Network Working Group K. Jaganathan
Request for Comments: 4757 L. Zhu
Category: Informational J. Brezak
Microsoft Corporation
December 2006
The RC4-HMAC Kerberos Encryption Types Used by Microsoft Windows
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2006).
IESG Note
This document documents the RC4 Kerberos encryption types first
introduced in Microsoft Windows 2000. Since then, these encryption
types have been implemented in a number of Kerberos implementations.
The IETF Kerberos community supports publishing this specification as
an informational document in order to describe this widely
implemented technology. However, while these encryption types
provide the operations necessary to implement the base Kerberos
specification [RFC4120], they do not provide all the required
operations in the Kerberos cryptography framework [RFC3961]. As a
result, it is not generally possible to implement potential
extensions to Kerberos using these encryption types. The Kerberos
encryption type negotiation mechanism [RFC4537] provides one approach
for using such extensions even when a Kerberos infrastructure uses
long-term RC4 keys. Because this specification does not implement
operations required by RFC 3961 and because of security concerns with
the use of RC4 and MD4 discussed in Section 8, this specification is
not appropriate for publication on the standards track.
Abstract
The Microsoft Windows 2000 implementation of Kerberos introduces a
new encryption type based on the RC4 encryption algorithm and using
an MD5 HMAC for checksum. This is offered as an alternative to using
the existing DES-based encryption types.
The RC4-HMAC encryption types are used to ease upgrade of existing
Windows NT environments, provide strong cryptography (128-bit key
lengths), and provide exportable (meet United States government
export restriction requirements) encryption. This document describes
the implementation of those encryption types.
Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................3
2. Key Generation ..................................................3
3. Basic Operations ................................................4
4. Checksum Types ..................................................5
5. Encryption Types ................................................6
6. Key Strength Negotiation ........................................8
7. GSS-API Kerberos V5 Mechanism Type ..............................8
7.1. Mechanism Specific Changes .................................8
7.2. GSS-API MIC Semantics ......................................9
7.3. GSS-API WRAP Semantics ....................................11
8. Security Considerations ........................................15
9. IANA Considerations ............................................15
10. Acknowledgements ..............................................15
11. References ....................................................16
11.1. Normative References .....................................16
11.2. Informative References ...................................16
1. Introduction
The Microsoft Windows 2000 implementation of Kerberos contains new
encryption and checksum types for two reasons. First, for export
reasons early in the development process, 56-bit DES encryption could
not be exported, and, second, upon upgrade from Windows NT 4.0 to
Windows 2000, accounts will not have the appropriate DES keying
material to do the standard DES encryption. Furthermore, 3DES was
not available for export when Windows 2000 was released, and there
was a desire to use a single flavor of encryption in the product for
both US and international products.
As a result, there are two new encryption types and one new checksum
type introduced in Microsoft Windows 2000.
Note that these cryptosystems aren't intended to be complete,
general-purpose Kerberos encryption or checksum systems as defined in
[RFC3961]: there is no one-one mapping between the operations in this
documents and the primitives described in [RFC3961].
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to
be interpreted as described in [RFC2119].
2. Key Generation
On upgrade from existing Windows NT domains, the user accounts would
not have a DES-based key available to enable the use of DES base
encryption types specified in [RFC4120] and [RFC3961]. The key used
for RC4-HMAC is the same as the existing Windows NT key (NT Password
Hash) for compatibility reasons. Once the account password is
changed, the DES-based keys are created and maintained. Once the DES
keys are available, DES-based encryption types can be used with
Kerberos.
The RC4-HMAC string to key function is defined as follows:
String2Key(password)
K = MD4(UNICODE(password))
The RC4-HMAC keys are generated by using the Windows UNICODE version
of the password. Each Windows UNICODE character is encoded in
little-endian format of 2 octets each. Then an MD4 [RFC1320] hash
operation is performed on just the UNICODE characters of the password
(not including the terminating zero octets).
For an account with a password of "foo", this String2Key("foo") will
return:
0xac, 0x8e, 0x65, 0x7f, 0x83, 0xdf, 0x82, 0xbe,
0xea, 0x5d, 0x43, 0xbd, 0xaf, 0x78, 0x00, 0xcc
3. Basic Operations
The MD5 HMAC function is defined in [RFC2104]. It is used in this
encryption type for checksum operations. Refer to [RFC2104] for
details on its operation. In this document, this function is
referred to as HMAC(Key, Data) returning the checksum using the
specified key on the data.
The basic MD5 hash operation is used in this encryption type and
defined in [RFC1321]. In this document, this function is referred to
as MD5(Data) returning the checksum of the data.
RC4 is a stream cipher licensed by RSA Data Security. In this
document, the function is referred to as RC4(Key, Data) returning the
encrypted data using the specified key on the data.
These encryption types use key derivation. With each message, the
message type (T) is used as a component of the keying material. The
following table summarizes the different key derivation values used
in the various operations. Note that these differ from the key
derivations used in other Kerberos encryption types. T = the message
type, encoded as a little-endian four-byte integer.
1. AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted with the
client key (T=1)
2. AS-REP Ticket and TGS-REP Ticket (includes TGS session key or
application session key), encrypted with the service key (T=2)
3. AS-REP encrypted part (includes TGS session key or application
session key), encrypted with the client key (T=8)
4. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the TGS
session key (T=4)
5. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the TGS
authenticator subkey (T=5)
6. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum, keyed
with the TGS session key (T=6)
7. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes TGS
authenticator subkey), encrypted with the TGS session key T=7)
8. TGS-REP encrypted part (includes application session key),
encrypted with the TGS session key (T=8)
9. TGS-REP encrypted part (includes application session key),
encrypted with the TGS authenticator subkey (T=9)
EID 2562 (Verified) is as follows:Section: 3
Original Text:
9. TGS-REP encrypted part (includes application session key),
encrypted with the TGS authenticator subkey (T=8)
Corrected Text:
9. TGS-REP encrypted part (includes application session key),
encrypted with the TGS authenticator subkey (T=9)
Notes:
Typo
10. AP-REQ Authenticator cksum, keyed with the application session
key (T=10)
11. AP-REQ Authenticator (includes application authenticator
subkey), encrypted with the application session key (T=11)
12. AP-REP encrypted part (includes application session subkey),
encrypted with the application session key (T=12)
13. KRB-PRIV encrypted part, encrypted with a key chosen by the
application. Also for data encrypted with GSS Wrap (T=13)
14. KRB-CRED encrypted part, encrypted with a key chosen by the
application (T=14)
15. KRB-SAFE cksum, keyed with a key chosen by the application.
Also for data signed in GSS MIC (T=15)
Relative to RFC-1964 key uses:
T = 0 in the generation of sequence number for the MIC token
T = 0 in the generation of sequence number for the WRAP token
T = 0 in the generation of encrypted data for the WRAPPED token
All strings in this document are ASCII unless otherwise specified.
The lengths of ASCII-encoded character strings include the trailing
terminator character (0). The concat(a,b,c,...) function will return
the logical concatenation (left to right) of the values of the
arguments. The nonce(n) function returns a pseudo-random number of
"n" octets.
4. Checksum Types
There is one checksum type used in this encryption type. The
Kerberos constant for this type is:
#define KERB_CHECKSUM_HMAC_MD5 (-138)
The function is defined as follows:
K = the Key
T = the message type, encoded as a little-endian four-byte integer
CHKSUM(K, T, data)
Ksign = HMAC(K, "signaturekey") //includes zero octet at end
tmp = MD5(concat(T, data))
CHKSUM = HMAC(Ksign, tmp)
5. Encryption Types
There are two encryption types used in these encryption types. The
Kerberos constants for these types are:
#define KERB_ETYPE_RC4_HMAC 23
#define KERB_ETYPE_RC4_HMAC_EXP 24
The basic encryption function is defined as follows:
T = the message type, encoded as a little-endian four-byte integer.
OCTET L40[14] = "fortybits";
The header field on the encrypted data in KDC messages is:
typedef struct _RC4_MDx_HEADER {
OCTET Checksum[16];
OCTET Confounder[8];
} RC4_MDx_HEADER, *PRC4_MDx_HEADER;
ENCRYPT (K, export, T, data)
{
struct EDATA {
struct HEADER {
OCTET Checksum[16];
OCTET Confounder[8];
} Header;
OCTET Data[0];
} edata;
if (export){
*((DWORD *)(L40+10)) = T;
K1 = HMAC(K, L40); // where the length of L40 in
// octets is 14
}
else
{
K1 = HMAC(K, &T); // where the length of T in octets
// is 4
}
memcpy (K2, K1, 16);
if (export) memset (K1+7, 0xAB, 9);
nonce (edata.Confounder, 8);
memcpy (edata.Data, data);
edata.Checksum = HMAC (K2, concat(edata.Confounder, edata.Data));
K3 = HMAC (K1, edata.Checksum);
RC4 (K3, edata.Confounder);
RC4 (K3, data.Data);
}
DECRYPT (K, export, T, edata)
{
// edata looks like
struct EDATA {
struct HEADER {
OCTET Checksum[16];
OCTET Confounder[8];
} Header;
OCTET Data[0];
} edata;
if (export){
*((DWORD *)(L40+10)) = T;
HMAC (K, L40, 14, K1);
}
else
{
HMAC (K, &T, 4, K1);
}
memcpy (K2, K1, 16);
if (export) memset (K1+7, 0xAB, 9);
K3 = HMAC (K1, edata.Checksum);
RC4 (K3, edata.Confounder);
RC4 (K3, edata.Data);
// verify generated and received checksums
checksum = HMAC (K2, concat(edata.Confounder, edata.Data));
if (checksum != edata.Checksum)
printf("CHECKSUM ERROR !!!!!!\n");
}
The KDC message is encrypted using the ENCRYPT function not including
the Checksum in the RC4_MDx_HEADER.
The character constant "fortybits" evolved from the time when a
40-bit key length was all that was exportable from the United States.
It is now used to recognize that the key length is of "exportable"
length. In this description, the key size is actually 56 bits.
The pseudo-random operation [RFC3961] for both enctypes above is
defined as follows:
pseudo-random(K, S) = HMAC-SHA1(K, S)
where K is the protocol key and S is the input octet string.
HMAC-SHA1 is defined in [RFC2104] and the output of HMAC-SHA1 is the
20-octet digest.
6. Key Strength Negotiation
A Kerberos client and server can negotiate over key length if they
are using mutual authentication. If the client is unable to perform
full-strength encryption, it may propose a key in the "subkey" field
of the authenticator, using a weaker encryption type. The server
must then either return the same key or suggest its own key in the
subkey field of the AP reply message. The key used to encrypt data
is derived from the key returned by the server. If the client is
able to perform strong encryption but the server is not, it may
propose a subkey in the AP reply without first being sent a subkey in
the authenticator.
7. GSS-API Kerberos V5 Mechanism Type
7.1. Mechanism Specific Changes
The Generic Security Service Application Program Interface (GSS-API)
per-message tokens also require new checksum and encryption types.
The GSS-API per-message tokens are adapted to support these new
encryption types. See [RFC1964] Section 1.2.2.
The only support quality of protection is:
#define GSS_KRB5_INTEG_C_QOP_DEFAULT 0x0
When using this RC4-based encryption type, the sequence number is
always sent in big-endian rather than little-endian order.
The Windows 2000 implementation also defines new GSS-API flags in the
initial token passed when initializing a security context. These
flags are passed in the checksum field of the authenticator. See
[RFC1964] Section 1.1.1.
GSS_C_DCE_STYLE - This flag was added for use with Microsoft's
implementation of Distributed Computing Environment Remote Procedure
Call (DCE RPC), which initially expected three legs of
authentication. Setting this flag causes an extra AP reply to be
sent from the client back to the server after receiving the server's
AP reply. In addition, the context negotiation tokens do not have
GSS-API per-message tokens -- they are raw AP messages that do not
include object identifiers.
#define GSS_C_DCE_STYLE 0x1000
GSS_C_IDENTIFY_FLAG - This flag allows the client to indicate to the
server that it should only allow the server application to identify
the client by name and ID, but not to impersonate the client.
#define GSS_C_IDENTIFY_FLAG 0x2000
GSS_C_EXTENDED_ERROR_FLAG - Setting this flag indicates that the
client wants to be informed of extended error information. In
particular, Windows 2000 status codes may be returned in the data
field of a Kerberos error message. This allows the client to
understand a server failure more precisely. In addition, the server
may return errors to the client that are normally handled at the
application layer in the server, in order to let the client try to
recover. After receiving an error message, the client may attempt to
resubmit an AP request.
#define GSS_C_EXTENDED_ERROR_FLAG 0x4000
These flags are only used if a client is aware of these conventions
when using the Security Support Provider Interface (SSPI) on the
Windows platform; they are not generally used by default.
When NetBIOS addresses are used in the GSS-API, they are identified
by the GSS_C_AF_NETBIOS value. This value is defined as:
#define GSS_C_AF_NETBIOS 0x14
NetBios addresses are 16-octet addresses typically composed of 1 to
15 characters, trailing blank (ASCII char 20) filled, with a 16th
octet of 0x0.
7.2. GSS-API MIC Semantics
EID 1647 (Verified) is as follows:Section: 7.2 and 7.3
Original Text:
In 7.2:
if (exportable)
{
Kseq = HMAC(Kss, "fortybits", (int32)0);
// len includes terminating null
memset(Kseq+7, 0xab, 7)
}
In 7.3:
if (exportable)
{
Kcrypt = HMAC(Klocal, "fortybits", (int32)0);
// len includes terminating null
memset(Kcrypt+7, 0xab, 7);
}
Again in 7.3:
if (exportable)
{
Kseq = HMAC(Kss, "fortybits", (int32)0);
// len includes terminating null
memset(Kseq+7, 0xab, 7)
}
Corrected Text:
In 7.2:
if (export)
{
Kseq = HMAC(Kss, "fortybits", (int32)0);
// len includes terminating null
memset(Kseq+7, 0xab, 7)
}
In 7.3:
if (export)
{
Kcrypt = HMAC(Klocal, "fortybits", (int32)0);
// len includes terminating null
memset(Kcrypt+7, 0xab, 7);
}
Again in 7.3:
if (export)
{
Kseq = HMAC(Kss, "fortybits", (int32)0);
// len includes terminating null
memset(Kseq+7, 0xab, 7)
}
Notes:
misnamed "export" argument . Larry Zhu confirmed this issue
Sean Turner add (as pointed out by Magnus Nystrom) that there were actually three exportable/export replacements needed: 1 in Section 7.2 and two in Section 7.3.
The GSS-API checksum type and algorithm are defined in Section 5.
Only the first 8 octets of the checksum are used. The resulting
checksum is stored in the SGN_CKSUM field. See [RFC1964] Section 1.2
for GSS_GetMIC() and GSS_Wrap(conf_flag=FALSE).
The GSS_GetMIC token has the following format:
Byte no Name Description
0..1 TOK_ID Identification field.
Tokens emitted by GSS_GetMIC() contain
the hex value 01 01 in this field.
2..3 SGN_ALG Integrity algorithm indicator.
11 00 - HMAC
4..7 Filler Contains ff ff ff ff
8..15 SND_SEQ Sequence number field.
16..23 SGN_CKSUM Checksum of "to-be-signed data",
calculated according to algorithm
specified in SGN_ALG field.
The MIC mechanism used for GSS-MIC-based messages is as follows:
GetMIC(Kss, direction, export, seq_num, data)
{
struct Token {
struct Header {
OCTET TOK_ID[2];
OCTET SGN_ALG[2];
OCTET Filler[4];
};
OCTET SND_SEQ[8];
OCTET SGN_CKSUM[8];
} Token;
Token.TOK_ID = 01 01;
Token.SGN_SLG = 11 00;
Token.Filler = ff ff ff ff;
// Create the sequence number
if (direction == sender_is_initiator)
{
memset(Token.SEND_SEQ+4, 0xff, 4)
}
else if (direction == sender_is_acceptor)
{
memset(Token.SEND_SEQ+4, 0, 4)
}
Token.SEND_SEQ[0] = (seq_num & 0xff000000) >> 24;
Token.SEND_SEQ[1] = (seq_num & 0x00ff0000) >> 16;
Token.SEND_SEQ[2] = (seq_num & 0x0000ff00) >> 8;
Token.SEND_SEQ[3] = (seq_num & 0x000000ff);
// Derive signing key from session key
Ksign = HMAC(Kss, "signaturekey");
// length includes terminating null
// Generate checksum of message - SGN_CKSUM
// Key derivation salt = 15
Sgn_Cksum = MD5((int32)15, Token.Header, data);
// Save first 8 octets of HMAC Sgn_Cksum
Sgn_Cksum = HMAC(Ksign, Sgn_Cksum);
memcpy(Token.SGN_CKSUM, Sgn_Cksum, 8);
// Encrypt the sequence number
// Derive encryption key for the sequence number
// Key derivation salt = 0
if (exportable)
{
Kseq = HMAC(Kss, "fortybits", (int32)0);
// len includes terminating null
memset(Kseq+7, 0xab, 9)
EID 1646 (Verified) is as follows:Section: 7.2 7.3
Original Text:
Kseq = HMAC(Kss, "fortybits", (int32)0);
// len includes terminating null
memset(Kseq+7, 0xab, 7)
Corrected Text:
Kseq = HMAC(Kss, "fortybits", (int32)0);
// len includes terminating null
memset(Kseq+7, 0xab, 9)
Notes:
applies both to section 7.2 and 7.3, confirmed by Larry Zhu
}
else
{
Kseq = HMAC(Kss, (int32)0);
}
Kseq = HMAC(Kseq, Token.SGN_CKSUM);
// Encrypt the sequence number
RC4(Kseq, Token.SND_SEQ);
}
7.3. GSS-API WRAP Semantics
There are two encryption keys for GSS-API message tokens, one that is
128 bits in strength and one that is 56 bits in strength as defined
in Section 6.
All padding is rounded up to 1 byte. One byte is needed to say that
there is 1 byte of padding. The DES-based mechanism type uses 8-byte
padding. See [RFC1964] Section 1.2.2.3.
The RC4-HMAC GSS_Wrap() token has the following format:
Byte no Name Description
0..1 TOK_ID Identification field.
Tokens emitted by GSS_Wrap() contain
the hex value 02 01 in this field.
2..3 SGN_ALG Checksum algorithm indicator.
11 00 - HMAC
4..5 SEAL_ALG ff ff - none
00 00 - DES-CBC
10 00 - RC4
6..7 Filler Contains ff ff
8..15 SND_SEQ Encrypted sequence number field.
16..23 SGN_CKSUM Checksum of plaintext padded data,
calculated according to algorithm
specified in SGN_ALG field.
24..31 Confounder Random confounder.
32..last Data Encrypted or plaintext padded data.
The encryption mechanism used for GSS-wrap-based messages is as
follows:
WRAP(Kss, encrypt, direction, export, seq_num, data)
{
struct Token { // 32 octets
struct Header {
OCTET TOK_ID[2];
OCTET SGN_ALG[2];
OCTET SEAL_ALG[2];
OCTET Filler[2];
};
OCTET SND_SEQ[8];
OCTET SGN_CKSUM[8];
OCTET Confounder[8];
} Token;
Token.TOK_ID = 02 01;
Token.SGN_SLG = 11 00;
Token.SEAL_ALG = (no_encrypt)? ff ff : 10 00;
Token.Filler = ff ff;
// Create the sequence number
if (direction == sender_is_initiator)
{
memset(&Token.SEND_SEQ[4], 0, 4)
}
else if (direction == sender_is_acceptor)
{
memset(&Token.SEND_SEQ[4], 0xff, 4)
}
EID 1675 (Verified) is as follows:Section: 7.3
Original Text:
// Create the sequence number
if (direction == sender_is_initiator)
{
memset(&Token.SEND_SEQ[4], 0xff, 4)
}
else if (direction == sender_is_acceptor)
{
memset(&Token.SEND_SEQ[4], 0, 4)
}
Corrected Text:
// Create the sequence number
if (direction == sender_is_initiator)
{
memset(&Token.SEND_SEQ[4], 0, 4)
}
else if (direction == sender_is_acceptor)
{
memset(&Token.SEND_SEQ[4], 0xff, 4)
}
EID 1372 (Verified) is as follows:Section: 7.3
Original Text:
// Generate checksum of message -
// SGN_CKSUM + Token.Confounder
// Key derivation salt = 15
Sgn_Cksum = MD5((int32)15, Token.Header,
Token.Confounder);
Corrected Text:
// Generate checksum of message -
// SGN_CKSUM + Token.Confounder
// Key derivation salt = 13
Sgn_Cksum = MD5((int32)13, Token.Header,
Token.Confounder);
Notes:
The final RFC appears to have cut-and-paste typo regarding the salt value used when generating the checksum for a WRAP token. The value used for a MIC token is 15, the value used for a WRAP token is 13.
Love Hörnquist Åstrand <lha@kth.se> pointed out that an earlier draft shows the values actually in use:
// Derive encryption key for data
// Key derivation salt = 0
for (i = 0; i < 16; i++) Klocal[i] = Kss[i] ^ 0xF0;
// XOR
if (exportable)
{
Kcrypt = HMAC(Klocal, "fortybits", (int32)0);
// len includes terminating null
memset(Kcrypt+7, 0xab, 7);
}
else
{
Kcrypt = HMAC(Klocal, (int32)0);
}
// new encryption key salted with seq
Kcrypt = HMAC(Kcrypt, (int32)seq_num);
EID 1651 (Verified) is as follows:Section: 7.3
Original Text:
// new encryption key salted with seq
Kcrypt = HMAC(Kcrypt, (int32)seq);
Corrected Text:
// new encryption key salted with seq
Kcrypt = HMAC(Kcrypt, (int32)seq_num);
Notes:
misnamed "seq" argument in HMAC function .
// Encrypt confounder (if encrypting)
// Sum the data buffer
Sgn_Cksum += MD5(data); // Append to checksum
// Encrypt the Confounder + data (if encrypting)
tmp=concat(Token.Confounder,data);
if (encrypt)
RC4(Kcrypt, tmp); /* tmp=Confounder + data */
memcpy(Token.Confounder,tmp,8);
memcpy(data,tmp+8,(tmp.len-8));
EID 1674 (Verified) is as follows:Section: 7.3
Original Text:
if (encrypt)
RC4(Kcrypt, Token.Confounder);
// Sum the data buffer
Sgn_Cksum += MD5(data); // Append to checksum
// Encrypt the data (if encrypting)
if (encrypt)
RC4(Kcrypt, data);
Corrected Text:
// Sum the data buffer
Sgn_Cksum += MD5(data); // Append to checksum
// Encrypt the Confounder + data (if encrypting)
tmp=concat(Token.Confounder,data);
if (encrypt)
RC4(Kcrypt, tmp); /* tmp=Confounder + data */
memcpy(Token.Confounder,tmp,8);
memcpy(data,tmp+8,(tmp.len-8));
Notes:
Notes : 1.Verified RC4 Encryption and Decryption on (Token.Confounder+Data) with Kcrypt key . 2.Verified RC4(K,x+y) !=RC4(K,x);RC4(K,y) 3.Reporting this issue after Larry's Feedback.
// Save first 8 octets of HMAC Sgn_Cksum
Sgn_Cksum = HMAC(Ksign, Sgn_Cksum);
memcpy(Token.SGN_CKSUM, Sgn_Cksum, 8);
// Derive encryption key for the sequence number
// Key derivation salt = 0
if (exportable)
{
Kseq = HMAC(Kss, "fortybits", (int32)0);
// len includes terminating null
memset(Kseq+7, 0xab, 7)
}
else
{
Kseq = HMAC(Kss, (int32)0);
}
Kseq = HMAC(Kseq, Token.SGN_CKSUM);
// Encrypt the sequence number
RC4(Kseq, Token.SND_SEQ);
// Encrypted message = Token + Data
}
The character constant "fortybits" evolved from the time when a
40-bit key length was all that was exportable from the United States.
It is now used to recognize that the key length is of "exportable"
length. In this description, the key size is actually 56 bits.
8. Security Considerations
Care must be taken in implementing these encryption types because
they use a stream cipher. If a different IV is not used in each
direction when using a session key, the encryption is weak. By using
the sequence number as an IV, this is avoided.
There are two classes of attack on RC4 described in [MIRONOV].
Strong distinguishers distinguish an RC4 keystream from randomness at
the start of the stream. Weak distinguishers can operate on any part
of the keystream, and the best ones, described in [FMcG] and
[MANTIN05], can exploit data from multiple, different keystreams. A
consequence of these is that encrypting the same data (for instance,
a password) sufficiently many times in separate RC4 keystreams can be
sufficient to leak information to an adversary. The encryption types
defined in this document defend against these by constructing a new
keystream for every message. However, it is RECOMMENDED not to use
the RC4 encryption types defined in this document for high-volume
connections.
Weaknesses in MD4 [BOER91] were demonstrated by den Boer and
Bosselaers in 1991. In August 2004, Xiaoyun Wang, et al., reported
MD4 collisions generated using hand calculation [WANG04].
Implementations based on Wang's algorithm can find collisions in real
time. However, the intended usage of MD4 described in this document
does not rely on the collision-resistant property of MD4.
Furthermore, MD4 is always used in the context of a keyed hash in
this document. Although no evidence has suggested keyed MD4 hashes
are vulnerable to collision-based attacks, no study has directly
proved that the HMAC-MD4 is secure: the existing study simply assumed
that the hash function used in HMAC is collision proof. It is thus
RECOMMENDED not to use the RC4 encryption types defined in this
document if alternative stronger encryption types, such as
aes256-cts-hmac-sha1-96 [RFC3962], are available.
9. IANA Considerations
Section 5 of this document defines two Kerberos encryption types
rc4-hmac (23) and rc4-hmac-exp (24). The Kerberos parameters
registration page at <http://www.iana.org/assignments/kerberos-
parameters> has been updated to reference this document for these two
encryption types.
10. Acknowledgements
The authors wish to thank Sam Hartman, Ken Raeburn, and Qunli Li for
their insightful comments.
11. References
11.1. Normative References
[RFC1320] Rivest, R., "The MD4 Message-Digest Algorithm", RFC 1320,
April 1992.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[RFC3962] Raeburn, K., "Advanced Encryption Standard (AES)
Encryption for Kerberos 5", RFC 3962, February 2005.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4537] Zhu, L., Leach, P., and K. Jaganathan, "Kerberos
Cryptosystem Negotiation Extension", RFC 4537, June 2006.
11.2. Informative References
[BOER91] den Boer, B. and A. Bosselaers, "An Attack on the Last Two
Rounds of MD4", Proceedings of the 11th Annual
International Cryptology Conference on Advances in
Cryptology, pages: 194 - 203, 1991.
[FMcG] Fluhrer, S. and D. McGrew, "Statistical Analysis of the
Alleged RC4 Keystream Generator", Fast Software
Encryption: 7th International Workshop, FSE 2000, April
2000, <http://www.mindspring.com/~dmcgrew/rc4-03.pdf>.
[MANTIN05] Mantin, I., "Predicting and Distinguishing Attacks on RC4
Keystream Generator", Advances in Cryptology -- EUROCRYPT
2005: 24th Annual International Conference on the Theory
and Applications of Cryptographic Techniques, May 2005.
[MIRONOV] Mironov, I., "(Not So) Random Shuffles of RC4", Advances
in Cryptology -- CRYPTO 2002: 22nd Annual International
Cryptology Conference, August 2002,
<http://eprint.iacr.org/2002/067.pdf>.
[WANG04] Wang, X., Lai, X., Feng, D., Chen, H., and X. Yu,
"Cryptanalysis of Hash functions MD4 and RIPEMD", August
2004, <http://www.infosec.sdu.edu.cn/paper/md4-ripemd-
attck.pdf>.
Authors' Addresses
Karthik Jaganathan
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: karthikj@microsoft.com
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: lzhu@microsoft.com
John Brezak
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: jbrezak@microsoft.com
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