Propping up SHA-1 (or MD5)
charliek at microsoft.com
Thu Mar 24 13:59:03 EST 2005
Whether these various tricks help depends on the technical details of
the attacks found. I hope that the bit twiddling crypto types who are
finding the attacks are going to propose something to fix them.
There are probably cheaper fixes than the 2x or 3x performance loss of
your algorithm down in the inner loops of these algorithms (such as the
change from SHA to SHA-1) and that these will come out. I'm reluctant to
jump on the SHA-256 bandwagon or to come up with some ad hoc fix until a
more thorough analysis is done. SHA-256 was designed before these
attacks were known and probably has related flaws (though they are even
less likely to be practically exploitable). We have the luxury of having
the current break being largely theoretical, so waiting even a year for
the mathematicians is probably OK. But it's never too early to start
preparing for a new algorithm - perhaps with a new hash size - in our
protocols. Further, given that lots of attacks (past and present) are
not exploitable if every hashed quantity includes some value chosen by a
trusted party and unpredictable by an attacker, it seems reasonable to
consider that as a desirable characteristic as we design our protocols.
p.s. Your formulae below have unbalanced parentheses, but I can guess
what you probably meant.
From: Ben Laurie [mailto:ben at algroup.co.uk]
Sent: Thursday, March 24, 2005 2:39 AM
To: Charlie Kaufman
Cc: Cryptography; saag at mit.edu
Subject: Re: Propping up SHA-1 (or MD5)
Charlie Kaufman wrote:
> All hash functions I'm aware of consist of an inner compression
> that hashes a fixed size block of data into a smaller fixed size block
> and an outer composition function that applies the inner function
> iteratively to the variable length data to be hashed. Essentially
> proposing a modification to the outer layer of the hash construction.
> All of the standard hash functions since MD4 have been constructed so
> that a collision in the inner compression function is likely to lead
> a collision in the hash function. MD2 did not have that property. It
> computed a cheap checksum of the variable length data in parallel with
> the digesting process and digested the checksum following the data. I
> have often wondered whether such a cheap addition would strengthen the
> newer hashes. (It would fix the suffixing attacks that motivated the
> development of HMAC).
> It's not obvious whether this would make the functions more secure or
> just make them harder to analyze. Perhaps someone from the research
> community could comment on why the checksum was removed in the
> from MD2 to MD4.
> Your proposed encoding has the disadvantage that it would require two
> passes over the message being digested. This would be bad news for
> hardware implementations and should be avoided if possible.
I suggested in a later version these two constructions:
H'(x)=H(H(x || H(0 || x) || H(0 || x))
H'(x)=H(H(x || H(0 || x) || H(1 || x))
which only require a single pass (but, unfortunately, two or three
different instances of the hash). This seems similar to the mechanism
used in MD2, except the checksum is expensive.
"There is no limit to what a man can do or how far he can go if he
doesn't mind who gets the credit." - Robert Woodruff
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