3c41966b9a
This new module exposes type constraints required by some hash algorithms and provides functions to check whether the constraints are satisfied with runtime values. Resolves #256.
64 lines
2.2 KiB
Haskell
64 lines
2.2 KiB
Haskell
-- |
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-- Module : Crypto.Number.Nat
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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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--
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-- Numbers at type level.
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--
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-- This module provides extensions to "GHC.TypeLits" and "GHC.TypeNats" useful
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-- to work with cryptographic algorithms parameterized with a variable bit
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-- length. Constraints like @'IsDivisibleBy8' n@ ensure that the type-level
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-- parameter is applicable to the algorithm.
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--
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-- Functions are also provided to test whether constraints are satisfied from
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-- values known at runtime. The following example shows how to discharge
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-- 'IsDivisibleBy8' in a computation @fn@ requiring this constraint:
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--
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-- > withDivisibleBy8 :: Integer
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-- > -> (forall proxy n . (KnownNat n, IsDivisibleBy8 n) => proxy n -> a)
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-- > -> Maybe a
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-- > withDivisibleBy8 len fn = do
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-- > SomeNat p <- someNatVal len
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-- > Refl <- isDivisibleBy8 p
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-- > pure (fn p)
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--
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-- Function @withDivisibleBy8@ above returns 'Nothing' when the argument @len@
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-- is negative or not divisible by 8.
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{-# LANGUAGE DataKinds #-}
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{-# LANGUAGE TypeOperators #-}
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module Crypto.Number.Nat
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( type IsDivisibleBy8
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, type IsAtMost, type IsAtLeast
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, isDivisibleBy8
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, isAtMost
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, isAtLeast
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) where
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import Data.Type.Equality
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import GHC.TypeLits
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import Unsafe.Coerce (unsafeCoerce)
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import Crypto.Internal.Nat
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-- | get a runtime proof that the constraint @'IsDivisibleBy8' n@ is satified
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isDivisibleBy8 :: KnownNat n => proxy n -> Maybe (IsDiv8 n n :~: 'True)
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isDivisibleBy8 n
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| mod (natVal n) 8 == 0 = Just (unsafeCoerce Refl)
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| otherwise = Nothing
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-- | get a runtime proof that the constraint @'IsAtMost' value bound@ is
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-- satified
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isAtMost :: (KnownNat value, KnownNat bound)
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=> proxy value -> proxy' bound -> Maybe ((value <=? bound) :~: 'True)
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isAtMost x y
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| natVal x <= natVal y = Just (unsafeCoerce Refl)
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| otherwise = Nothing
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-- | get a runtime proof that the constraint @'IsAtLeast' value bound@ is
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-- satified
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isAtLeast :: (KnownNat value, KnownNat bound)
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=> proxy value -> proxy' bound -> Maybe ((bound <=? value) :~: 'True)
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isAtLeast = flip isAtMost
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