[TLS] Query regarding generating client_write_key.

[nilesh <[e-mail]>]

Hi,

I am new to the TLS/Cryptography and started working on decrypting the 
packets over SSL/TLS.
I have followed the RFC2246, 5246.

To start decoding the packets, I need to have symmetric keys 
(client_write_key/server_write_key).
I have the key_block, IV and MAC. As per the rfc:

client_write_key = PRF{key_block[32..36], client_random, server_random, 
some_string}[0..31];
And I have started writing PRF (pseudo-random function) somewhat like:

<snip>
/*
  input : has 5bytes data i.e. key_block[32..36];
  salt1 == client_random, salt2 == server_random, salt = 'A'.
*/
for (i = 0; i < 2; i++)
{
	MD5_Init(&md5);
	MD5_Update(&md5, (unsigned char *) &input[0], 3);
	// MD5_Update(&md5, shasig, 20);
	MD5_Final((unsigned char *)&md, &md5);
	
	SHA1_Init(&sha);
	SHA1_Update(&sha, pad, i + 1);
	SHA1_Update(&sha, (unsigned char *) &input[2], 3);
	SHA1_Update(&sha, salt1, 32);
	SHA1_Update(&sha, salt2, 32);
	SHA1_Final(shasig, &sha);
}
</snip>

Now, we need to XOR the md and shasig to get the bytes in client_write_key.

Queries:
1. The md5 generates 16bytes and SHA1 generates 20bytes so xor will be 
20bytes.
But the key we expect should be 32bytes (iterated the MD5/SHA1 twice for 
that). How could this be achieved?
2. Any suggestions on if the PRF() is designed somewhere? Pointers on 
how it could be designed?

I tried several ways, but getting the wrong key. Could someone please 
advice?

-- 
Thanks,
Nilesh