Charles Miller wrote:
> On 04/01/2006, at 12:18 PM, Jeff Robertson wrote:
>
>> Assume that a password between 1 and 24 ASCII characters was stored as
>> an MD5 hash. No salt. What is the probability that someone cracking the
>> password will find not the password that the user originally chose, but
>> a different password that happens to collide with it? Intuitively it
>> seems so unlikely that you wouldn't ever expect to see it. But what is
>> the probability really?
>
>
> From my back-of-the-envelope calculation, your intuition is misplaced. :)
>
> Even if you assume only 6 bits of variance per password character
> (which is just a-zA-Z0-9 plus two punctuation chars), that's 2^144
> possible 24-character passwords. MD5 is a 128 bit hash, so that's 2^16
> passwords for every hash value, or only a 1 in 65,000 chance that the
> first matching hash you come across in the password space is, in fact,
> the correct password.
>
Without knowing the correct password there is no way of knowing that the
collision isn't it, and from a practical point of view it doesn't matter
in the slightest.
Considering the fact that MD5 has been broken though, I'm fairly
surprised it even came up to discussion. It's not exactly hard to find
info or even collision-generators.
As for not using salts, read this. You'll change your mind.
http://discuss.develop.com/archives/wa.exe?A2=ind0301b&L=advanced-dotnet&T=0&F=&S=&P=4424
Here are some (good) links I found fairly quickly on MD5 being broken.
Google has lots more.
http://www.schneier.com/blog/archives/2005/03/more_hash_funct.html
http://www.schneier.com/crypto-gram-0409.html#3
http://www.schneier.com/blog/archives/2005/06/more_md5_collis.html
http://it.slashdot.org/article.pl?sid=05/11/15/2037232&tid=172&tid=93&tid=228
http://www.cs.bham.ac.uk/~mdr/teaching/modules04/security/lectures/hash.html
http://www.chiark.greenend.org.uk/pipermail/ukcrypto/2004-August/074400.html
http://csrc.nist.gov/hash_standards_comments.pdf
/exon
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Received on Jan 06 2006
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