Re: HMAC vs hash+salt

[Alexander Klimov <alserkli () inbox ru>]

On Wed, 11 Jan 2006 vdhieu84 () gmail com wrote:
> I don't really understand the advantage of HMAC over hash+salt
>
> As my understand, MAC is a function of 3 inputs:
>  - authentication scheme
>  - key
>  - message
>
> As in HMAC, the authentication scheme is
>    hash1(key XOR opad + hash2(key XOR ipad + message))
>
> In hash+salt, if we consider salt is key then authentication scheme is
>    hash1(hash2(message)+salt)
>
> So the only difference is the authentication scheme. In this case,
> HMAC is more complicated than hash+salt.
>
> However, what I don't understand is what is the weaknesses of
> hash+salt? Why HMAC is preferable to use?

To get any meaningful answer you have to specify what you are
actually doing and what are the possible attacks you care about.

MAC is designed to protect a message agains modifications by somebody
who does not know the key. For this HMAC is better than your scheme
because it requires less from the hash algorithms: If there are
*known* messages AAA and ABA, such that
  hash2(AAA) = hash2(ABA),
then an attacker can replace AAA with ABA and reuse your `MAC'. OTOH
with the real HMAC the attacker needs a collision with partially
*unknown* messages:
  hash2(secret + AAA) = hash2(secret + ABA).

See also,

  M. Bellare, R. Canetti, and H. Krawczyk,
  ``Keying hash functions for message authentication,''
  <http://www-cse.ucsd.edu/users/mihir/papers/kmd5.pdf>

-- 
Regards,
ASK

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