Indispensable.hs

{-# LANGUAGE OverloadedStrings #-}
module Voting.Protocol.Trustee.Indispensable where

import Control.Monad (Monad(..), foldM, unless)
import Control.Monad.Trans.Except (ExceptT(..), throwE)
import Data.Eq (Eq(..))
import Data.Function (($))
import Data.Maybe (maybe)
import Data.Semigroup (Semigroup(..))
import Text.Show (Show(..))
import qualified Control.Monad.Trans.State.Strict as S
import qualified Data.ByteString as BS
import qualified Data.List as List

import Voting.Protocol.Utils
import Voting.Protocol.Arithmetic
import Voting.Protocol.Credential
import Voting.Protocol.Election
import Voting.Protocol.Tally

-- * Type 'TrusteePublicKey'
data TrusteePublicKey q = TrusteePublicKey
 { trustee_PublicKey      :: PublicKey q
 , trustee_SecretKeyProof :: Proof q
	-- ^ NOTE: It is important to ensure
	-- that each trustee generates its key pair independently
	-- of the 'PublicKey's published by the other trustees.
	-- Otherwise, a dishonest trustee could publish as 'PublicKey'
	-- its genuine 'PublicKey' divided by the 'PublicKey's of the other trustees.
	-- This would then lead to the 'election_PublicKey'
	-- being equal to this dishonest trustee's 'PublicKey',
	-- which means that knowing its 'SecretKey' would be sufficient
	-- for decrypting messages encrypted to the 'election_PublicKey'.
	-- To avoid this attack, each trustee publishing a 'PublicKey'
	-- must 'prove' knowledge of the corresponding 'SecretKey'.
	-- Which is done in 'proveIndispensableTrusteePublicKey'
	-- and 'verifyIndispensableTrusteePublicKey'.
 } deriving (Eq,Show)

-- ** Type 'ErrorTrusteePublicKey'
data ErrorTrusteePublicKey
 =   ErrorTrusteePublicKey_Wrong
     -- ^ The 'trustee_SecretKeyProof' is wrong.
 deriving (Eq,Show)

-- | @('proveIndispensableTrusteePublicKey' trustSecKey)@
-- returns the 'PublicKey' associated to 'trustSecKey'
-- and a 'Proof' of its knowledge.
proveIndispensableTrusteePublicKey ::
 Monad m => RandomGen r => SubGroup q =>
 SecretKey q -> S.StateT r m (TrusteePublicKey q)
proveIndispensableTrusteePublicKey trustSecKey = do
	let trustee_PublicKey = publicKey trustSecKey
	trustee_SecretKeyProof <-
		prove trustSecKey [groupGen] $
			hash (indispensableTrusteePublicKeyStatement trustee_PublicKey)
	return TrusteePublicKey{..}

-- | @('verifyIndispensableTrusteePublicKey' trustPubKey)@
-- returns 'True' iif. the given 'trustee_SecretKeyProof'
-- does 'prove' that the 'SecretKey' associated with
-- the given 'trustee_PublicKey' is known by the trustee.
verifyIndispensableTrusteePublicKey ::
 Monad m => SubGroup q =>
 TrusteePublicKey q ->
 ExceptT ErrorTrusteePublicKey m ()
verifyIndispensableTrusteePublicKey TrusteePublicKey{..} =
	unless ((proof_challenge trustee_SecretKeyProof ==) $
	hash
	 (indispensableTrusteePublicKeyStatement trustee_PublicKey)
	 [commit trustee_SecretKeyProof groupGen trustee_PublicKey]) $
		throwE ErrorTrusteePublicKey_Wrong

-- ** Hashing
indispensableTrusteePublicKeyStatement :: PublicKey q -> BS.ByteString
indispensableTrusteePublicKeyStatement trustPubKey = "pok|"<>bytesNat trustPubKey<>"|"

-- * 'Election''s 'PublicKey'

combineIndispensableTrusteePublicKeys ::
 SubGroup q => [TrusteePublicKey q] -> PublicKey q
combineIndispensableTrusteePublicKeys =
	List.foldr (\TrusteePublicKey{..} -> (trustee_PublicKey *)) one

verifyIndispensableDecryptionShareByTrustee ::
 SubGroup q => Monad m =>
 EncryptedTally q -> [PublicKey q] -> [DecryptionShare q] ->
 ExceptT ErrorDecryptionShare m ()
verifyIndispensableDecryptionShareByTrustee encTally =
	isoZipWithM_ (throwE $ ErrorDecryptionShare_Invalid "verifyIndispensableDecryptionShareByTrustee")
	 (verifyDecryptionShare encTally)

-- | @('combineDecryptionShares' pubKeyByTrustee decShareByTrustee)@
-- returns the 'DecryptionFactor's by choice by 'Question'
combineIndispensableDecryptionShares ::
 SubGroup q => [PublicKey q] -> EncryptedTally q -> DecryptionShareCombinator q
combineIndispensableDecryptionShares pubKeyByTrustee encTally decShareByTrustee = do
	verifyIndispensableDecryptionShareByTrustee encTally pubKeyByTrustee decShareByTrustee
	(d0,ds) <- maybe err return $ List.uncons decShareByTrustee
	foldM
	 (\decFactorByChoiceByQuest DecryptionShare{..} ->
		isoZipWithM err
		 (\acc df -> maybe err return $ isoZipWith (*) acc df)
		 decFactorByChoiceByQuest decryptionShare_factors)
	 (decryptionShare_factors d0) ds
	where err = throwE $ ErrorDecryptionShare_Invalid "combineIndispensableDecryptionShares"