61 lines
2.1 KiB
Haskell
61 lines
2.1 KiB
Haskell
-- |
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-- Module : Crypto.Number.Generate
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-- License : BSD-style
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-- Maintainer : Vincent Hanquez <vincent@snarc.org>
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-- Stability : experimental
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-- Portability : Good
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module Crypto.Number.Generate
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( generateMax
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, generateBetween
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, generateOfSize
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, generateBits
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) where
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import Control.Applicative
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import Crypto.Number.Basic
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import Crypto.Number.Serialize
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import Crypto.Random.Types
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import qualified Data.ByteString as B
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import Data.Bits ((.|.), (.&.), shiftR)
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-- | generate a positive integer x, s.t. 0 <= x < m
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generateMax :: MonadRandom m => Integer -> m Integer
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generateMax 1 = return 0
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generateMax m
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| m <= 0 = error "negative value for generateMax"
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| otherwise = do
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result <- randomInt bytesLength
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let result' = result `shiftR` bitsPoppedOff
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if result' >= m
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then generateMax m
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else return result'
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where
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bytesLength = lengthBytes m
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bitsLength = log2 (m-1) + 1
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bitsPoppedOff = 8 - (bitsLength `mod` 8)
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randomInt nbBytes = os2ip <$> getRandomBytes nbBytes
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-- | generate a number between the inclusive bound [low,high].
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generateBetween :: MonadRandom m => Integer -> Integer -> m Integer
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generateBetween low high = (low +) <$> generateMax (high - low + 1)
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-- | generate a positive integer of a specific size in bits.
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-- the number of bits need to be multiple of 8. It will always returns
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-- an integer that is close to 2^(1+bits/8) by setting the 2 highest bits to 1.
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generateOfSize :: MonadRandom m => Int -> m Integer
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generateOfSize bits = toInteger <$> getRandomBytes (bits `div` 8)
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where
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toInteger bs = os2ip $ snd $ B.mapAccumL (\acc w -> (0, w .|. acc)) 0xc0 bs
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-- | Generate a number with the specified number of bits
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generateBits :: MonadRandom m => Int -> m Integer
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generateBits nbBits = modF . os2ip <$> getRandomBytes nbBytes'
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where (nbBytes, strayBits) = nbBits `divMod` 8
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nbBytes' | strayBits == 0 = nbBytes
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| otherwise = nbBytes + 1
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modF | strayBits == 0 = id
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| otherwise = (.&.) (2^nbBits - 1)
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