9 ms·

I don't really understand your phrasing, "auto_hash is an inferior password hash", its a rails plugin that uses SHA2 (256). If you mean "auto_hash" uses an inf

• tptacek 16y ago
  There's no such thing as a "security hash". You mean to say "secure hash function". The term "security" refers to the integrity of the hash itself; SHA256 is "secure" against attempts to cause collisions (and several other attacks). None of that has anything to do with the security that SHA256 provides to your application. SHA256 knows how to take care of itself. It doesn't know a damn thing about passwords. SHA256 isn't a secure password hash. You already know that, because you're not just using SHA256; you're also adding a random salt to it. That's because if you didn't do that, attackers could just aim a precomputed dictionary at your database rows and smite it. So your hash isn't SHA256; it's a construction called "salted SHA256" (or "SHA256 with a nonce"). Nonced SHA256 still sucks, though, because SHA256 is lightning fast. Attackers can't throw precomputed dictionaries at your password hash, because you randomized them. But they can take each hash in turn and run a large dictionary against each hash. This is a brute force attack, but it's not a brute force against the 256 bits of the hash; it's an attack against the (low) entropy of the passwords themselves. This is how all password cracking got done in the '90s, which was a bit of a golden age for password cracking. It's how John the Ripper (the most famous password cracker) works. John the Ripper is extraordinarily fast against Unix password hashes, which are slower (by a lot) than SHA256. Incremental crackers tear through naked SHA256 hashes like a knife through butter. That you don't know any of this --- and I say this respectfully --- tells me that maybe you should be using someone else's password hashing library instead of reinventing your own. But you're welcome anyways for the quick and dirty lesson in password hashing.
  • generalk 16y ago
    The deal with bcrypt is that you can increase the number of work units, so that bcrypt gets slower and slower as you like. That defeats the ability to very quickly iterate over a dictionary, because it simply takes too long to do effectively. (Please correct any of this if I'm wrong. IANACryptographer.)
  • seiji 16y ago
    Short version (you almost got around to it, but didn't explicitly state it :)) -- Don't use straight hash functions because they are made to be fast. Even if you salt/nonce them, you can generate private rainbow tables quickly. Use a password hiding method that's slow to compute (slow being "this operation isn't implemented in hardware or optimized assembly") which is what bcrypt or scrypt provides. Differences in speed/effort are mentioned at http://www.tarsnap.com/scrypt.html http://www.tarsnap.com/scrypt.html -- scrypt enc is approximately 100 billion times more than the cost of cracking the same password on a file encrypted by openssl enc
    • lonestar 16y ago
      Actually, slow in this case does not at all mean "this operation isn't implemented in hardware or optimized assembly". The slowness of an algorithm like bcrypt is a tunable property of the key expansion algorithm. A step in this process will be repeated 2^n times, where n can be configured by the user. If a password function was only slow because it wasn't implemented in assembly on your server, an attacker would obviously just go implement it in assembly for his brute-force crack.
      • tptacek 16y ago
        Actually, the major contribution in Colin's scrypt work is to find primitives that don't admit as well to hardware acceleration; his hypothesis is that one of bcrypt's weaknesses is that even though it's very slow in software, it might be possible to massively accelerate it with FPGAs.
  • KevinMS 16y ago
    You sure are reading a lot into what I said, and coming off as pretty arrogant trying to correct me. So because I haven't yet offered other hashing algorithm options yet, since this plugin is about a week old and nobody has used it, and nobody has requested options, I have no right to be offering this solution for public use and scrutiny? Looks like authlogic uses sha512 by default and clearance and devise use SHA1 by default. If these are their defaults, as you would say, I say this respectfully, maybe those guys should be using somebody else's password hashing library. Also, could I not continue using SHA2 256, and add an option to specify the number of hashing repetitions, which would increase the time to compute, as well as frustrate dictionary attacks?