Quote from: crazyates on June 17, 2012, 02:58:42 AM

Quote from: MrTeal on June 17, 2012, 02:52:10 AM

Someone more knowledgeable than I am will probably chime in here, but they don't crack passwords like this. For cracking, you have the hash of the password. You then take your password guess, hash it, then compare that hash to the value you have. Rinse and repeat several trillion times.

Bold added for emphasis Wink

I'm not too up on my cryptology and password hacking, so I would not be surprised if you are right!

This is correct, in the case we're referring to. (Certain methods more efficient than guessing and checking are usable with some hashing algorithms, but not SHA256 to my knowledge)

Since raw passwords are never (should never be) stored on the server, when a user signs up, their password is hashed at least once and hopefully salted (though not as frequently as they should be) before being put in a database. Through various methods irrelevant to this discussion, a hacker can gain access to that database and consequentially the hashed passwords.

Hopefully the server uses a good hashing algorithm such as bcrypt (multi-layered salted SHA256/512 isn't as good, but is usually sufficient). In this case, the hacker can't do much of anything with the database, as it is inefficient to brute force and impossible to reverse-hash.

However, in a startling number of cases (the MtGox hack for example), passwords are not well-hashed and salted, and are stored in a hashing algorithm such as SHA256, SHA1, or (heaven forbid) MD5 that is efficient. (read: hashes can be calculated quickly)

The hacker then proceeds to find a hash he/she wishes to crack and then attempts to brute force it by hashing standard alphanumeric strings until he finds the one giving the hash he has obtained. He can then use that string (which should be the user's password, except in rare cases) in order to log in to the website and wreak havoc.

Now, for some math. In the 95 character ASCII printable keyspace (usually around the number of different characters allowed in a password), checking every possible password up to 8 characters would take around 6.6 e+15 tries. While that seems like a lot, a 1 TH/s (1 e+12 hashes/sec) unit could process all of those in under two hours. (this is hypothetical, I'm not accounting for bandwidth limitations and the like)

Sorry, that was a bit long winded. Hopefully it's at least moderately understandable.