See this script more as a fun little hack rather than a "Formal proof". Thanks aparadja for sharing. That being said, using MD5 without salt is asking for trouble. I mean, I know security is usually just a time vs $ vs quality problem, but it costs almost nothing more to add a salt in front of the password. Why not do it?
Using SHA256 with a salt is asking for trouble.
Use bcrypt, scrypt, or PBKDF2. Do not DIY your password hash.
If somebody got read-level access to your password hashes, it follows (based purely on the assumption that any app with the rights to read the hash will probably have the right to change it when applicable) that one could simply overwrite the password hash with a new one that is already known, gain unauthorized access, and change the hash back to prevent the user from finding out. Unless you really need that password to crack multiple accounts that might be reusing it, it seems unnecessary. (And honestly I wouldn't care to implement these special password hashes just to protect extraneous accounts of my customers which I don't control)
edit I should clarify that while I agree that expensive compute time for password hashes helps prevent the ultimate compromise of a user's password, I find it a much more worrisome prospect that somebody got access to the database in the first place. To me, a person's password strength is almost irrelevant compared to the importance of preventing brute-force attacks on a login API or ensuring the integrity of the password database and db apps.
That being said, for those wishing to implement a bcrypt-type hash:
perl -le'print crypt("something", "\$2a\$random")'
They protect the (majority of) users that re-use passwords on multiple services, and, more importantly, they protect you from the PR shitstorm of having dumps of cracked passwords posted to Pastebin after a compromise.
Also, a note to anyone reading the above post, that is not how bcrypt works and is incredibly insecure.
1. http://www.postgresql.org/docs/current/static/pgcrypto.html
For instance, I've got lots of password with over 20 characters mixed with upper/lower cases with a few symbols in it. (i.e. Seems hard but: phzb0xis@mynickname.on.hackernews.com is already pretty long and hard to crack but still easy for me to type/remember), btw that is not my password ;) I wonder if I encrypt that with MD5.. Could it be contained in a rainbow table somewhere?
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