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

Portability | portable
Stability | beta
Maintainer | Thomas.DuBuisson@gmail.com

Crypto.Random

Contents

- Basic Interface
- Helper functions and expanded interface
- Instances

Description

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

Synopsis

- class CryptoRandomGen g where
  - newGen :: ByteString -> Either GenError g
  - genSeedLength :: Tagged g ByteLength
  - genBytes :: ByteLength -> g -> Either GenError (ByteString, g)
  - genBytesWithEntropy :: ByteLength -> ByteString -> g -> Either GenError (ByteString, g)
  - reseed :: ByteString -> g -> Either GenError g
  - newGenIO :: IO g
- data GenError
  - = GenErrorOther String
  - | RequestedTooManyBytes
  - | RangeInvalid
  - | NeedReseed
  - | NotEnoughEntropy
  - | NeedsInfiniteSeed
- splitGen :: CryptoRandomGen g => g -> Either GenError (g, g)
- data SystemRandom

# Basic Interface

class CryptoRandomGen g where

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`.

Methods

newGen :: ByteString -> Either GenError g

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 ByteLength

Length of input entropy necessary to instantiate or reseed a generator

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

`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)

`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.

Default:

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

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

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 g

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

Instances

CryptoRandomGen SystemRandom |

data GenError

Generator failures should always return the appropriate GenError.

Constructors

GenErrorOther String | Misc
RequestedTooManyBytes | Requested more bytes than a single pass can generate (The maximum request is generator dependent)
RangeInvalid | When using `genInteger g (l,h)` and `logBase 2 (h - l) > (maxBound :: Int)`.
NeedReseed | Some generators cease operation after too high a count without a reseed (ex: NIST SP 800-90)
NotEnoughEntropy | For instantiating new generators (or reseeding)
NeedsInfiniteSeed | This generator can not be instantiated or reseeded with a finite seed (ex: `SystemRandom`)

Instances

Eq GenError |
Ord GenError |
Show GenError |

# Helper functions and expanded interface

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

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 libraries@haskell.org discussion around Sept, Oct 2010)

# Instances

data SystemRandom

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

Instances

CryptoRandomGen SystemRandom |