hi Sylvain,
> ...
> btw, i wander if, from security point of view, a 160-bit hash (SHA1,
> RIPE-MD160) signed with a 1024-bit key is more or less robust than a
> 128-bit hash (MD5, RIPE-MD128) signed with a 2048 bit ... or in
> other word
> is it easier to factorize a key or to built a plain message given a
> specific hash ... someone can brief me ?
>
in short (and not very exact):
to find a collosion in a 160-bit hash is considered approximately as hard as
to break a 80-bit symmetric key.
a 80-bit symmetric key is considered as hard to break as an 760-bit RSA key.
thus, in both cases you mentioned, it will be "easier" to find a collision
for the hash algorithm.
have a look at http://www.rsalabs.com/bulletins/bulletin13.html
Karl
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