Re: Values to use for a salt?

From: Brian Hatch (bri_at_ifokr.org)
Date: 12/19/03

  • Next message: Michael Wojcik: "RE: Values to use for a salt?" 
    Date: Thu, 18 Dec 2003 18:25:07 -0800
To: Scott Cleven-Mulcahy <scottcm3@hotmail.com>

    > thanks for correcting my overly broad statement that salt must be secret -
    > there are definitely situations where salt is not secret. Incidentally,
    > Windows 2000 and later (not sure about NTLMv2 under NT4) use a method other
    > than salt to provide uniqueness and randomization.
    >
    > Still, the point remains that salt can provide two functions: increased
    > randomness and uniqueness of a password's hash. Highly random salt, if
    > used for all password's, only increases the randomness of a password. It
    > does not solve the uniqueness problem - two people with the same password
    > would still have the same password hash code. To make the point more
    > relevant, consider authentication for a web application.

    Disclaimer: I know nothing about windows hashing in any way.

    What you say above makes no sense. If two people have the same
    password, and the hash function used two different salts for
    them

            The salt for each user needs to be stored somewhere
            (Specifically, it's stored in the resulting hash
            for unix-style password hashing ala DES, MD5, BSD
            blowfish, etc)

            The resulting hash should be different for these
            two users with the same password.

            This provides 'uniqueness of a passwords hash' as
            stated above. Or, more accurately, you have more
            potential hashes for the same password. 4096
            for DES for example.

    If the resulting hash isn't different, then the salt was
    irrelevant, and your hash function sucks.

    If the hash function uses the same salt for every user, then

            The system-wide salt needs to be stored somewhere
            (in the registry, etc) and could be secret if you
            want for additional obscurity.

            Two users with the same password will have the
            same hash, but two users on different systems
            (which use different salts) would have different
            hashes.

    The first part listed above, 'increased randomness ... of a password's
    hash' doesn't make much sense to me. If you mean the hash will
    be more random, then you have a bad hash function. The whole point
    of a good hash algorithm is that the result should be totally
    unlike the input, small changes in the input result in widely different
    hash results, and the results should be uniformly distributed over
    the resulting space (ie you should have no 'clumping' of results.)
    So salts have nothing to do with this, since it shouldn't be an
    issue if you have a proper hash function.

    I haven't read the rest of the email, but there's clearly
    some confusion in the original paragraph above. 

    --
Brian Hatch                  "Fashion is what one
   Systems and                wears oneself.  What
   Security Engineer          is unfashionable is
http://www.ifokr.org/bri/     what other people wear."
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  • Next message: Michael Wojcik: "RE: Values to use for a salt?"

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