CBOR Object Signing and Encryption (COSE): Structures and Process

1.1. Requirements Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

1.3. Design Changes from JOSE

• Define a single overall message structure so that encrypted, signed, and MACed messages can easily be identified and still have a consistent view.¶
• Signed messages distinguish between the protected and unprotected header parameters that relate to the content and those that relate to the signature.¶
• MACed messages are separated from signed messages.¶
• MACed messages have the ability to use the same set of recipient algorithms as enveloped messages for obtaining the MAC authentication key.¶
• Use binary encodings, rather than base64url encodings, to encode binary data.¶
• Combine the authentication tag for encryption algorithms with the ciphertext.¶
• The set of cryptographic algorithms has been expanded in some directions and trimmed in others.¶

1.4. CBOR Grammar

There was not a standard CBOR grammar available when COSE was originally written. For that reason the CBOR data objects defined here are described in prose. Since that time CBOR Data Definition Language (CDDL) [RFC8610] has been published as an RFC. The CBOR grammar presented in this document is compatible with CDDL.¶

The document was developed by first working on the grammar and then developing the prose to go with it. An artifact of this is that the prose was written using the primitive type strings defined by CBOR Data Definition Language (CDDL) [RFC8610]. In this specification, the following primitive types are used:¶

• any -- non-specific value that permits all CBOR values to be placed here.¶
• bool -- a boolean value (true: major type 7, value 21; false: major type 7, value 20).¶
• bstr -- byte string (major type 2).¶
• int -- an unsigned integer or a negative integer.¶
• nil -- a null value (major type 7, value 22).¶
• nint -- a negative integer (major type 1).¶
• tstr -- a UTF-8 text string (major type 3).¶
• uint -- an unsigned integer (major type 0).¶

Two syntaxes from CDDL appear in this document as shorthand. These are:¶

• FOO / BAR -- indicates that either FOO or BAR can appear here.¶
• [+ FOO] -- indicates that the type FOO appears one or more times in an array.¶
• * FOO -- indicates that the type FOO appears zero or more times.¶

Two of the constraints defined by CDDL are also used in this document. These are:¶

• type1 .cbor type2 -- indicates that the contents of type1, usually bstr, contains a value of type2.¶
• type1 .size integer -- indicates that the contents of type1 is integer bytes long¶

As well as the prose description, a version of a CBOR grammar is presented in CDDL. The CDDL grammar is informational; the prose description is normative.¶

The collected CDDL can be extracted from the XML version of this document via the following XPath expression below. (Depending on the XPath evaluator one is using, it may be necessary to deal with > as an entity.)¶

//sourcecode[@type='CDDL']/text()

CDDL expects the initial non-terminal symbol to be the first symbol in the file. For this reason, the first fragment of CDDL is presented here.¶

start = COSE_Messages / COSE_Key / COSE_KeySet / Internal_Types

; This is defined to make the tool quieter:
Internal_Types = Sig_structure / Enc_structure / MAC_structure

The non-terminal Internal_Types is defined for dealing with the automated validation tools used during the writing of this document. It references those non-terminals that are used for security computations but are not emitted for transport.¶

1.5. CBOR-Related Terminology

In JSON, maps are called objects and only have one kind of map key: a text string. In COSE, we use text strings, negative integers, and unsigned integers as map keys. The integers are used for compactness of encoding and easy comparison. The inclusion of text strings allows for an additional range of short encoded values to be used as well. Since the word "key" is mainly used in its other meaning, as a cryptographic key, we use the term "label" for this usage as a map key.¶

The presence a label that is neither a text string or an integer, in a CBOR map, is an error. Applications can either fail processing or process messages by ignoring incorrect labels; however, they MUST NOT create messages with incorrect labels.¶

A CDDL grammar fragment defines the non-terminal 'label', as in the previous paragraph, and 'values', which permits any value to be used.¶

label = int / tstr
values = any

3.1. Common COSE Header Parameters

This section defines a set of common header parameters. A summary of these header parameters can be found in Table 3. This table should be consulted to determine the value of label and the type of the value.¶

The set of header parameters defined in this section are:¶

alg:

This header parameter is used to indicate the algorithm used for the security processing. This header parameter MUST be authenticated where the ability to do so exists. This support is provided by AEAD algorithms or construction (e.g. COSE_Sign and COSE_Mac0). This authentication can be done either by placing the header parameter in the protected header parameter bucket or as part of the externally supplied data Section 4.3). The value is taken from the "COSE Algorithms" registry (see [COSE.Algorithms]).¶

crit:

This header parameter is used to indicate which protected header parameters an application that is processing a message is required to understand. Header parameters defined in this document do not need to be included as they should be understood by all implementations. When present, this the 'crit' header parameter MUST be placed in the protected header parameter bucket. The array MUST have at least one value in it.¶

Not all header parameter labels need to be included in the 'crit' header parameter. The rules for deciding which header parameters are placed in the array are:¶

• Integer labels in the range of 0 to 7 SHOULD be omitted.¶
• Integer labels in the range -1 to -128 can be omitted. Algorithms can assign labels in this range where the ability to process the content of the label is considered to be core to implementing the algorithm. Algorithms can assign labels outside of this range where the ability to process the content of the label is not considered to be core, but needs to be understood to correctly process this instance. Integer labels in the range -129 to -65536 SHOULD be included as these would be less common header parameters that might not be generally supported.¶
• Labels for header parameters required for an application MAY be omitted. Applications should have a statement if the label can be omitted.¶

The header parameters indicated by 'crit' can be processed by either the security library code or an application using a security library; the only requirement is that the header parameter is processed. If the 'crit' value list includes a label for which the header parameter is not in the protected header parameters bucket, this is a fatal error in processing the message.¶

content type:

This header parameter is used to indicate the content type of the data in the payload or ciphertext fields. Integers are from the "CoAP Content-Formats" IANA registry table [COAP.Formats]. Text values following the syntax of "<type-name>/<subtype-name>" where <type-name> and <subtype-name> are defined in Section 4.2 of [RFC6838]. Leading and trailing whitespace is also omitted. Textual content values along with parameters and subparameters can be located using the IANA "Media Types" registry. Applications SHOULD provide this header parameter if the content structure is potentially ambiguous.¶

kid:

This header parameter identifies one piece of data that can be used as input to find the needed cryptographic key. The value of this header parameter can be matched against the 'kid' member in a COSE_Key structure. Other methods of key distribution can define an equivalent field to be matched. Applications MUST NOT assume that 'kid' values are unique. There may be more than one key with the same 'kid' value, so all of the keys associated with this 'kid' may need to be checked. The internal structure of 'kid' values is not defined and cannot be relied on by applications. Key identifier values are hints about which key to use. This is not a security-critical field. For this reason, it can be placed in the unprotected header parameters bucket.¶

IV:

This header parameter holds the Initialization Vector (IV) value. For some symmetric encryption algorithms, this may be referred to as a nonce. The IV can be placed in the unprotected bucket as modifying the IV will cause the decryption to yield plaintext that is readily detectable as garbled.¶

Partial IV:

This header parameter holds a part of the IV value. When using the COSE_Encrypt0 structure, a portion of the IV can be part of the context associated with the key (Context IV) while a portion can be changed with each message (Partial IV). This field is used to carry a value that causes the IV to be changed for each message. The Partial IV can be placed in the unprotected bucket as modifying the value will cause the decryption to yield plaintext that is readily detectable as garbled. The 'Initialization Vector' and 'Partial Initialization Vector' header parameters MUST NOT both be present in the same security layer.¶

The message IV is generated by the following steps:¶

1. Left-pad the Partial IV with zeros to the length of IV (determined by the algorithm).¶
2. XOR the padded Partial IV with the context IV.¶

The CDDL fragment that represents the set of header parameters defined in this section is given below. Each of the header parameters is tagged as optional because they do not need to be in every map; header parameters required in specific maps are discussed above.¶

Generic_Headers = (
    ? 1 => int / tstr,  ; algorithm identifier
    ? 2 => [+label],    ; criticality
    ? 3 => tstr / int,  ; content type
    ? 4 => bstr,        ; key identifier
    ? 5 => bstr,        ; IV
    ? 6 => bstr         ; Partial IV
)

4.1. Signing with One or More Signers

The COSE_Sign structure allows for one or more signatures to be applied to a message payload. Header parameters relating to the content and header parameters relating to the signature are carried along with the signature itself. These header parameters may be authenticated by the signature, or just present. An example of a header parameter about the content is the content type header parameter. An example of a header parameter about the signature would be the algorithm and key used to create the signature.¶

RFC 5652 indicates that:¶

When more than one signature is present, the successful validation of one signature associated with a given signer is usually treated as a successful signature by that signer. However, there are some application environments where other rules are needed. An application that employs a rule other than one valid signature for each signer must specify those rules. Also, where simple matching of the signer identifier is not sufficient to determine whether the signatures were generated by the same signer, the application specification must describe how to determine which signatures were generated by the same signer. Support for different communities of recipients is the primary reason that signers choose to include more than one signature.¶

For example, the COSE_Sign structure might include signatures generated with the Edwards-curve Digital Signature Algorithm (EdDSA) [RFC8032] and with the Elliptic Curve Digital Signature Algorithm (ECDSA) [DSS]. This allows recipients to verify the signature associated with one algorithm or the other. More-detailed information on multiple signature evaluations can be found in [RFC5752].¶

The signature structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Sign structure is identified by the CBOR tag 98. The CDDL fragment that represents this is:¶

COSE_Sign_Tagged = #6.98(COSE_Sign)

A COSE Signed Message is defined in two parts. The CBOR object that carries the body and information about the body is called the COSE_Sign structure. The CBOR object that carries the signature and information about the signature is called the COSE_Signature structure. Examples of COSE Signed Messages can be found in Appendix C.1.¶

The COSE_Sign structure is a CBOR array. The fields of the array in order are:¶

protected:

This is as described in Section 3.¶

unprotected:

This is as described in Section 3.¶

payload:

This field contains the serialized content to be signed. If the payload is not present in the message, the application is required to supply the payload separately. The payload is wrapped in a bstr to ensure that it is transported without changes. If the payload is transported separately ("detached content"), then a nil CBOR object is placed in this location, and it is the responsibility of the application to ensure that it will be transported without changes.¶

Note: When a signature with a message recovery algorithm is used (Section 8.1), the maximum number of bytes that can be recovered is the length of the original payload. The size of the encoded payload is reduced by the number of bytes that will be recovered. If all of the bytes of the original payload are consumed, then the transmitted payload is encoded as a zero-length byte string rather than as being absent.¶

signatures:

This field is an array of signatures. Each signature is represented as a COSE_Signature structure.¶

The CDDL fragment that represents the above text for COSE_Sign follows.¶

COSE_Sign = [
    Headers,
    payload : bstr / nil,
    signatures : [+ COSE_Signature]
]

The COSE_Signature structure is a CBOR array. The fields of the array in order are:¶

protected:

This is as described in Section 3.¶

unprotected:

This is as described in Section 3.¶

signature:

This field contains the computed signature value. The type of the field is a bstr. Algorithms MUST specify padding if the signature value is not a multiple of 8 bits.¶

The CDDL fragment that represents the above text for COSE_Signature follows.¶

COSE_Signature =  [
    Headers,
    signature : bstr
]

4.3. Externally Supplied Data

One of the features offered in the COSE document is the ability for applications to provide additional data to be authenticated, but that is not carried as part of the COSE object. The primary reason for supporting this can be seen by looking at the CoAP message structure [RFC7252], where the facility exists for options to be carried before the payload. Examples of data that can be placed in this location would be the CoAP code or CoAP options. If the data is in the headers of the CoAP message, then it is available for proxies to help in performing its operations. For example, the Accept Option can be used by a proxy to determine if an appropriate value is in the proxy's cache. But the sender can cause a failure at the server if a proxy, or an attacker, changes the set of accept values by including the field in the externally supplied data.¶

This document describes the process for using a byte array of externally supplied authenticated data; the method of constructing the byte array is a function of the application. Applications that use this feature need to define how the externally supplied authenticated data is to be constructed. Such a construction needs to take into account the following issues:¶

• If multiple items are included, applications need to ensure that the same byte string cannot be produced if there are different inputs. This would occur by concatenating the text strings 'AB' and 'CDE' or by concatenating the text strings 'ABC' and 'DE'. This is usually addressed by making fields a fixed width and/or encoding the length of the field as part of the output. Using options from CoAP [RFC7252] as an example, these fields use a TLV structure so they can be concatenated without any problems.¶
• If multiple items are included, an order for the items needs to be defined. Using options from CoAP as an example, an application could state that the fields are to be ordered by the option number.¶
• Applications need to ensure that the byte string is going to be the same on both sides. Using options from CoAP might give a problem if the same relative numbering is kept. An intermediate node could insert or remove an option, changing how the relative number is done. An application would need to specify that the relative number must be re-encoded to be relative only to the options that are in the external data.¶

4.4. Signing and Verification Process

In order to create a signature, a well-defined byte string is needed. The Sig_structure is used to create the canonical form. This signing and verification process takes in the body information (COSE_Sign or COSE_Sign1), the signer information (COSE_Signature), and the application data (external source). A Sig_structure is a CBOR array. The fields of the Sig_structure in order are:¶

1. A context text string identifying the context of the signature. The context text string is:¶

   • "Signature" for signatures using the COSE_Signature structure.¶
   • "Signature1" for signatures using the COSE_Sign1 structure.¶
2. The protected attributes from the body structure encoded in a bstr type. If there are no protected attributes, a zero-length byte string is used.¶
3. The protected attributes from the signer structure encoded in a bstr type. If there are no protected attributes, a zero-length byte string is used. This field is omitted for the COSE_Sign1 signature structure.¶
4. The externally supplied data from the application encoded in a bstr type. If this field is not supplied, it defaults to a zero-length byte string. (See Section 4.3 for application guidance on constructing this field.)¶
5. The payload to be signed encoded in a bstr type. The payload is placed here independent of how it is transported.¶

The CDDL fragment that describes the above text is:¶

Sig_structure = [
    context : "Signature" / "Signature1",
    body_protected : empty_or_serialized_map,
    ? sign_protected : empty_or_serialized_map,
    external_aad : bstr,
    payload : bstr
]

How to compute a signature:¶

1. Create a Sig_structure and populate it with the appropriate fields.¶
2. Create the value ToBeSigned by encoding the Sig_structure to a byte string, using the encoding described in Section 9.¶
3. Call the signature creation algorithm passing in K (the key to sign with), alg (the algorithm to sign with), and ToBeSigned (the value to sign).¶
4. Place the resulting signature value in the correct location. This is the 'signature' field of the COSE_Signature or COSE_Sign1 structure.¶

The steps for verifying a signature are:¶

1. Create a Sig_structure and populate it with the appropriate fields.¶
2. Create the value ToBeSigned by encoding the Sig_structure to a byte string, using the encoding described in Section 9.¶
3. Call the signature verification algorithm passing in K (the key to verify with), alg (the algorithm used sign with), ToBeSigned (the value to sign), and sig (the signature to be verified).¶

In addition to performing the signature verification, the application performs the appropriate checks to ensure that the key is correctly paired with the signing identity and that the signing identity is authorized before performing actions.¶

5.1. Enveloped COSE Structure

The enveloped structure allows for one or more recipients of a message. There are provisions for header parameters about the content and header parameters about the recipient information to be carried in the message. The protected header parameters associated with the content are authenticated by the content encryption algorithm. The protected header parameters associated with the recipient are authenticated by the recipient algorithm (when the algorithm supports it). Examples of header parameters about the content are the type of the content and the content encryption algorithm. Examples of header parameters about the recipient are the recipient's key identifier and the recipient's encryption algorithm.¶

The same techniques and nearly the same structure are used for encrypting both the plaintext and the keys. This is different from the approach used by both "Cryptographic Message Syntax (CMS)" [RFC5652] and "JSON Web Encryption (JWE)" [RFC7516] where different structures are used for the content layer and for the recipient layer. Two structures are defined: COSE_Encrypt to hold the encrypted content and COSE_recipient to hold the encrypted keys for recipients. Examples of encrypted messages can be found in Appendix C.3.¶

The COSE_Encrypt structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Encrypt structure is identified by the CBOR tag 96. The CDDL fragment that represents this is:¶

COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)

The COSE_Encrypt structure is a CBOR array. The fields of the array in order are:¶

protected:

This is as described in Section 3.¶

unprotected:

This is as described in Section 3.¶

ciphertext:

This field contains the ciphertext encoded as a bstr. If the ciphertext is to be transported independently of the control information about the encryption process (i.e., detached content), then the field is encoded as a nil value.¶

recipients:

This field contains an array of recipient information structures. The type for the recipient information structure is a COSE_recipient.¶

The CDDL fragment that corresponds to the above text is:¶

COSE_Encrypt = [
    Headers,
    ciphertext : bstr / nil,
    recipients : [+COSE_recipient]
]

The COSE_recipient structure is a CBOR array. The fields of the array in order are:¶

protected:

This is as described in Section 3.¶

unprotected:

This is as described in Section 3.¶

ciphertext:

This field contains the encrypted key encoded as a bstr. All encoded keys are symmetric keys; the binary value of the key is the content. If there is not an encrypted key, then this field is encoded as a nil value.¶

recipients:

This field contains an array of recipient information structures. The type for the recipient information structure is a COSE_recipient (an example of this can be found in Appendix B). If there are no recipient information structures, this element is absent.¶

The CDDL fragment that corresponds to the above text for COSE_recipient is:¶

COSE_recipient = [
    Headers,
    ciphertext : bstr / nil,
    ? recipients : [+COSE_recipient]
]

5.2. Single Recipient Encrypted

The COSE_Encrypt0 encrypted structure does not have the ability to specify recipients of the message. The structure assumes that the recipient of the object will already know the identity of the key to be used in order to decrypt the message. If a key needs to be identified to the recipient, the enveloped structure ought to be used.¶

Examples of encrypted messages can be found in Appendix C.3.¶

The COSE_Encrypt0 structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Encrypt0 structure is identified by the CBOR tag 16. The CDDL fragment that represents this is:¶

COSE_Encrypt0_Tagged = #6.16(COSE_Encrypt0)

The COSE_Encrypt0 structure is a CBOR array. The fields of the array in order are:¶

protected:

This is as described in Section 3.¶

unprotected:

This is as described in Section 3.¶

ciphertext:

This is as described in Section 5.1.¶

The CDDL fragment for COSE_Encrypt0 that corresponds to the above text is:¶

COSE_Encrypt0 = [
    Headers,
    ciphertext : bstr / nil,
]

5.3. How to Encrypt and Decrypt for AEAD Algorithms

The encryption algorithm for AEAD algorithms is fairly simple. The first step is to create a consistent byte string for the authenticated data structure. For this purpose, we use an Enc_structure. The Enc_structure is a CBOR array. The fields of the Enc_structure in order are:¶

1. A context text string identifying the context of the authenticated data structure. The context text string is:¶

   • "Encrypt0" for the content encryption of a COSE_Encrypt0 data structure.¶
   • "Encrypt" for the first layer of a COSE_Encrypt data structure (i.e., for content encryption).¶
   • "Enc_Recipient" for a recipient encoding to be placed in an COSE_Encrypt data structure.¶
   • "Mac_Recipient" for a recipient encoding to be placed in a MACed message structure.¶
   • "Rec_Recipient" for a recipient encoding to be placed in a recipient structure.¶
2. The protected attributes from the body structure encoded in a bstr type. If there are no protected attributes, a zero-length byte string is used.¶
3. The externally supplied data from the application encoded in a bstr type. If this field is not supplied, it defaults to a zero-length byte string. (See Section 4.3 for application guidance on constructing this field.)¶

The CDDL fragment that describes the above text is:¶

Enc_structure = [
    context : "Encrypt" / "Encrypt0" / "Enc_Recipient" /
        "Mac_Recipient" / "Rec_Recipient",
    protected : empty_or_serialized_map,
    external_aad : bstr
]

How to encrypt a message:¶

1. Create an Enc_structure and populate it with the appropriate fields.¶
2. Encode the Enc_structure to a byte string (Additional Authenticated Data (AAD)), using the encoding described in Section 9.¶

3. Determine the encryption key (K). This step is dependent on the class of recipient algorithm being used. For:¶

   No Recipients:

   The key to be used is determined by the algorithm and key at the current layer. Examples are key transport keys (Section 8.5.3), key wrap keys (Section 8.5.2), or pre-shared secrets.¶

   Direct Encryption and Direct Key Agreement:

   The key is determined by the key and algorithm in the recipient structure. The encryption algorithm and size of the key to be used are inputs into the KDF used for the recipient. (For direct, the KDF can be thought of as the identity operation.) Examples of these algorithms are found in Sections 6.1.2 and 6.3 of [I-D.ietf-cose-rfc8152bis-algs].¶

   Other:

   The key is randomly or pseudo-randomly generated.¶

4. Call the encryption algorithm with K (the encryption key), P (the plaintext), and AAD. Place the returned ciphertext into the 'ciphertext' field of the structure.¶
5. For recipients of the message, recursively perform the encryption algorithm for that recipient, using K (the encryption key) as the plaintext.¶

How to decrypt a message:¶

1. Create an Enc_structure and populate it with the appropriate fields.¶
2. Encode the Enc_structure to a byte string (AAD), using the encoding described in Section 9.¶

3. Determine the decryption key. This step is dependent on the class of recipient algorithm being used. For:¶

   No Recipients:

   The key to be used is determined by the algorithm and key at the current layer. Examples are key transport keys (Section 8.5.3), key wrap keys (Section 8.5.2), or pre-shared secrets.¶

   Direct Encryption and Direct Key Agreement:

   The key is determined by the key and algorithm in the recipient structure. The encryption algorithm and size of the key to be used are inputs into the KDF used for the recipient. (For direct, the KDF can be thought of as the identity operation.)¶

   Other:

   The key is determined by decoding and decrypting one of the recipient structures.¶

4. Call the decryption algorithm with K (the decryption key to use), C (the ciphertext), and AAD.¶

5.4. How to Encrypt and Decrypt for AE Algorithms

How to encrypt a message:¶

1. Verify that the 'protected' field is empty.¶
2. Verify that there was no external additional authenticated data supplied for this operation.¶

3. Determine the encryption key. This step is dependent on the class of recipient algorithm being used. For:¶

   No Recipients:

   The key to be used is determined by the algorithm and key at the current layer. Examples are key transport keys (Section 8.5.3), key wrap keys (Section 8.5.2), or pre-shared secrets.¶

   Direct Encryption and Direct Key Agreement:

   The key is determined by the key and algorithm in the recipient structure. The encryption algorithm and size of the key to be used are inputs into the KDF used for the recipient. (For direct, the KDF can be thought of as the identity operation.) Examples of these algorithms are found in Sections 6.1.2 and 6.3 of [I-D.ietf-cose-rfc8152bis-algs].¶

   Other:

   The key is randomly generated.¶

4. Call the encryption algorithm with K (the encryption key to use) and P (the plaintext). Place the returned ciphertext into the 'ciphertext' field of the structure.¶
5. For recipients of the message, recursively perform the encryption algorithm for that recipient, using K (the encryption key) as the plaintext.¶

How to decrypt a message:¶

1. Verify that the 'protected' field is empty.¶
2. Verify that there was no external additional authenticated data supplied for this operation.¶

3. Determine the decryption key. This step is dependent on the class of recipient algorithm being used. For:¶

   No Recipients:

   The key to be used is determined by the algorithm and key at the current layer. Examples are key transport keys (Section 8.5.3), key wrap keys (Section 8.5.2), or pre-shared secrets.¶

   Direct Encryption and Direct Key Agreement:

   The key is determined by the key and algorithm in the recipient structure. The encryption algorithm and size of the key to be used are inputs into the KDF used for the recipient. (For direct, the KDF can be thought of as the identity operation.) Examples of these algorithms are found in Sections 6.1.2 and 6.3 of [I-D.ietf-cose-rfc8152bis-algs].¶

   Other:

   The key is determined by decoding and decrypting one of the recipient structures.¶

4. Call the decryption algorithm with K (the decryption key to use) and C (the ciphertext).¶

6.1. MACed Message with Recipients

The multiple recipient MACed message uses two structures: the COSE_Mac structure defined in this section for carrying the body and the COSE_recipient structure (Section 5.1) to hold the key used for the MAC computation. Examples of MACed messages can be found in Appendix C.5.¶

The MAC structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Mac structure is identified by the CBOR tag 97. The CDDL fragment that represents this is:¶

COSE_Mac_Tagged = #6.97(COSE_Mac)

The COSE_Mac structure is a CBOR array. The fields of the array in order are:¶

protected:

This is as described in Section 3.¶

unprotected:

This is as described in Section 3.¶

payload:

This field contains the serialized content to be MACed. If the payload is not present in the message, the application is required to supply the payload separately. The payload is wrapped in a bstr to ensure that it is transported without changes. If the payload is transported separately (i.e., detached content), then a nil CBOR value is placed in this location, and it is the responsibility of the application to ensure that it will be transported without changes.¶

tag:

This field contains the MAC value.¶

recipients:

This is as described in Section 5.1.¶

The CDDL fragment that represents the above text for COSE_Mac follows.¶

COSE_Mac = [
   Headers,
   payload : bstr / nil,
   tag : bstr,
   recipients :[+COSE_recipient]
]

7.1. COSE Key Common Parameters

This document defines a set of common parameters for a COSE Key object. Table 4 provides a summary of the parameters defined in this section. There are also parameters that are defined for specific key types. Key-type-specific parameters can be found in [I-D.ietf-cose-rfc8152bis-algs].¶

kty:

This parameter is used to identify the family of keys for this structure and, thus, the set of key-type-specific parameters to be found. The set of values defined in this document can be found in [COSE.KeyTypes]. This parameter MUST be present in a key object. Implementations MUST verify that the key type is appropriate for the algorithm being processed. The key type MUST be included as part of the trust decision process.¶

alg:

This parameter is used to restrict the algorithm that is used with the key. If this parameter is present in the key structure, the application MUST verify that this algorithm matches the algorithm for which the key is being used. If the algorithms do not match, then this key object MUST NOT be used to perform the cryptographic operation. Note that the same key can be in a different key structure with a different or no algorithm specified; however, this is considered to be a poor security practice.¶

kid:

This parameter is used to give an identifier for a key. The identifier is not structured and can be anything from a user-provided byte string to a value computed on the public portion of the key. This field is intended for matching against a 'kid' parameter in a message in order to filter down the set of keys that need to be checked. The value of the identifier is not a unique value and can occur in other key objects, even for different keys.¶

key_ops:

This parameter is defined to restrict the set of operations that a key is to be used for. The value of the field is an array of values from Table 5. Algorithms define the values of key ops that are permitted to appear and are required for specific operations. The set of values matches that in [RFC7517] and [W3C.WebCrypto].¶

Base IV:

This parameter is defined to carry the base portion of an IV. It is designed to be used with the Partial IV header parameter defined in Section 3.1. This field provides the ability to associate a Base IV with a key that is then modified on a per message basis with the Partial IV.¶

Extreme care needs to be taken when using a Base IV in an application. Many encryption algorithms lose security if the same IV is used twice.¶

If different keys are derived for each sender, starting at the same Base IV is likely to satisfy this condition. If the same key is used for multiple senders, then the application needs to provide for a method of dividing the IV space up between the senders. This could be done by providing a different base point to start from or a different Partial IV to start with and restricting the number of messages to be sent before rekeying.¶

11.1. COSE Header Parameters Registry

IANA created a registry titled "COSE Header Parameters" as part of processing [RFC8152]. IANA is requested to update the reference for this registry from [RFC8152] to this document. IANA is also requested to update the reference for all entries, except "counter signature" and "CounterSignature0", in the table from [RFC8152] to this document. The reference for "counter signature" and "CounterSignature0" are to be left as-is.¶

11.2. COSE Key Common Parameters Registry

IANA created a registry titled "COSE Key Common Parameters" as part of the processing of [RFC8152]. IANA is requested to update the reference for this registry from [RFC8152] to this document. IANA is also requested to update the reference for entries in the table from [RFC8152] to this document.¶

11.6. Expert Review Instructions

All of the IANA registries established by [RFC8152] are, at least in part, defined as expert review. This section gives some general guidelines for what the experts should be looking for, but they are being designated as experts for a reason, so they should be given substantial latitude.¶

Expert reviewers should take into consideration the following points:¶

• Point squatting should be discouraged. Reviewers are encouraged to get sufficient information for registration requests to ensure that the usage is not going to duplicate one that is already registered, and that the point is likely to be used in deployments. The zones tagged as private use are intended for testing purposes and closed environments; code points in other ranges should not be assigned for testing.¶
• Specifications are required for the standards track range of point assignment. Specifications should exist for specification required ranges, but early assignment before a specification is available is considered to be permissible. Specifications are needed for the first-come, first-serve range if they are expected to be used outside of closed environments in an interoperable way. When specifications are not provided, the description provided needs to have sufficient information to identify what the point is being used for.¶
• Experts should take into account the expected usage of fields when approving point assignment. The fact that there is a range for standards track documents does not mean that a standards track document cannot have points assigned outside of that range. The length of the encoded value should be weighed against how many code points of that length are left, the size of device it will be used on, and the number of code points left that encode to that size.¶
• When algorithms are registered, vanity registrations should be discouraged. One way to do this is to require registrations to provide additional documentation on security analysis of the algorithm. Another thing that should be considered is requesting an opinion on the algorithm from the Crypto Forum Research Group (CFRG). Algorithms that do not meet the security requirements of the community and the messages structures should not be registered.¶

C.1. Examples of Signed Messages

98(
  [
    / protected / h'',
    / unprotected / {},
    / payload / 'This is the content.',
    / signatures / [
      [
        / protected h'a10126' / << {
            / alg / 1:-7 / ECDSA 256 /
          } >>,
        / unprotected / {
          / kid / 4:'11'
        },
        / signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
98f53afd2fa0f30a'
      ]
    ]
  ]
)

98(
  [
    / protected / h'',
    / unprotected / {},
    / payload / 'This is the content.',
    / signatures / [
      [
        / protected h'a10126' / << {
            / alg / 1:-7 / ECDSA 256 /
          } >>,
        / unprotected / {
          / kid / 4:'11'
        },
        / signature / h'e2aeafd40d69d19dfe6e52077c5d7ff4e408282cbefb
5d06cbf414af2e19d982ac45ac98b8544c908b4507de1e90b717c3d34816fe926a2b
98f53afd2fa0f30a'
      ],
      [
        / protected h'a1013823' / << {
            / alg / 1:-36 / ECDSA 521 /
          } >> ,
        / unprotected / {
          / kid / 4:'bilbo.baggins@hobbiton.example'
        },
        / signature / h'00a2d28a7c2bdb1587877420f65adf7d0b9a06635dd1
de64bb62974c863f0b160dd2163734034e6ac003b01e8705524c5c4ca479a952f024
7ee8cb0b4fb7397ba08d009e0c8bf482270cc5771aa143966e5a469a09f613488030
c5b07ec6d722e3835adb5b2d8c44e95ffb13877dd2582866883535de3bb03d01753f
83ab87bb4f7a0297'
      ]
    ]
  ]
)

98(
  [
    / protected h'a2687265736572766564f40281687265736572766564' /
    << {
        "reserved":false,
        / crit / 2:[
          "reserved"
        ]
      } >>,
    / unprotected / {},
    / payload / 'This is the content.',
    / signatures / [
      [
        / protected h'a10126' / << {
            / alg / 1:-7 / ECDSA 256 /
          } >>,
        / unprotected / {
          / kid / 4:'11'
        },
        / signature / h'3fc54702aa56e1b2cb20284294c9106a63f91bac658d
69351210a031d8fc7c5ff3e4be39445b1a3e83e1510d1aca2f2e8a7c081c7645042b
18aba9d1fad1bd9c'
      ]
    ]
  ]
)

C.2. Single Signer Examples

18(
  [
    / protected h'a10126' / << {
        / alg / 1:-7 / ECDSA 256 /
      } >>,
    / unprotected / {
      / kid / 4:'11'
    },
    / payload / 'This is the content.',
    / signature / h'8eb33e4ca31d1c465ab05aac34cc6b23d58fef5c083106c4
d25a91aef0b0117e2af9a291aa32e14ab834dc56ed2a223444547e01f11d3b0916e5
a4c345cacb36'
  ]
)

C.3. Examples of Enveloped Messages

96(
  [
    / protected h'a10101' / << {
        / alg / 1:1 / AES-GCM 128 /
      } >>,
    / unprotected / {
      / iv / 5:h'c9cf4df2fe6c632bf7886413'
    },
    / ciphertext / h'7adbe2709ca818fb415f1e5df66f4e1a51053ba6d65a1a0
c52a357da7a644b8070a151b0',
    / recipients / [
      [
        / protected h'a1013818' / << {
            / alg / 1:-25 / ECDH-ES + HKDF-256 /
          } >>,
        / unprotected / {
          / ephemeral / -1:{
            / kty / 1:2,
            / crv / -1:1,
            / x / -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbf
bf054e1c7b4d91d6280',
            / y / -3:true
          },
          / kid / 4:'meriadoc.brandybuck@buckland.example'
        },
        / ciphertext / h''
      ]
    ]
  ]
)

96(
  [
    / protected h'a1010a' / << {
        / alg / 1:10 / AES-CCM-16-64-128 /
      } >>,
    / unprotected / {
      / iv / 5:h'89f52f65a1c580933b5261a76c'
    },
    / ciphertext / h'753548a19b1307084ca7b2056924ed95f2e3b17006dfe93
1b687b847',
    / recipients / [
      [
        / protected  h'a10129' / << {
            / alg / 1:-10
          } >>,
        / unprotected / {
          / salt / -20:'aabbccddeeffgghh',
          / kid / 4:'our-secret'
        },
        / ciphertext / h''
      ]
    ]
  ]
)

96(
  [
    / protected h'a10101' / << {
        / alg / 1:1 / AES-GCM 128 /
      } >> ,
    / unprotected / {
      / iv / 5:h'02d1f7e6f26c43d4868d87ce'
    },
    / ciphertext / h'64f84d913ba60a76070a9a48f26e97e863e28529d8f5335
e5f0165eee976b4a5f6c6f09d',
    / recipients / [
      [
        / protected / h'a101381f' / {
            \ alg \ 1:-32 \ ECHD-SS+A128KW \
          } / ,
        / unprotected / {
          / static kid / -3:'peregrin.took@tuckborough.example',
          / kid / 4:'meriadoc.brandybuck@buckland.example',
          / U nonce / -22:h'0101'
        },
        / ciphertext / h'41e0d76f579dbd0d936a662d54d8582037de2e366fd
e1c62'
      ]
    ]
  ]
)

C.4. Examples of Encrypted Messages

16(
  [
    / protected h'a1010a' / << {
        / alg / 1:10 / AES-CCM-16-64-128 /
      } >> ,
    / unprotected / {
      / iv / 5:h'89f52f65a1c580933b5261a78c'
    },
    / ciphertext / h'5974e1b99a3a4cc09a659aa2e9e7fff161d38ce71cb45ce
460ffb569'
  ]
)

16(
  [
    / protected  h'a1010a' / << {
        / alg / 1:10 / AES-CCM-16-64-128 /
      } >> ,
    / unprotected / {
      / partial iv / 6:h'61a7'
    },
    / ciphertext / h'252a8911d465c125b6764739700f0141ed09192de139e05
3bd09abca'
  ]
)

C.5. Examples of MACed Messages

97(
  [
    / protected  h'a1010f' / << {
        / alg / 1:15 / AES-CBC-MAC-256//64 /
      } >> ,
    / unprotected / {},
    / payload / 'This is the content.',
    / tag / h'9e1226ba1f81b848',
    / recipients / [
      [
        / protected / h'',
        / unprotected / {
          / alg / 1:-6 / direct /,
          / kid / 4:'our-secret'
        },
        / ciphertext / h''
      ]
    ]
  ]
)

97(
  [
    / protected  h'a10105' / << {
        / alg / 1:5 / HMAC 256//256 /
      } >> ,
    / unprotected / {},
    / payload / 'This is the content.',
    / tag / h'81a03448acd3d305376eaa11fb3fe416a955be2cbe7ec96f012c99
4bc3f16a41',
    / recipients / [
      [
        / protected  h'a101381a' / << {
            / alg / 1:-27 / ECDH-SS + HKDF-256 /
          } >> ,
        / unprotected / {
          / static kid / -3:'peregrin.took@tuckborough.example',
          / kid / 4:'meriadoc.brandybuck@buckland.example',
          / U nonce / -22:h'4d8553e7e74f3c6a3a9dd3ef286a8195cbf8a23d
19558ccfec7d34b824f42d92bd06bd2c7f0271f0214e141fb779ae2856abf585a583
68b017e7f2a9e5ce4db5'
        },
        / ciphertext / h''
      ]
    ]
  ]
)

97(
  [
    / protected  h'a1010e' / << {
        / alg / 1:14 / AES-CBC-MAC-128//64 /
      } >> ,
    / unprotected / {},
    / payload / 'This is the content.',
    / tag / h'36f5afaf0bab5d43',
    / recipients / [
      [
        / protected / h'',
        / unprotected / {
          / alg / 1:-5 / A256KW /,
          / kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037'
        },
        / ciphertext / h'711ab0dc2fc4585dce27effa6781c8093eba906f227
b6eb0'
      ]
    ]
  ]
)

97(
  [
    / protected h'a10105' / << {
        / alg / 1:5 / HMAC 256//256 /
      } >> ,
    / unprotected / {},
    / payload / 'This is the content.',
    / tag / h'bf48235e809b5c42e995f2b7d5fa13620e7ed834e337f6aa43df16
1e49e9323e',
    / recipients / [
      [
        / protected h'a101381c' / << {
            / alg / 1:-29 / ECHD-ES+A128KW /
          } >> ,
        / unprotected / {
          / ephemeral / -1:{
            / kty / 1:2,
            / crv / -1:3,
            / x / -2:h'0043b12669acac3fd27898ffba0bcd2e6c366d53bc4db
71f909a759304acfb5e18cdc7ba0b13ff8c7636271a6924b1ac63c02688075b55ef2
d613574e7dc242f79c3',
            / y / -3:true
          },
          / kid / 4:'bilbo.baggins@hobbiton.example'
        },
        / ciphertext / h'339bc4f79984cdc6b3e6ce5f315a4c7d2b0ac466fce
a69e8c07dfbca5bb1f661bc5f8e0df9e3eff5'
      ],
      [
        / protected / h'',
        / unprotected / {
          / alg / 1:-5 / A256KW /,
          / kid / 4:'018c0ae5-4d9b-471b-bfd6-eef314bc7037'
        },
        / ciphertext / h'0b2c7cfce04e98276342d6476a7723c090dfdd15f9a
518e7736549e998370695e6d6a83b4ae507bb'
      ]
    ]
  ]
)

C.6. Examples of MAC0 Messages

17(
  [
    / protected h'a1010f' / << {
        / alg / 1:15 / AES-CBC-MAC-256//64 /
      } >> ,
    / unprotected / {},
    / payload / 'This is the content.',
    / tag / h'726043745027214f'
  ]
)

C.7. COSE Keys

[
  {
    -1:1,
    -2:h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c0
8551d',
    -3:h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd008
4d19c',
    1:2,
    2:'meriadoc.brandybuck@buckland.example'
  },
  {
    -1:1,
    -2:h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a
09eff',
    -3:h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbf
c117e',
    1:2,
    2:'11'
  },
  {
    -1:3,
    -2:h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737bf5de
7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620085e5c8
f42ad',
    -3:h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e247e
60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f3fe1ea1
d9475',
    1:2,
    2:'bilbo.baggins@hobbiton.example'
  },
  {
    -1:1,
    -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91
d6280',
    -3:h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e03bf
822bb',
    1:2,
    2:'peregrin.took@tuckborough.example'
  }
]

[
  {
    1:2,
    2:'meriadoc.brandybuck@buckland.example',
    -1:1,
    -2:h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c0
8551d',
    -3:h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd008
4d19c',
    -4:h'aff907c99f9ad3aae6c4cdf21122bce2bd68b5283e6907154ad911840fa
208cf'
  },
  {
    1:2,
    2:'11',
    -1:1,
    -2:h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a
09eff',
    -3:h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbf
c117e',
    -4:h'57c92077664146e876760c9520d054aa93c3afb04e306705db609030850
7b4d3'
  },
  {
    1:2,
    2:'bilbo.baggins@hobbiton.example',
    -1:3,
    -2:h'0072992cb3ac08ecf3e5c63dedec0d51a8c1f79ef2f82f94f3c737bf5de
7986671eac625fe8257bbd0394644caaa3aaf8f27a4585fbbcad0f2457620085e5c8
f42ad',
    -3:h'01dca6947bce88bc5790485ac97427342bc35f887d86d65a089377e247e
60baa55e4e8501e2ada5724ac51d6909008033ebc10ac999b9d7f5cc2519f3fe1ea1
d9475',
    -4:h'00085138ddabf5ca975f5860f91a08e91d6d5f9a76ad4018766a476680b
55cd339e8ab6c72b5facdb2a2a50ac25bd086647dd3e2e6e99e84ca2c3609fdf177f
eb26d'
  },
  {
    1:4,
    2:'our-secret',
    -1:h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dcea6c4
27188'
  },
  {
    1:2,
    -1:1,
    2:'peregrin.took@tuckborough.example',
    -2:h'98f50a4ff6c05861c8860d13a638ea56c3f5ad7590bbfbf054e1c7b4d91
d6280',
    -3:h'f01400b089867804b8e9fc96c3932161f1934f4223069170d924b7e03bf
822bb',
    -4:h'02d1f7e6f26c43d4868d87ceb2353161740aacf1f7163647984b522a848
df1c3'
  },
  {
    1:4,
    2:'our-secret2',
    -1:h'849b5786457c1491be3a76dcea6c4271'
  },
  {
    1:4,
    2:'018c0ae5-4d9b-471b-bfd6-eef314bc7037',
    -1:h'849b57219dae48de646d07dbb533566e976686457c1491be3a76dcea6c4
27188'
  }
]