1.2 SSL Record Data Format
The data portion of an SSL record is composed of three components (transmitted and received in the order shown):
MAC-DATA[MAC-SIZE]
ACTUAL-DATA[N]
PADDING-DATA[PADDING]
ACTUAL-DATA is the actual data being transmitted (the message payload). PADDING-DATA is the padding data sent when a block cipher is used and padding is needed. Finally, MAC-DATA is the Message Authentication Code.
When SSL records are sent in the clear, no cipher is used. Consequently the amount of PADDING-DATA will be zero and the amount of MAC-DATA will be zero. When encryption is in effect, the PADDING-DATA will be a function of the cipher block size. The MAC-DATA is a function of the CIPHER-CHOICE (more about that later).
The MAC-DATA is computed as follows:
MAC-DATA = HASH[ SECRET, ACTUAL-DATA, PADDING-DATA, SEQUENCE-NUMBER ]
Where the SECRET data is fed to the hash function first, followed by the ACTUAL-DATA, which is followed by the PADDING-DATA which is finally followed by the SEQUENCE-NUMBER. The SEQUENCE-NUMBER is a 32 bit value which is presented to the hash function as four bytes, with the first byte being the most significant byte of the sequence number, the second byte being the next most significant byte of the sequence number, the third byte being the third most significant byte, and the fourth byte being the least significant byte (that is, in network byte order or "big endian" order).
MAC-SIZE is a function of the digest algorithm being used. For MD2 and MD5 the MAC-SIZE will be 16 bytes (128 bits).
The SECRET value is a function of which party is sending the message. If the client is sending the message then the SECRET is the CLIENT-WRITE-KEY (the server will use the SERVER-READ-KEY to verify the MAC). If the client is receiving the message then the SECRET is the CLIENT-READ-KEY (the server will use the SERVER-WRITE-KEY to generate the MAC).
The SEQUENCE-NUMBER is a counter which is incremented by both the sender and the receiver. For each transmission direction, a pair of counters is kept (one by the sender, one by the receiver). Every time a message is sent by a sender the counter is incremented. Sequence numbers are 32 bit unsigned quantities and must wrap to zero after incrementing past 0xFFFFFFFF.
The receiver of a message uses the expected value of the sequence number as input into the MAC HASH function (the HASH function is chosen from the CIPHER-CHOICE). The computed MAC-DATA must agree bit for bit with the transmitted MAC-DATA. If the comparison is not identity then the record is considered damaged, and it is to be treated as if an "I/O Error" had occurred (i.e. an unrecoverable error is asserted and the connection is closed).
A final consistency check is done when a block cipher is used and the protocol is using encryption. The amount of data present in a record (RECORD-LENGTH))must be a multiple of the cipher's block size. If the received record is not a multiple of the cipher's block size then the record is considered damaged, and it is to be treated as if an "I/O Error" had occurred (i.e. an unrecoverable error is asserted and the connection is closed).
The SSL Record Layer is used for all SSL communications, including handshake messages, security escapes and application data transfers. The SSL Record Layer is used by both the client and the server at all times.
For a two byte header, the maximum record length is 32767 bytes. For the three byte header, the maximum record length is 16383 bytes. The SSL Handshake Protocol messages are constrained to fit in a single SSL Record Protocol record. Application protocol messages are allowed to consume multiple SSL Record Protocol record's.
Before the first record is sent using SSL all sequence numbers are initialized to zero. The transmit sequence number is incremented after every message sent, starting with the CLIENT-HELLO and SERVER-HELLO messages.
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