SHA-based encryption function in Python

[Richard Parker]

Paul Rubin at phr-n2002a at nightsong.com wrote on 4/24/02 4:42 PM:

> Do the keys really need to be independent?

As I recall, it was essential to the HMAC proof of security that the keys be
independent.  I don't know if is possible to prove HMAC secure with
dependent keys, but it would almost certainly require stronger assumptions
about the underlying hash function.

> How is H(K || (H || 'a' || K || X))?

I am concerned about it.  You're basically using HMAC with a weaker
pseudorandom key derivation.  If you absolutely can not use the xor of ipad
and opad, make sure you use a key derivation for both K1 and K2.  Perhaps
the following:

  H(c1 || K || H(c2 || K || x))

I strongly recommend you just use HMAC, rather than inventing your own MAC
construction.

-Richard