practices This section describes practices that contribute to the secure, effective operation of the mechanisms defined in this memo. - An SNMP engine must discard SNMP Response messages that do not correspond to any currently outstanding Request message. It is the responsibility of the Message Processing module to take care of this. For example it can use a msgID for that. An SNMP Command Generator Application must discard any Response PDU for which there is no currently outstanding Request PDU; for example for SNMPv2 [RFC1905] PDUs, the request-id component in the PDU can be used to correlate Responses to outstanding Requests. Although it would be typical for an SNMP engine and an SNMP Command Generator Application to do this as a matter of course, when using these security protocols it is significant due to the possibility of message duplication (malicious or otherwise). - If an SNMP engine uses a msgID for correlating Response messages to outstanding Request messages, then it MUST use different msgIDs in all such Request messages that it sends out during a Time Window (150 seconds) period. Blumenthal & Wijnen Standards Track [Page 64]
RFC 2274 USM for SNMPv3 January 1998 A Command Generator or Notification Originator Application MUST use different request-ids in all Request PDUs that it sends out during a TimeWindow (150 seconds) period. This must be done to protect against the possibility of message duplication (malicious or otherwise). For example, starting operations with a msgID and/or request-id value of zero is not a good idea. Initializing them with an unpredictable number (so they do not start out the same after each reboot) and then incrementing by one would be acceptable. - An SNMP engine should perform time synchronization using authenticated messages in order to protect against the possibility of message duplication (malicious or otherwise). - When sending state altering messages to a managed authoritative SNMP engine, a Command Generator Application should delay sending successive messages to that managed SNMP engine until a positive acknowledgement is received for the previous message or until the previous message expires. No message ordering is imposed by the SNMP. Messages may be received in any order relative to their time of generation and each will be processed in the ordered received. Note that when an authenticated message is sent to a managed SNMP engine, it will be valid for a period of time of approximately 150 seconds under normal circumstances, and is subject to replay during this period. Indeed, an SNMP engine and SNMP Command Generator Applications must cope with the loss and re-ordering of messages resulting from anomalies in the network as a matter of course. However, a managed object, snmpSetSerialNo [RFC1907], is specifically defined for use with SNMP Set operations in order to provide a mechanism to ensure that the processing of SNMP messages occurs in a specific order. - The frequency with which the secrets of a User-based Security Model user should be changed is indirectly related to the frequency of their use. Protecting the secrets from disclosure is critical to the overall security of the protocols. Frequent use of a secret provides a continued source of data that may be useful to a cryptanalyst in exploiting known or perceived weaknesses in an algorithm. Frequent changes to the secret avoid this vulnerability. Blumenthal & Wijnen Standards Track [Page 65]
RFC 2274 USM for SNMPv3 January 1998 Changing a secret after each use is generally regarded as the most secure practice, but a significant amount of overhead may be associated with that approach. Note, too, in a local environment the threat of disclosure may be less significant, and as such the changing of secrets may be less frequent. However, when public data networks are used as the communication paths, more caution is prudent. 11.2 Defining Users The mechanisms defined in this document employ the notion of users on whose behalf messages are sent. How "users" are defined is subject to the security policy of the network administration. For example, users could be individuals (e.g., "joe" or "jane"), or a particular role (e.g., "operator" or "administrator"), or a combination (e.g., "joe-operator", "jane-operator" or "joe-admin"). Furthermore, a user may be a logical entity, such as an SNMP Application or a set of SNMP Applications, acting on behalf of an individual or role, or set of individuals, or set of roles, including combinations. Appendix A describes an algorithm for mapping a user "password" to a 16 octet value for use as either a user's authentication key or privacy key (or both). Note however, that using the same password (and therefore the same key) for both authentication and privacy is very poor security practice and should be strongly discouraged. Passwords are often generated, remembered, and input by a human. Human-generated passwords may be less than the 16 octets required by the authentication and privacy protocols, and brute force attacks can be quite easy on a relatively short ASCII character set. Therefore, the algorithm is Appendix A performs a transformation on the password. If the Appendix A algorithm is used, SNMP implementations (and SNMP configuration applications) must ensure that passwords are at least 8 characters in length. Because the Appendix A algorithm uses such passwords (nearly) directly, it is very important that they not be easily guessed. It is suggested that they be composed of mixed-case alphanumeric and punctuation characters that don't form words or phrases that might be found in a dictionary. Longer passwords improve the security of the system. Users may wish to input multiword phrases to make their password string longer while ensuring that it is memorable. Since it is infeasible for human users to maintain different passwords for every SNMP engine, but security requirements strongly discourage having the same key for more than one SNMP engine, the User-based Security Model employs a compromise proposed in [Localized-key]. It derives the user keys for the SNMP engines from Blumenthal & Wijnen Standards Track [Page 66]
RFC 2274 USM for SNMPv3 January 1998 user's password in such a way that it is practically impossible to either determine the user's password, or user's key for another SNMP engine from any combination of user's keys on SNMP engines. Note however, that if user's password is disclosed, then key localization will not help and network security may be compromised in this case. Therefore a user's password or non-localized key MUST NOT be stored on a managed device/node. Instead the localized key SHALL be stored (if at all) , so that, in case a device does get compromised, no other managed or managing devices get compromised. 11.3. Conformance To be termed a "Secure SNMP implementation" based on the User-based Security Model, an SNMP implementation MUST: - implement one or more Authentication Protocol(s). The HMAC-MD5-96 and HMAC-SHA-96 Authentication Protocols defined in this memo are examples of such protocols. - to the maximum extent possible, prohibit access to the secret(s) of each user about which it maintains information in a Local Configuration Datastore (LCD) under all circumstances except as required to generate and/or validate SNMP messages with respect to that user. - implement the key-localization mechanism. - implement the SNMP-USER-BASED-SM-MIB. In addition, an authoritative SNMP engine SHOULD provide initial configuration in accordance with Appendix A.1. Implementation of a Privacy Protocol (the DES Symmetric Encryption Protocol defined in this memo is one such protocol) is optional. 12. References [RFC1321] Rivest, R., "Message Digest Algorithm MD5", RFC 1321, April 1992. [RFC1903] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser, "Textual Conventions for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1903, January 1996. [RFC1905] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser, "Protocol Operations for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1905, January 1996. Blumenthal & Wijnen Standards Track [Page 67]
RFC 2274 USM for SNMPv3 January 1998 [RFC1906] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser, "Transport Mappings for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1906, January 1996. [RFC1907] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser, "Management Information Base for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1907 January 1996. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. [RFC2028] Hovey, R., and S. Bradner, "The Organizations Involved in the IETF Standards Process", BCP 11, RFC 2028, October 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2271] Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for describing SNMP Management Frameworks", RFC 2271, January 1998. [RFC2272] Case, J., Harrington, D., Presuhn, R., and B. Wijnen, "Message Processing and Dispatching for the Simple Network Management Protocol (SNMP)", RFC 2272, January 1998. [Localized-Key] U. Blumenthal, N. C. Hien, B. Wijnen "Key Derivation for Network Management Applications" IEEE Network Magazine, April/May issue, 1997. [DES-NIST] Data Encryption Standard, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 46-1. Supersedes FIPS Publication 46, (January, 1977; reaffirmed January, 1988). [DES-ANSI] Data Encryption Algorithm, American National Standards Institute. ANSI X3.92-1981, (December, 1980). [DESO-NIST] DES Modes of Operation, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 81, (December, 1980). [DESO-ANSI] Data Encryption Algorithm - Modes of Operation, American National Standards Institute. ANSI X3.106-1983, (May 1983). [DESG-NIST] Guidelines for Implementing and Using the NBS Data Encryption Standard, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Blumenthal & Wijnen Standards Track [Page 68]
RFC 2274 USM for SNMPv3 January 1998 Publication 74, (April, 1981). [DEST-NIST] Validating the Correctness of Hardware Implementations of the NBS Data Encryption Standard, National Institute of Standards and Technology. Special Publication 500-20. [DESM-NIST] Maintenance Testing for the Data Encryption Standard, National Institute of Standards and Technology. Special Publication 500-61, (August, 1980). [SHA-NIST] Secure Hash Algorithm. NIST FIPS 180-1, (April, 1995) http://csrc.nist.gov/fips/fip180-1.txt (ASCII) http://csrc.nist.gov/fips/fip180-1.ps (Postscript) 13. Editors' Addresses Uri Blumenthal IBM T. J. Watson Research 30 Saw Mill River Pkwy, Hawthorne, NY 10532 USA EMail: uri@watson.ibm.com Phone: +1-914-784-7064 Bert Wijnen IBM T. J. Watson Research Schagen 33 3461 GL Linschoten Netherlands EMail: wijnen@vnet.ibm.com Phone: +31-348-432-794 Blumenthal & Wijnen Standards Track [Page 69]
RFC 2274 USM for SNMPv3 January 1998 APPENDIX A - Installation A.1. SNMP engine Installation Parameters During installation, an authoritative SNMP engine SHOULD (in the meaning as defined in [RFC2119]) be configured with several initial parameters. These include: 1) A security posture The choice of security posture determines if initial configuration is implemented and if so how. One of three possible choices is selected: minimum-secure, semi-secure, very-secure (i.e., no-initial-configuration) In the case of a very-secure posture, there is no initial configuration, and so the following steps are irrelevant. 2) one or more secrets These are the authentication/privacy secrets for the first user to be configured. One way to accomplish this is to have the installer enter a "password" for each required secret. The password is then algorithmically converted into the required secret by: - forming a string of length 1,048,576 octets by repeating the value of the password as often as necessary, truncating accordingly, and using the resulting string as the input to the MD5 algorithm [MD5]. The resulting digest, termed "digest1", is used in the next step. - a second string is formed by concatenating digest1, the SNMP engine's snmpEngineID value, and digest1. This string is used as input to the MD5 algorithm [MD5]. The resulting digest is the required secret (see Appendix A.2). With these configured parameters, the SNMP engine instantiates the following usmUserEntry in the usmUserTable: Blumenthal & Wijnen Standards Track [Page 70]
RFC 2274 USM for SNMPv3 January 1998 no privacy support privacy support ------------------ --------------- usmUserEngineID localEngineID localEngineID usmUserName "initial" "initial" usmUserSecurityName "initial" "initial" usmUserCloneFrom ZeroDotZero ZeroDotZero usmUserAuthProtocol usmHMACMD5AuthProtocol usmHMACMD5AuthProtocol usmUserAuthKeyChange "" "" usmUserOwnAuthKeyChange "" "" usmUserPrivProtocol none usmDESPrivProtocol usmUserPrivKeyChange "" "" usmUserOwnPrivKeyChange "" "" usmUserPublic "" "" usmUserStorageType anyValidStorageType anyValidStorageType usmUserStatus active active A.2. Password to Key Algorithm A sample code fragment (section A.2.1) demonstrates the password to key algorithm which can be used when mapping a password to an authentication or privacy key using MD5. The reference source code of MD5 is available in [RFC1321]. Another sample code fragment (section A.2.2) demonstrates the password to key algorithm which can be used when mapping a password to an authentication or privacy key using SHA (documented in SHA- NIST). An example of the results of a correct implementation is provided (section A.3) which an implementor can use to check if his implementation produces the same result. A.2.1. Password to Key Sample Code for MD5 void password_to_key_md5( u_char *password, /* IN */ u_int passwordlen, /* IN */ u_char *engineID, /* IN - pointer to snmpEngineID */ u_int engineLength /* IN - length of snmpEngineID */ u_char *key) /* OUT - pointer to caller 16-octet buffer */ { MD5_CTX MD; u_char *cp, password_buf[64]; u_long password_index = 0; u_long count = 0, i; MD5Init (&MD); /* initialize MD5 */ Blumenthal & Wijnen Standards Track [Page 71]
RFC 2274 USM for SNMPv3 January 1998 /**********************************************/ /* Use while loop until we've done 1 Megabyte */ /**********************************************/ while (count < 1048576) { cp = password_buf; for (i = 0; i < 64; i++) { /*************************************************/ /* Take the next octet of the password, wrapping */ /* to the beginning of the password as necessary.*/ /*************************************************/ *cp++ = password[password_index++ % passwordlen]; } MD5Update (&MD, password_buf, 64); count += 64; } MD5Final (key, &MD); /* tell MD5 we're done */ /*****************************************************/ /* Now localize the key with the engineID and pass */ /* through MD5 to produce final key */ /* May want to ensure that engineLength <= 32, */ /* otherwise need to use a buffer larger than 64 */ /*****************************************************/ memcpy(password_buf, key, 16); memcpy(password_buf+16, engineID, engineLength); memcpy(password_buf+engineLength, key, 16); MD5Init(&MD); MD5Update(&MD, password_buf, 32+engineLength); MD5Final(key, &MD); return; } A.2.2. Password to Key Sample Code for SHA void password_to_key_sha( u_char *password, /* IN */ u_int passwordlen, /* IN */ u_char *engineID, /* IN - pointer to snmpEngineID */ u_int engineLength /* IN - length of snmpEngineID */ u_char *key) /* OUT - pointer to caller 20-octet buffer */ { SHA_CTX SH; u_char *cp, password_buf[72]; u_long password_index = 0; u_long count = 0, i; Blumenthal & Wijnen Standards Track [Page 72]
RFC 2274 USM for SNMPv3 January 1998 SHAInit (&SH); /* initialize SHA */ /**********************************************/ /* Use while loop until we've done 1 Megabyte */ /**********************************************/ while (count < 1048576) { cp = password_buf; for (i = 0; i < 64; i++) { /*************************************************/ /* Take the next octet of the password, wrapping */ /* to the beginning of the password as necessary.*/ /*************************************************/ *cp++ = password[password_index++ % passwordlen]; } SHAUpdate (&SH, password_buf, 64); count += 64; } SHAFinal (key, &SH); /* tell SHA we're done */ /*****************************************************/ /* Now localize the key with the engineID and pass */ /* through SHA to produce final key */ /* May want to ensure that engineLength <= 32, */ /* otherwise need to use a buffer larger than 72 */ /*****************************************************/ memcpy(password_buf, key, 20); memcpy(password_buf+20, engineID, engineLength); memcpy(password_buf+engineLength, key, 20); SHAInit(&SH); SHAUpdate(&SH, password_buf, 40+engineLength); SHAFinal(key, &SH); return; } A.3. Password to Key Sample Results A.3.1. Password to Key Sample Results using MD5 The following shows a sample output of the password to key algorithm for a 16-octet key using MD5. With a password of "maplesyrup" the output of the password to key algorithm before the key is localized with the SNMP engine's snmpEngineID is: '9f af 32 83 88 4e 92 83 4e bc 98 47 d8 ed d9 63'H Blumenthal & Wijnen Standards Track [Page 73]
RFC 2274 USM for SNMPv3 January 1998 After the intermediate key (shown above) is localized with the snmpEngineID value of: '00 00 00 00 00 00 00 00 00 00 00 02'H the final output of the password to key algorithm is: '52 6f 5e ed 9f cc e2 6f 89 64 c2 93 07 87 d8 2b'H A.3.2. Password to Key Sample Results using SHA The following shows a sample output of the password to key algorithm for a 20-octet key using SHA. With a password of "maplesyrup" the output of the password to key algorithm before the key is localized with the SNMP engine's snmpEngineID is: 'f1 be a9 ae 66 7f 4f b6 34 1e 51 af 06 80 7e 91 e4 3b 01 ac'H After the intermediate key (shown above) is localized with the snmpEngineID value of: '00 00 00 00 00 00 00 00 00 00 00 02'H the final output of the password to key algorithm is: '8a a3 d9 9e 3e 30 56 f2 bf e3 a9 ee f3 45 d5 39 54 91 12 be'H A.4. Sample encoding of msgSecurityParameters The msgSecurityParameters in an SNMP message are represented as an OCTET STRING. This OCTET STRING should be considered opaque outside a specific Security Model. The User-based Security Model defines the contents of the OCTET STRING as a SEQUENCE (see section 2.4). Given these two properties, the following is an example of the msgSecurityParameters for the User-based Security Model, encoded as an OCTET STRING: 04 <length> 30 <length> 04 <length> <msgAuthoritativeEngineID> 02 <length> <msgAuthoritativeEngineBoots> 02 <length> <msgAuthoritativeEngineTime> 04 <length> <msgUserName> Blumenthal & Wijnen Standards Track [Page 74]
RFC 2274 USM for SNMPv3 January 1998 04 0c <HMAC-MD5-96-digest> 04 08 <salt> Here is the example once more, but now with real values (except for the digest in msgAuthenticationParameters and the salt in msgPrivacyParameters, which depend on variable data that we have not defined here): Hex Data Description -------------- ----------------------------------------------- 04 39 OCTET STRING, length 57 30 37 SEQUENCE, length 55 04 0c 80000002 msgAuthoritativeEngineID: IBM 01 IPv4 address 09840301 9.132.3.1 02 01 01 msgAuthoritativeEngineBoots: 1 02 02 0101 msgAuthoritativeEngineTime: 257 04 04 62657274 msgUserName: bert 04 0c 01234567 msgAuthenticationParameters: sample value 89abcdef fedcba98 04 08 01234567 msgPrivacyParameters: sample value 89abcdef Blumenthal & Wijnen Standards Track [Page 75]
RFC 2274 USM for SNMPv3 January 1998 B. Full Copyright Statement Copyright (C) The Internet Society (1998). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Blumenthal & Wijnen Standards Track [Page 76]
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