Subject: DIGEST-MD5 Confidentiality Protection
Author: SteveHB
In response to: DIGEST-MD5 Integrity Protection
Posted on: 06/13/2006 06:28:52 PM
If the server sent a "cipher-opts" directive and the client responded with a "cipher" directive, then subsequent messages between the client and the server MUST be confidentiality protected. Using as a base session key the value of H(A1) as defined above the client and server calculate a pair of message integrity keys as follows.
The key for confidentiality protecting messages from client to server is:
Kcc = MD5({H(A1)[0..n],
"Digest H(A1) to client-to-server sealing key magic constant"})
The key for confidentiality protecting messages from server to client is:
Kcs = MD5({H(A1)[0..n],
"Digest H(A1) to server-to-client sealing key magic constant"})
where MD5 is as specified in [RFC 1321]. For cipher "rc4-40" n is 5; for "rc4-56" n is 7; for the rest n is 16. The key for the "rc-*" ciphers is all 16 bytes of Kcc or Kcs; the key for "des" is the first 7 bytes; the key for "3des" is the first 14 bytes. The IV for "des" and "3des" is the last 8 bytes of Kcc or Kcs.
If message confidentiality is negotiated, each message is encrypted with the chosen cipher and a MAC block is appended to the message.
The MAC block is a variable length padding prefix followed by 16 bytes formatted as follows: the first 10 bytes of the HMAC-MD5 [RFC 2104] of the message, a 2-byte message type number in network byte order with value 1, and the 4-byte sequence number in network byte order. If the blocksize of the chosen cipher is not 1 byte, the padding prefix is one or more octets each containing the number of padding bytes, such that total length of the encrypted part of the message is a multiple of the blocksize. The padding and first 10 bytes of the MAC block are encrypted along with the message.
SEAL(Ki, Kc, SeqNum, msg) =
{CIPHER(Kc, {msg, pad, HMAC(Ki, {SeqNum, msg})[0..9])}), 0x0001,
SeqNum}
where CIPHER is the chosen cipher, Ki and Kc are Kic and Kcc for messages sent by the client and Kis and Kcs for those sent by the server. The sequence number is initialized to zero, and incremented by one for each message sent.
Upon receipt, the message is decrypted, HMAC(Ki, {SeqNum, msg}) is computed and compared with the received value; the message is discarded if they differ.
>
> On 06/13/2006 06:20:48 PM
SteveHB wrote:
If the server offered "qop=auth-int" and the client responded "qop=auth-int", then subsequent messages, up to but not including the next subsequent authentication, between the client and the server MUST be integrity protected. Using as a base session key the value of H(A1) as defined above the client and server calculate a pair of message integrity keys as follows.
The key for integrity protecting messages from client to server is:
Kic = MD5({H(A1),
"Digest session key to client-to-server signing key magic constant"})
The key for integrity protecting messages from server to client is:
Kis = MD5({H(A1),
"Digest session key to server-to-client signing key magic constant"})
where MD5 is as specified in [RFC 1321]. If message integrity is negotiated, a MAC block for each message is appended to the message. The MAC block is 16 bytes: the first 10 bytes of the HMAC-MD5 [RFC 2104] of the message, a 2-byte message type number in network byte order with value 1, and the 4-byte sequence number in network byte order. The message type is to allow for future extensions such as rekeying.
MAC(Ki, SeqNum, msg) = (HMAC(Ki, {SeqNum, msg})[0..9], 0x0001,
SeqNum)
where Ki is Kic for messages sent by the client and Kis for those sent by the server. The sequence number is initialized to zero, and incremented by one for each message sent.
Upon receipt, MAC(Ki, SeqNum, msg) is computed and compared with the received value; the message is discarded if they differ.
References:
