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I can't seem to correctly decrypt a SSL 1.2 Finished message.

I have correctly generated the master key, and therefore I am confident that my PRF implementation is correct.

To generate the keying material:

PRF(master_secret, 'key expansion', seed)

  • I take 16 bytes (128 bits) for client_write_MAC_key (MD5)
  • I then take 16 bytes (128 bits) for server_write_MAC_key (MD5)
  • I then take 5 bytes (40 bits) for client_write_key (RC4-40)
  • I then take 5 bytes (40 bits) for server_write_key (RC4-40)

I extract the encrypted handshake from Wireshark and attempt to decrypt it using RC4 with a 5 byte key (EXP-RC4-MD5). The decrypted output is not formatted as expected. The first byte of data I should see is the content type, which should be 14 for the Finished message, I do not see this. Is there anything incorrect about what I have said thus far?

1 Answer 1

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The key generation function described in RFC 5246 is not applicable to this cipher suite, which is why I was unable to decrypt the message. Having reimplemented the key generation function described in RFC 2246 (TLS 1.0) I am now able to successfully decrypt the Finished message.

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