Cryptography Reference
In-Depth Information
You can reproduce the master secret with this information:
[jdavies@localhost ssl]$ ./prf \
0x030102030405060708090a0b0c0d0e0f101112131415161718\
191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f “master secret” \
0x4c4f193f00000000000000000000000000000000000000000000000000000000\
4c4f193fe58513ecbaf3966b6ab96fbfbee41052ae1b643c93174f38a6548e53 48
2dee06e1ba5e41722dd1c24286ae5a0cfbd89b38bd4688fa
fb97c3dc05a2647be55490ba733406807df8023ae75d0a0a
The master secret is always exactly 48 bytes long.
The master secret itself isn't used for key material; instead, it's used as a seed
into the PRF again. The PRF also needs to know how many bytes to generate in
order to determine how many times to iterate. How many bytes are needed for
the selected cipher suite? AES-128 uses 16-byte keys, and a 16-byte block size, so
you need 32 bytes of keying material and 32 bytes of IV — one of each for each
side of the conversation. SHA-1 uses a 20-byte MAC key, so you need 40 bytes
of MAC secret; this works out to 104 bytes of keying material.
You can reproduce the keys by running the PRF algorithm with this input:
[jdavies@localhost ssl]$ ./prf \
0x2dee06e1ba5e41722dd1c24286ae5a0cfbd89b38bd4688\
fafb97c3dc05a2647be55490ba733406807df8023ae75d0a0a “key expansion” \
0x4c4f193fe58513ecbaf3966b6ab96fbfbee41052ae1b643c93174f38a6548e\
534c4f193f00000000000000000000000000000000000000000000000000000000 104
3a1ee25b3fa7efb9a2c8f112de47c3276917a2bbb0f81a9a389dbc82c3fc2a073e97aa31087f312
96dbb1276d318c6551ef8245888420cf4c2a545f7a8515c42c367599cdd52cf6ef6bb0cc22615db
9c0d93ad3c21d2f58ed18324dbfb7645103f191455421cceca
Notice that the seed in this case is the server random, followed by the client
random, whereas for the master secret expansion, it was the other way around.
If you overlook this fact, you will end up tearing your hair out for days trying
to fi gure out why your code isn't working. (Don't ask me how I know).
The key material block starts with the MAC secrets, then the keys, then the
initialization vectors, so this works out to what is shown in Table 6-1.
Table 6-1:
The Key Material Block
PURPOSE
CLIENT/WRITE
SERVER/READ
MAC secret
3a1ee25b3fa7efb9a2c8f112de
47c3276917a2bb
b0f81a9a389dbc82c3f
c2a073e97aa31087f3129
Encryption Key
6dbb1276d318c6551e
f8245888420cf4
c2a545f7a8515c42c367599cd
d52cf6e
Initialization
Vector
f6bb0cc22615db9c
0d93ad3c21d2f58e
d18324dbfb7645103f
191455421cceca
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