Cryptography Reference
In-Depth Information
rc4_40_decrypt, new_md5_digest },
{ TLS_RSA_WITH_RC4_128_MD5, 0, 0, 16, MD5_BYTE_SIZE, rc4_128_encrypt,
rc4_128_decrypt, new_md5_digest },
{ TLS_RSA_WITH_RC4_128_SHA, 0, 0, 16, SHA1_BYTE_SIZE, rc4_128_encrypt,
rc4_128_decrypt, new_sha1_digest },
{ TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5, 0, 0, 0, MD5_BYTE_SIZE, NULL, NULL,
new_md5_digest },
{ TLS_RSA_WITH_IDEA_CBC_SHA, 0, 0, 0, SHA1_BYTE_SIZE, NULL, NULL,
new_sha1_digest },
{ TLS_RSA_EXPORT_WITH_DES40_CBC_SHA, 0, 0, 0, SHA1_BYTE_SIZE, NULL, NULL,
new_sha1_digest },
{ TLS_RSA_WITH_DES_CBC_SHA, 8, 8, 8, SHA1_BYTE_SIZE, des_encrypt, des_decrypt,
new_sha1_digest },
{ TLS_RSA_WITH_3DES_EDE_CBC_SHA, 8, 8, 24, SHA1_BYTE_SIZE, des3_encrypt,
des3_decrypt, new_sha1_digest },
…
Because these instances are referred to by position, you have to list each one,
even if it's not supported. Notice, for example, that
TLS_RSA_WITH_IDEA_CBC_
SHA
is declared, but left empty. It is never used by this implementation, but by
allocating space for it, the rest of the code is allowed to refer to elements in the
CipherSuite
structure by just referencing the
suites
array.
If you wanted to create a key for a 3DES cipher suite, for example, you could
invoke
suites[ TLS_RSA_WITH_3DES_EDE_CBC_SHA ].key_size
In fact, because the
CipherSuiteIdentifier
was added to
ProtectionParameters
,
the key computation code can just invoke
suites[ parameters->suite ].key_size
when it needs to know how much keying material to retrieve from the master
secret.
Now, recall that
MASTER_SECRET_LENGTH
is 48 bytes, regardless of cipher suite.
If the selected cipher suite is AES 256, CBC, with SHA-1, you need 136 bytes of
keying material — 32 bytes each for the client and server keys, 16 bytes each for
the initialization vectors, and 20 bytes each for the MAC secrets. Therefore, the
master secret itself must be expanded. As you can probably guess, this is done
via the PRF; the only difference between the use of the PRF in key calculation
and the use of the PRF in master secret expansion is that the label passed in is
“key expansion”
rather than
“master secret”
.
The key calculation routine is shown in Listing 6-41.
Listing 6-41:
“tls.c” calculate_keys
/**
6.3: Compute a key block, including MAC secrets, keys, and IVs for client & server.
Notice that the seed is server random followed by client random (whereas for master
secret computation, it's client random followed by server random). Sheesh!
(Continued)
Search WWH ::
Custom Search