Re: SHA1 and MD5 of passphrases

> I've seen several suggestions that for large data, taking both MD5 and 
> SHA-1 hashes would increase the overall trust of the original data. My 
> unrelated question is regarding hashes of smaller data such as passphrases.
>
> Does storing both the SHA-1 and the MD5 of a passphrase weaken the strength 
> of the passphrase? That is, if both hashes are available, does that 
> introduce an additional avenue of attack on the original passphrase?


What you are talking about is pre-image attacks.  AFAIK, weak collision
resistance in a hash does not imply weak pre-image resistance.

In general, outside of this issue, multiple different hashes of your
passwords will be only as secure as your least secure hash.  For
instance, in some versions of windows, both your NT and your LM hash are
stored in the SAM.  (Someone, please correct me if I am wrong on any of
these details.)  Great, so we can crack the LM password and never attack
the NT password, if we can get the SAM.  Good thing Microsoft thought
that one through...

I think salted SHA1 for the time being is plenty secure for passwords.
The real problem with password hash storage isn't the hash anyway,
provided you are using a halfway decent hash... The big problem is how
the users select their passwords...

cheers,
tim

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