Re: OMAC help

From: Tom St Denis (tomstdenis_at_iahu.ca)
Date: 06/14/03 

Date: Sat, 14 Jun 2003 16:58:14 GMT

Jack Lloyd wrote:
> On Fri, 13 Jun 2003 18:29:05 -0400, Brian Gladman wrote:
>
>
>>Maybe I am missing something but they seemed almost the same in software
>>- just two small code sequences (gf_mulx & gfdivx) in place of one
>>(gf_mulx). Is there more to it than this?
>
>
> Nope. The part I like about it is you don't have to remember (or
> calculate) what the inverse of the polynomial is, only the poly
> itself. Perhaps I overstate OMAC1's advantages, but I've always
> had a hard time with polynomial arithmetic for some reason, so
> anything that reduces how much of it I have to think about is a
> good thing to me. (Also, anything that reduces code size at no cost ==
> good).

Personally I don't see the benefit of OMAC at all. It is *not* a
combined encrypt/mac algorithm [it requires you to decrypt and
re-encrypt to verify the mac as far as I can tell].

The specs are horrible. Admitedly I don't have 30 yrs in the field but
I have implemented some hairy algorithms [Twofish, CAST5, etc..] before
and this one perplexes me. I spent a good 8 hours toying with it and
everytime I failed to produce correct output.

Personally I just think the specs are half-assed. They spent too much
time analyzing the algorithm and not enough describing how to implement
it [hint: reference source code would have gone a long way!]

I don't see the advantage over say HMAC. In fact I was quite horribly
surprised that a non-encrypt mode was picked....

Oh well back to CTR+HMAC for me :-)

Tom

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