Crypto.Random

crypto-api-0.13.2: A generic interface for cryptographic operations

Maintainer | Thomas.DuBuisson@gmail.com
Stability | beta
Portability | portable
Safe Haskell | None
Language | Haskell98

Crypto.Random

Contents

- Basic Interface
- Helper functions and expanded interface
- Instances

Description

This module is for instantiating cryptographicly strong determinitic random bit generators (DRBGs, aka PRNGs) For the simple use case of using the system random number generator (`Entropy`) 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)
  - reseedInfo :: g -> ReseedInfo
  - reseedPeriod :: g -> ReseedInfo
  - 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
- data ReseedInfo
  - = InXBytes !Word64
  - | InXCalls !Word64
  - | NotSoon
  - | Never
- splitGen :: CryptoRandomGen g => g -> Either GenError (g, g)
- throwLeft :: Exception e => Either e a -> a
- 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`, `reseedInfo`, `reseedPeriod`.

Minimal complete definition

newGen, genSeedLength, genBytes, reseedInfo, reseedPeriod, 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`

reseedInfo :: g -> ReseedInfo

Indicates how soon a reseed is needed

reseedPeriod :: g -> ReseedInfo

Indicates the period between reseeds (constant for most generators).

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 return `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.Entropy to obtain entropy for `newGen`.

Instances

CryptoRandomGen SystemRandom |

data GenError

Generator failures should always return the appropriate GenError. Note `GenError` in an instance of exception but wether or not an exception is thrown depends on if the selected generator (read: if you don't want execptions from code that uses `throw` then pass in a generator that never has an error for the used functions)

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 |
Data GenError |
Ord GenError |
Read GenError |
Show GenError |
Exception GenError |

data ReseedInfo

Constructors

InXBytes !Word64 | Generator needs reseeded in X bytes
InXCalls !Word64 | Generator needs reseeded in X calls
NotSoon | The bound is over 2^64 bytes or calls
Never | This generator never reseeds (ex: `SystemRandom`)

Instances

Eq ReseedInfo |
Data ReseedInfo |
Ord ReseedInfo |
Read ReseedInfo |
Show ReseedInfo |

# 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). You can find implementations of such generators in the `DRBG` package.

throwLeft :: Exception e => Either e a -> a

Useful utility to extract the result of a generator operation and translate error results to exceptions.

# 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 |