5812-DS01-R datasheet(21/84 Pages) BOARDCOM

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Advance Data Sheet

BCM5812

3/11/03

B roa dcom Co rpo rat ion

Document

5812-DS01-405-R

Cryptographic Operations

Page 13

SSL MAC

The BCM5812 computes the SSL-specific MAC function used in processing SSL protocol records. Figure 11 shows the SSL

Record Structure, consisting of a five byte header, application data, and an authentication code computed using the SSL

MAC. Normally, the application data and MAC are also encrypted. If a block cipher is used, 1 or more pad bytes would follow

the MAC to ensure that the encrypted data is a multiple of the blocksize.

The SSL-specified MAC can be computed using either MD5 or SHA-1. Figure 12 diagrams the SSL MAC computation using

MD5. A two level hash scheme is used. First, a buffer is constructed that begins with the MAC secret, which for MD5 is 16

bytes. The MAC secret is followed by 48-pad bytes containing 0x36, followed by a 64-bit record sequence number, followed

by the content type from the record header. This is followed by a 16-bit length value, and then the application data payload

from the record. The length value is the length of the application data proper, in bytes.

Figure 11: SSL Record Structure

This buffer is hashed using MD5. A second buffer is constructed using the MAC secret and a 48-byte pad field, this time

using 0x5C as the pad value, followed by the 16 byte output from the first MD5 hash. This is then hashed using MD5, and

the 16-byte output used as the SSL MAC.

Version

SSL/TLS Record

PAD (BLOCK CIPHER)

Application Data

Length From

Header

MAC

Content Type

Length (MSB)

Length (LSB)

Authenticated Length

(for MAC)