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From Chip Turner <>
Subject Re: mac_check in eagle book
Date Mon, 16 Apr 2001 05:08:56 GMT
Abhijit Menon-Sen <> writes:

> On 2001-04-15 23:52:38, wrote:
> >
> > > I was wondering if someone could explain to me why in the eagle book
> > > it is necessary to perform an md5 twice before sending a mac_check
> > > to a user [...]
> >
> > Any hashing algorithm worth its salt shouldn't have to be done twice.
> > And doing it twice may in fact expose weaknesses in the algorithm
> > (though I have no evidence to support this).
> Doesn't the Eagle book mention somewhere that this is done because of a
> known weakness in the MD5 algorithm?

There is a theoretical weakness in md5 if the attacker can create both
sets of data that are hashed.  Under some strict circumstances, he
could get two different files with the same hash value.  However, the
real world risk of this is supposedly quite low and the attack is
computationally difficult to perform.  The double hashing reduces the
risk further.  The modperl book mentions it double hashes to prevent a
malicious user from concatenating data onto the values being checked.
I don't know if they are referring to this weakness, but I suspect
they are.  Sadly, the book doesn't seem to offer a reference for the
claim as to the specific md5 vulnderability.  (Hey Doug, wanna shed
some light on that somewhat cryptic passage? :)

It's been a while, but I believe SHA1 has yet to have a weakness
found.  md5 is probably secure enough for websites though.


Chip Turner         
                              RHN Web Engineer

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