crypto-api-0.8: A generic interface for cryptographic operations





This module is for instantiating cryptographically strong determinitic random bit generators (DRBGs, aka PRNGs) For the simple use case of using the system random number generator (System.Crypto.Random) to seed the DRBG:

   g <- newGenIO

Users needing to provide their own entropy can call newGen directly

    entropy <- getEntropy nrBytes
    let generator = newGen entropy


Basic Interface

class CryptoRandomGen g whereSource

A class of random bit generators that allows for the possibility of failure, reseeding, providing entropy at the same time as requesting bytes

Minimum complete definition: newGen, genSeedLength, genBytes, reseed.


newGen :: ByteString -> Either GenError gSource

Instantiate a new random bit generator. The provided bytestring should be of length >= genSeedLength. If the bytestring is shorter then the call may fail (suggested error: NotEnoughEntropy). If the bytestring is of sufficent length the call should always succeed.

genSeedLength :: Tagged g ByteLengthSource

Length of input entropy necessary to instantiate or reseed a generator

genBytes :: ByteLength -> g -> Either GenError (ByteString, g)Source

genBytes len g generates a random ByteString of length len and new generator. The MonadCryptoRandom package has routines useful for converting the ByteString to commonly needed values (but cereal or other deserialization libraries would also work).

This routine can fail if the generator has gone too long without a reseed (usually this is in the ball-park of 2^48 requests). Suggested error in this cases is NeedReseed

genBytesWithEntropy :: ByteLength -> ByteString -> g -> Either GenError (ByteString, g)Source

genBytesWithEntropy g i entropy generates i random bytes and use the additional input entropy in the generation of the requested data to increase the confidence our generated data is a secure random stream.

Some generators use entropy to perturb the state of the generator, meaning:

     (_,g2') <- genBytesWithEntropy len g1 ent
     (_,g2 ) <- genBytes len g1
     g2 /= g2'

But this is not required.


     genBytesWithEntropy g bytes entropy = xor entropy (genBytes g bytes)

reseed :: ByteString -> g -> Either GenError gSource

If the generator has produced too many random bytes on its existing seed it will throw NeedReseed. In that case, reseed the generator using this function and a new high-entropy seed of length >= genSeedLength. Using bytestrings that are too short can result in an error (NotEnoughEntropy).

newGenIO :: IO gSource

By default this uses System.Crypto.Random to obtain entropy for newGen.

data GenError Source

Generator failures should always return the appropriate GenError.


GenErrorOther String



Requested more bytes than a single pass can generate (The maximum request is generator dependent)


When using genInteger g (l,h) and logBase 2 (h - l) > (maxBound :: Int).


Some generators cease operation after too high a count without a reseed (ex: NIST SP 800-90)


For instantiating new generators (or reseeding)


This generator can not be instantiated or reseeded with a finite seed (ex: SystemRandom)

Helper functions and expanded interface

splitGen :: CryptoRandomGen g => g -> Either GenError (g, g)Source

While the safety and wisdom of a splitting function depends on the properties of the generator being split, several arguments from informed people indicate such a function is safe for NIST SP 800-90 generators. (see discussion around Sept, Oct 2010)


data SystemRandom Source

Not that it is technically correct as an instance of CryptoRandomGen, but simply because it's a reasonable engineering choice here is a CryptoRandomGen which streams the system randoms. Take note:

  • It uses the default definition of genByteWithEntropy
  • newGen will always fail!
  • reseed will always fail!
  • the handle to the system random is never closed