TLS HMAC key bit-length

Sat Mar 3 22:11:56 UTC 2007

On Sat, Mar 03, 2007 at 12:23:56PM -0500, James Muir wrote:
> Does anyone know the bit-length of the symmetric keys used in HMAC after 
> two nodes establish a TLS session?  I've tried to discover this from the 
> specs, source code and using various "openssl s_client" commands, but no 
> luck.

Check out section 6.3 of RFC2246: the MAC secrets are derived from the
first 2*SecurityParmeters.hash_size bytes of the generated key block.
So this will be 20 bytes if the hash is SHA-1, etc.

hth,
-- 
Nick Mathewson
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