System.Entropy

Obtain entropy from system sources or x86 RDRAND when available.

Currently supporting:

• Windows via CryptoAPI
• *nix systems via /dev/urandom
• Includes QNX
• Xen (only when RDRAND is available)

Documentation

getEntropy

Arguments

:: Int

Number of bytes

-> IO ByteString

Get a specific number of bytes of cryptographically secure random data using the *system-specific* sources. (As of 0.4. Verions <0.4 mixed system and hardware sources)

The returned random value is considered cryptographically secure but not true entropy.

On some platforms this requies a file handle which can lead to resource exhaustion in some situations.

getHardwareEntropy

Arguments

:: Int

Number of bytes

-> IO (Maybe ByteString)

Get a specific number of bytes of cryptographically secure random data using a supported *hardware* random bit generator.

If there is no hardware random number generator then Nothing is returned. If any call returns non-Nothing then it should never be Nothing unless there has been a hardware failure.

If trust of the CPU allows it and no context switching is important, a bias to the hardware rng with system rng as fall back is trivial:

let fastRandom nr = maybe (getEntropy nr) pure =<< getHardwareEntropy nr

The old, <0.4, behavior is possible using xor from Bits:

let oldRandom nr =
     do hwRnd  maybe (replicate nr 0) BS.unpack <$ getHardwareEntropy nr
        sysRnd BS.unpack <$ getEntropy nr
        pure $ BS.pack $ zipWith xor sysRnd hwRnd

A less maliable mixing can be accomplished by replacing xor with a composition of concat and cryptographic hash.

data CryptHandle

Handle for manual resource management

openHandle :: IO CryptHandle

Open a CryptHandle

hGetEntropy :: CryptHandle -> Int -> IO ByteString

Read random data from a CryptHandle

closeHandle :: CryptHandle -> IO ()

Close the CryptHandle