[gnutls-help] Obtaining client_write_{MAC_secret, key, IV} via gnutls_prf() function

[Nikos Mavrogiannopoulos]

On Mon, 2014-08-04 at 23:18 +0900, Youngsok Kim wrote:
> Hello,
> Could anyone provide assistance with obtaining
> client_write_{MAC_secret,key,IV} of an active TLS/SSL session? I'm
> trying to obtain these values for offload the data encryption and
> decryption routines to an accelerator without modifying GnuTLS's
> source codes.

There is no straightforward way to export the TLS session secrets. Which
accelerator do you use for off-loading? gnutls supports cryptodev and
more can be registered using gnutls_crypto_single_cipher_register(),
gnutls_crypto_single_mac_register() etc. If you are using one with free
software drivers consider contributing it back with that API.

A direct way to extract the calculated secrets (which is recommended for
debugging only as it will break on any future upgrade of gnutls) is to
export gnutls_session_int and security_parameters in your program.

>    // obtain 'key_block'
>    unsigned long key_block_size = hash_size * 2 + key_size * 2 +
> IV_size * 2;
>    unsigned char key_block[key_block_size];
>    err = gnutls_prf(m_session, 13, "key expansion", 1, 0, NULL,
> key_block_size, (char *)key_block);

I believe the 4th variable should be zero (the client random is first).
Note also that this construction will only work if you negotiate TLS 1.0
or later.

regards,
Nikos