Crypto.PasswordStore

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.

Registering and verifying passwords

makePassword :: ByteString -> Int -> IO ByteString

Hash a password with a given strength (12 is a good default). The output of this function can be written directly to a password file or database. Generates a salt using high-quality randomness from /dev/urandom or (if that is not available) System.Random, which is included in the hashed output.

makePasswordSalt :: ByteString -> Salt -> Int -> ByteString

Hash a password with a given strength (12 is a good default), using a given salt. The output of this function can be written directly to a password file or database. Example:

 >>> makePasswordSalt "hunter2" "72cd18b5ebfe6e96" 12
 "sha256|12|72cd18b5ebfe6e96|Xkki10Vus/a2SN/LgCVLTT5R30lvHSCCxH6QboV+U3E="

verifyPassword :: ByteString -> ByteString -> Bool

verifyPassword userInput pwHash verifies the password userInput given by the user against the stored password hash pwHash. Returns True if the given password is correct, and False if it is not.

Updating password hash strength

strengthenPassword :: ByteString -> Int -> ByteString

Try to strengthen a password hash, by hashing it some more times. strengthenPassword pwHash new_strength will return a new password hash with strength at least new_strength. If the password hash already has strength greater than or equal to new_strength, then it is returned unmodified. If the password hash is invalid and does not parse, it will be returned without comment.

This function can be used to periodically update your password database when computers get faster, in order to keep up with Moore's law. This isn't hugely important, but it's a good idea.

passwordStrength :: ByteString -> Int

Return the strength of a password hash.

Utilities

data Salt

A salt is a unique random value which is stored as part of the password hash. You can generate a salt with genSaltIO or genSaltRandom, or if you really know what you're doing, you can create them from your own ByteString values with makeSalt.

Instances

Eq Salt

Ord Salt

Show Salt

isPasswordFormatValid :: ByteString -> Bool

Is the format of a password hash valid? Attempts to parse a given password hash. Returns True if it parses correctly, and False otherwise.

genSaltIO :: IO Salt

Generate a Salt from 128 bits of data from /dev/urandom, with the system RNG as a fallback. This is the function used to generate salts by makePassword.

genSaltRandom :: RandomGen b => b -> (Salt, b)

Generate a Salt with 128 bits of data taken from a given random number generator. Returns the salt and the updated random number generator. This is meant to be used with makePasswordSalt by people who would prefer to either use their own random number generator or avoid the IO monad.

makeSalt :: ByteString -> Salt

Create a Salt from a ByteString. The input must be at least 8 characters, and can contain arbitrary bytes. Most users will not need to use this function.