Arith.hs

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE Rank2Types #-} -- for ReifyCrypto
{-# LANGUAGE UndecidableInstances #-} -- for Reifies instances
{-# OPTIONS_GHC -fno-warn-orphans #-}
-- | Arithmetic
module Voting.Protocol.Arith where

import Control.Arrow (first)
import Control.DeepSeq (NFData)
import Control.Monad (Monad(..))
import Data.Aeson (ToJSON(..),FromJSON(..))
import Data.Bits
import Data.Bool
import Data.Eq (Eq(..))
import Data.Foldable (Foldable, foldl')
import Data.Function (($), (.), id)
import Data.Functor ((<$>))
import Data.Int (Int)
import Data.Maybe (Maybe(..), fromJust)
import Data.Ord (Ord(..))
import Data.Proxy (Proxy(..))
import Data.Reflection (Reifies(..))
import Data.Semigroup (Semigroup(..))
import Data.String (IsString(..))
import Data.Text (Text)
import GHC.Generics (Generic)
import GHC.Natural (minusNaturalMaybe)
import Numeric.Natural (Natural)
import Prelude (Integer, Integral(..), fromIntegral, Enum(..))
import Text.Read (readMaybe)
import Text.Show (Show(..))
import qualified Control.Monad.Trans.State.Strict as S
import qualified Crypto.Hash as Crypto
import qualified Data.Aeson as JSON
import qualified Data.Aeson.Types as JSON
import qualified Data.ByteArray as ByteArray
import qualified Data.ByteString as BS
import qualified Data.ByteString.Base64 as BS64
import qualified Data.Char as Char
import qualified Data.List as List
import qualified Data.Text as Text
import qualified Data.Text.Encoding as Text
import qualified Data.Text.Lazy as TL
import qualified Data.Text.Lazy.Builder as TLB
import qualified Data.Text.Lazy.Builder.Int as TLB
import qualified Prelude as Num
import qualified System.Random as Random

-- * Class 'Additive'
class Additive a where
	zero :: a
	(+) :: a -> a -> a; infixl 6 +
	sum :: Foldable f => f a -> a
	sum = foldl' (+) zero
instance Additive Natural where
	zero = 0
	(+)  = (Num.+)
instance Additive Integer where
	zero = 0
	(+)  = (Num.+)
instance Additive Int where
	zero = 0
	(+)  = (Num.+)

-- ** Class 'Negable'
class Additive a => Negable a where
	neg :: a -> a
	(-) :: a -> a -> a; infixl 6 -
	x-y = x + neg y
instance Negable Integer where
	neg  = Num.negate
instance Negable Int where
	neg  = Num.negate

-- * Class 'Multiplicative'
class Multiplicative a where
	one :: a
	(*) :: a -> a -> a; infixl 7 *
instance Multiplicative Natural where
	one = 1
	(*) = (Num.*)
instance Multiplicative Integer where
	one = 1
	(*) = (Num.*)
instance Multiplicative Int where
	one = 1
	(*) = (Num.*)

-- ** Class 'Invertible'
class Multiplicative a => Invertible a where
	inv :: a -> a
	(/) :: a -> a -> a; infixl 7 /
	x/y = x * inv y

-- | @(b '^' e)@ returns the modular exponentiation of base 'b' by exponent 'e'.
(^) ::
 forall crypto c.
 Reifies c crypto =>
 Multiplicative (G crypto c) =>
 G crypto c -> E crypto c -> G crypto c
(^) b (E e)
 | e == 0 = one
 | otherwise = t * (b*b) ^ E (e`shiftR`1)
	where t | testBit e 0 = b
	        | otherwise   = one
infixr 8 ^

-- * Class 'GroupParams' where
class
 ( Multiplicative (G crypto c)
 , Invertible     (G crypto c)
 , FromNatural    (G crypto c)
 , ToNatural      (G crypto c)
 , Eq             (G crypto c)
 , Ord            (G crypto c)
 , Show           (G crypto c)
 , NFData         (G crypto c)
 , FromJSON       (G crypto c)
 , ToJSON         (G crypto c)
 , Reifies c crypto
 ) => GroupParams crypto c where
	-- | A generator of the subgroup.
	groupGen   :: G crypto c
	-- | The order of the subgroup.
	groupOrder :: Proxy c -> Natural
	
	-- | 'groupGenPowers' returns the infinite list
	-- of powers of 'groupGen'.
	--
	-- NOTE: In the 'GroupParams' class to keep
	-- computed values in memory across calls to 'groupGenPowers'.
	groupGenPowers :: [G crypto c]
	groupGenPowers = go one
		where go g = g : go (g * groupGen)
	
	-- | 'groupGenInverses' returns the infinite list
	-- of 'inv'erse powers of 'groupGen':
	-- @['groupGen' '^' 'neg' i | i <- [0..]]@,
	-- but by computing each value from the previous one.
	--
	-- NOTE: In the 'GroupParams' class to keep
	-- computed values in memory across calls to 'groupGenInverses'.
	--
	-- Used by 'intervalDisjunctions'.
	groupGenInverses :: [G crypto c]
	groupGenInverses = go one
		where
		invGen = inv $ groupGen
		go g = g : go (g * invGen)

-- ** Class 'ReifyCrypto'
class ReifyCrypto crypto where
	-- | Like 'reify' but augmented with the 'GroupParams' constraint.
	reifyCrypto :: crypto -> (forall c. Reifies c crypto => GroupParams crypto c => Proxy c -> r) -> r

-- ** Type 'G'
-- | The type of the elements of a subgroup of a field.
newtype G crypto c = G { unG :: FieldElement crypto }

-- *** Type family 'FieldElement'
type family FieldElement crypto :: *

-- ** Type 'E'
-- | An exponent of a (cyclic) subgroup of a field.
-- The value is always in @[0..'groupOrder'-1]@.
newtype E crypto c = E { unE :: Natural }
 deriving (Eq,Ord,Show)
 deriving newtype NFData
instance ToJSON (E crypto c) where
	toJSON = JSON.toJSON . show . unE
instance GroupParams crypto c => FromJSON (E crypto c) where
	parseJSON (JSON.String s)
	 | Just (c0,_) <- Text.uncons s
	 , c0 /= '0'
	 , Text.all Char.isDigit s
	 , Just x <- readMaybe (Text.unpack s)
	 , x < groupOrder (Proxy @c)
	 = return (E x)
	parseJSON json = JSON.typeMismatch "Exponent" json
instance GroupParams crypto c => FromNatural (E crypto c) where
	fromNatural i =
		E $ abs $ i `mod` groupOrder (Proxy @c)
		where
		abs x | x < 0 = x + groupOrder (Proxy @c)
		      | otherwise = x
instance ToNatural (E crypto c) where
	nat = unE
instance GroupParams crypto c => Additive (E crypto c) where
	zero = E zero
	E x + E y = E $ (x + y) `mod` groupOrder (Proxy @c)
instance GroupParams crypto c => Negable (E crypto c) where
	neg (E x)
	 | x == 0 = zero
	 | otherwise = E $ fromJust $ nat (groupOrder (Proxy @c))`minusNaturalMaybe`x
instance GroupParams crypto c => Multiplicative (E crypto c) where
	one = E one
	E x * E y = E $ (x * y) `mod` groupOrder (Proxy @c)
instance GroupParams crypto c => Random.Random (E crypto c) where
	randomR (E lo, E hi) =
		first (E . fromIntegral) .
		Random.randomR
		 ( 0`max`toInteger lo
		 , toInteger hi`min`(toInteger (groupOrder (Proxy @c)) - 1) )
	random =
		first (E . fromIntegral) .
		Random.randomR (0, toInteger (groupOrder (Proxy @c)) - 1)
instance GroupParams crypto c => Enum (E crypto c) where
	toEnum = fromNatural . fromIntegral
	fromEnum = fromIntegral . nat
	enumFromTo lo hi = List.unfoldr
	 (\i -> if i<=hi then Just (i, i+one) else Nothing) lo

-- * Class 'FromNatural'
class FromNatural a where
	fromNatural :: Natural -> a

-- * Class 'ToNatural'
class ToNatural a where
	nat :: a -> Natural
instance ToNatural Natural where
	nat = id

-- | @('bytesNat' x)@ returns the serialization of 'x'.
bytesNat :: ToNatural n => n -> BS.ByteString
bytesNat = fromString . show . nat

-- * Type 'Hash'
newtype Hash crypto c = Hash (E crypto c)
 deriving newtype (Eq,Ord,Show,NFData)

-- | @('hash' bs gs)@ returns as a number in 'GroupExponent'
-- the 'Crypto.SHA256' hash of the given 'BS.ByteString' 'bs'
-- prefixing the decimal representation of given subgroup elements 'gs',
-- with a comma (",") intercalated between them.
--
-- NOTE: to avoid any collision when the 'hash' function is used in different contexts,
-- a message 'gs' is actually prefixed by a 'bs' indicating the context.
--
-- Used by 'proveEncryption' and 'verifyEncryption',
-- where the 'bs' usually contains the 'statement' to be proven,
-- and the 'gs' contains the 'commitments'.
hash :: GroupParams crypto c => BS.ByteString -> [G crypto c] -> E crypto c
hash bs gs = do
	let s = bs <> BS.intercalate (fromString ",") (bytesNat <$> gs)
	let h = Crypto.hashWith Crypto.SHA256 s
	fromNatural $
		decodeBigEndian $ ByteArray.convert h

-- | @('decodeBigEndian' bs)@ interpret @bs@ as big-endian number.
decodeBigEndian :: BS.ByteString -> Natural
decodeBigEndian =
	BS.foldl'
	 (\acc b -> acc`shiftL`8 + fromIntegral b)
	 (0::Natural)

-- ** Type 'Base64SHA256'
newtype Base64SHA256 = Base64SHA256 Text
 deriving (Eq,Ord,Show,Generic)
 deriving anyclass (ToJSON,FromJSON)
 deriving newtype NFData

-- | @('base64SHA256' bs)@ returns the 'Crypto.SHA256' hash
-- of the given 'BS.ByteString' 'bs',
-- as a 'Text' escaped in @base64@ encoding
-- (<https://tools.ietf.org/html/rfc4648 RFC 4648>).
base64SHA256 :: BS.ByteString -> Base64SHA256
base64SHA256 bs =
	let h = Crypto.hashWith Crypto.SHA256 bs in
	Base64SHA256 $
		Text.takeWhile (/= '=') $ -- NOTE: no padding.
		Text.decodeUtf8 $ BS64.encode $ ByteArray.convert h

-- ** Type 'HexSHA256'
newtype HexSHA256 = HexSHA256 Text
 deriving (Eq,Ord,Show,Generic)
 deriving anyclass (ToJSON,FromJSON)
 deriving newtype NFData
-- | @('hexSHA256' bs)@ returns the 'Crypto.SHA256' hash
-- of the given 'BS.ByteString' 'bs', escaped in hexadecimal
-- into a 'Text' of 32 lowercase characters.
--
-- Used (in retro-dependencies of this library) to hash
-- the 'PublicKey' of a voter or a trustee.
hexSHA256 :: BS.ByteString -> Text
hexSHA256 bs =
	let h = Crypto.hashWith Crypto.SHA256 bs in
	let n = decodeBigEndian $ ByteArray.convert h in
	-- NOTE: always set the 256 bit then remove it
	-- to always have leading zeros,
	-- and thus always 64 characters wide hashes.
	TL.toStrict $
	TL.tail $ TLB.toLazyText $ TLB.hexadecimal $
	setBit n 256

-- * Random

-- | @('randomR' i)@ returns a random integer in @[0..i-1]@.
randomR ::
 Monad m =>
 Random.RandomGen r =>
 Random.Random i =>
 Negable i =>
 Multiplicative i =>
 i -> S.StateT r m i
randomR i = S.StateT $ return . Random.randomR (zero, i-one)

-- | @('random')@ returns a random integer
-- in the range determined by its type.
random ::
 Monad m =>
 Random.RandomGen r =>
 Random.Random i =>
 Negable i =>
 Multiplicative i =>
 S.StateT r m i
random = S.StateT $ return . Random.random

instance Random.Random Natural where
	randomR (mini,maxi) =
		first (fromIntegral::Integer -> Natural) .
		Random.randomR (fromIntegral mini, fromIntegral maxi)
	random = first (fromIntegral::Integer -> Natural) . Random.random