Securely store hashed, salted passwords. If you need to store and verify passwords, there are many wrong ways to do it, most of them all too common. Some people store users' passwords in plain text. Then, when an attacker manages to get their hands on this file, they have the passwords for every user's account. One step up, but still wrong, is to simply hash all passwords with SHA1 or something. This is vulnerable to rainbow table and dictionary attacks. One step up from that is to hash the password along with a unique salt value. This is vulnerable to dictionary attacks, since guessing a password is very fast. The right thing to do is to use a slow hash function, to add some small but significant delay, that will be negligible for legitimate users but prohibitively expensive for someone trying to guess passwords by brute force. That is what this library does. It iterates a SHA256 hash, with a random salt, a few thousand times. This scheme is known as PBKDF1, and is generally considered secure; there is nothing innovative happening here.

The API here is very simple. What you store are called password hashes. They are strings (technically, ByteStrings) that look like this:

 "sha256|12|Ge9pg8a/r4JW356Uux2JHg==|Fdv4jchzDlRAs6WFNUarxLngaittknbaHFFc0k8hAy0="

Each password hash shows the algorithm, the strength (more on that later), the salt, and the hashed-and-salted password. You store these on your server, in a database, for when you need to verify a password. You make a password hash with the makePassword function. Here's an example:

 >>> makePassword "hunter2" 12
 "sha256|12|lMzlNz0XK9eiPIYPY96QCQ==|1ZJ/R3qLEF0oCBVNtvNKLwZLpXPM7bLEy/Nc6QBxWro="

This will hash the password "hunter2", with strength 12, which is a good default value. The strength here determines how long the hashing will take. When doing the hashing, we iterate the SHA256 hash function 2^strength times, so increasing the strength by 1 makes the hashing take twice as long. When computers get faster, you can bump up the strength a little bit to compensate. You can strengthen existing password hashes with the strengthenPassword function. Note that makePassword needs to generate random numbers, so its return type is IO ByteString. If you want to avoid the IO monad, you can generate your own salt and pass it to makePasswordSalt.

Your strength value should not be less than 10, and 12 is a good default value at the time of this writing, in 2011.

Once you've got your password hashes, the second big thing you need to do with them is verify passwords against them. When a user gives you a password, you compare it with a password hash using the verifyPassword function:

 >>> verifyPassword "wrong guess" passwordHash
 False
 >>> verifyPassword "hunter2" passwordHash
 True

These two functions are really all you need. If you want to make existing password hashes stronger, you can use strengthenPassword. Just pass it an existing password hash and a new strength value, and it will return a new password hash with that strength value, which will match the same password as the old password hash.