I was reading a few articles on salts and password hashes and a few people were mentioning rainbow attacks. What exactly is a rainbow attack and what are the best methods to prevent it?
What exactly is a rainbow attack?
cryptographyrainbowattacksaltedhash
Related Solutions
A public salt will not make dictionary attacks harder when cracking a single password. As you've pointed out, the attacker has access to both the hashed password and the salt, so when running the dictionary attack, she can simply use the known salt when attempting to crack the password.
A public salt does two things: makes it more time-consuming to crack a large list of passwords, and makes it infeasible to use a rainbow table.
To understand the first one, imagine a single password file that contains hundreds of usernames and passwords. Without a salt, I could compute "md5(attempt[0])", and then scan through the file to see if that hash shows up anywhere. If salts are present, then I have to compute "md5(salt[a] . attempt[0])", compare against entry A, then "md5(salt[b] . attempt[0])", compare against entry B, etc. Now I have n
times as much work to do, where n
is the number of usernames and passwords contained in the file.
To understand the second one, you have to understand what a rainbow table is. A rainbow table is a large list of pre-computed hashes for commonly-used passwords. Imagine again the password file without salts. All I have to do is go through each line of the file, pull out the hashed password, and look it up in the rainbow table. I never have to compute a single hash. If the look-up is considerably faster than the hash function (which it probably is), this will considerably speed up cracking the file.
But if the password file is salted, then the rainbow table would have to contain "salt . password" pre-hashed. If the salt is sufficiently random, this is very unlikely. I'll probably have things like "hello" and "foobar" and "qwerty" in my list of commonly-used, pre-hashed passwords (the rainbow table), but I'm not going to have things like "jX95psDZhello" or "LPgB0sdgxfoobar" or "dZVUABJtqwerty" pre-computed. That would make the rainbow table prohibitively large.
So, the salt reduces the attacker back to one-computation-per-row-per-attempt, which, when coupled with a sufficiently long, sufficiently random password, is (generally speaking) uncrackable.
The point of rainbow tables is that they're created in advance and distributed en masse to save calculation time for others - it takes just as long to generate rainbow tables on the fly as it would to just crack the password+salt combination directly (since effectively what's being done when generating rainbow tables is pre-running the calculations for brute-forcing the hash), thus the argument that by knowing the salt someone could "generate a rainbow table" is spurious.
There's no real point in storing salts in a separate file as long as they're on a per-user basis - the point of the salt is simply to make it so that one rainbow table can't break every password in the DB.
Best Answer
The wikipedia article is a bit difficult to understand. In a nutshell, you can think of a Rainbow Table as a large dictionary with pre-calculated hashes and the passwords from which they were calculated.
The difference between Rainbow Tables and other dictionaries is simply in the method how the entries are stored. The Rainbow table is optimized for hashes and passwords, and thus achieves great space optimization while still maintaining good look-up speed. But in essence, it's just a dictionary.
When an attacker steals a long list of password hashes from you, he can quickly check if any of them are in the Rainbow Table. For those that are, the Rainbow Table will also contain what string they were hashed from.
Of course, there are just too many hashes to store them all in a Rainbow Table. So if a hash is not in the particular table, the hacker is out of luck. But if your users use simple english words and you have hashed them just once, there is a large possibility that a good Rainbow Table will contain the password.