7/29/2023 0 Comments Retrospect client 7It is present in the "handshake finished" messages (sent once by each side at the end of the handshake) and every "data" message sent from either end. It is an HMAC (HMAC-SHA2-256 in this case) computed using a MAC-specific key (one of two actually client and server each have a different key they use for generating the HMAC, though of course both know the other's key so they can verify the HMAC).īecause the MAC is within the encrypted part of the message, it's not visible until the message is decrypted, at which point it's verified but then generally discarded (no further use). To see it in Wireshark or a similar program, you'd need a view that shows you the raw decrypted version of the encrypted portion of each record (rather than showing you just the decrypted payload of the record, or the decrypted payload reconstructed into the higher-level protocol such as HTTP, or of course the still-encrypted payload+MAC). Key: determined from the exchanged pre-master secret client and server use different keys to create the MAC (and the other's key to verify the MAC).The MAC is an HMAC (Hash-based Message Authentication Code) using the hash algorithm specified in the cipher suite (SHA1 in the example, SHA2-256 in your question) as the pseudo-random function, and computed from the following data: Padding using a modified version (every padding byte is reduced by one) of the padding defined in PKCS#7( ) (brings the plaintext length to a multiple of the block size which for AES is 16 bytes always at least one byte of padding to avoid ambiguity so will be 1-16 bytes inclusive 8 bytes in the example).The MAC (in the example, HMAC-SHA1, which is 20 bytes with your cipher suite, HMAC-SHA2-256, which is 32 bytes).The actual message payload (In the example, the ASCII string "ping", 4 bytes).The plaintext of the message is as follows:Įncrypted payload (AES in CBC mode always produces a multiple of the block length which is 16 encrypted plaintext includes the MAC example is 32 bytes long).Encryption IV (16 bytes for AES, unique for every encryption).big-endian, does not include the header length but does include the IV) Record payload length (2 bytes, network byte order a.k.a.TLS version (2 bytes, major and minor version, 1-indexed, includes SSL versions, so TLS 1.2 is 0x03, 0x03).Record type (1 byte, indicating "client data").You can see that, from start to finish, a TLS 1.2 client message using this cipher suite looks like this: I found this great site for breaking down the TLS packet structure:, and will refer to it throughout this answer as "the example". Data (in the example, the 4-byte ASCII string "ping").Data length (2 bytes, network byte order).Protocol version (2 bytes, as in header).Sequence number (unsigned 64-bit integer, network byte order uniquely identifies this record in this direction counts from zero for all encrypted traffic including the handshake-finished messages, each direction has independent sequence numbers).Message (the following fields concatenated without spacing).The key is the same for all messages sent in a given direction within a particular session. You can look up the details of the HMAC construction if you want, though any cryptography library should be capable of computing HMACs given a pseudorandom function (typically a member of the Secure Hash Algorithm a.k.a.
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