Re: MD5 is 1=>1?

[Derek Atkins <warlord@MIT.EDU>]

>     Are there 128 bit messages 
> in MD4 which hash to the same value, and if so, what insight into the 
> cycle leingth vs string leingth would it give us.

If there are, then you have broken MD4!  This is the definition of
breaking a Hash: finding two strings (of *any* size) that hash to the
same value.

Let me comment on something you wrote:

> hash(a,b)=a+b,a-b;
> now hash^2(a,b)=2a,2b.
> 
> so here you have lost 1 bit of information when  you start to itterate 
> the hash function, and will be left with exactly 1  option after 128 
> iterations of this function in every case.

If we make a small adjustment to the definition of this hash routine,
and define the hash to be:
	hash(a,b) = (a+b)mod 2^64, (a-b)mod 2^64

Then I argue that you will not lose that bit of information, since it
will just wrap around the 64-bit values instead of just doing a
bit-shift.

The point here is that if MD5 lost entropy, it would probably make it
easier to find two strings to hash to the same value, which, by
definition, breaks that hash.

> I would recomend replacing that option or discarding it, that is unless 
> hash functions never throw away bits in sizes smaller than their output size.
> (again, that was my question)

They shouldn't.  I refer back to my last statement, that if they did,
it would make breaking the hash much easier.

I hope this helps.

-derek