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1 {-# LANGUAGE DeriveAnyClass #-}
2 {-# LANGUAGE DeriveGeneric #-}
3 {-# LANGUAGE OverloadedStrings #-}
4 {-# LANGUAGE UndecidableInstances #-} -- for Reifies instances
5 module Voting.Protocol.Trustee.Indispensable where
6
7 import Control.DeepSeq (NFData)
8 import Control.Monad (Monad(..), foldM, unless)
9 import Control.Monad.Trans.Except (ExceptT(..), throwE)
10 import Data.Aeson (ToJSON(..), FromJSON(..), (.:), (.=))
11 import Data.Eq (Eq(..))
12 import Data.Function (($))
13 import Data.Functor ((<$>))
14 import Data.Maybe (maybe)
15 import Data.Reflection (Reifies(..))
16 import Data.Semigroup (Semigroup(..))
17 import Data.Tuple (fst)
18 import GHC.Generics (Generic)
19 import System.Random (RandomGen)
20 import Text.Show (Show(..))
21 import qualified Control.Monad.Trans.State.Strict as S
22 import qualified Data.Aeson as JSON
23 import qualified Data.ByteString as BS
24 import qualified Data.List as List
25
26 import Voting.Protocol.Utils
27 import Voting.Protocol.Arithmetic
28 import Voting.Protocol.Version
29 import Voting.Protocol.Cryptography
30 import Voting.Protocol.Credential
31 import Voting.Protocol.Tally
32
33 -- * Type 'TrusteePublicKey'
34 data TrusteePublicKey crypto v c = TrusteePublicKey
35 { trustee_PublicKey :: !(PublicKey crypto c)
36 , trustee_SecretKeyProof :: !(Proof crypto v c)
37 -- ^ NOTE: It is important to ensure
38 -- that each trustee generates its key pair independently
39 -- of the 'PublicKey's published by the other trustees.
40 -- Otherwise, a dishonest trustee could publish as 'PublicKey'
41 -- its genuine 'PublicKey' divided by the 'PublicKey's of the other trustees.
42 -- This would then lead to the 'election_PublicKey'
43 -- being equal to this dishonest trustee's 'PublicKey',
44 -- which means that knowing its 'SecretKey' would be sufficient
45 -- for decrypting messages encrypted to the 'election_PublicKey'.
46 -- To avoid this attack, each trustee publishing a 'PublicKey'
47 -- must 'prove' knowledge of the corresponding 'SecretKey'.
48 -- Which is done in 'proveIndispensableTrusteePublicKey'
49 -- and 'verifyIndispensableTrusteePublicKey'.
50 } deriving (Generic)
51 deriving instance Eq (G crypto c) => Eq (TrusteePublicKey crypto v c)
52 deriving instance (Show (G crypto c), Show (PublicKey crypto c)) => Show (TrusteePublicKey crypto v c)
53 deriving instance NFData (G crypto c) => NFData (TrusteePublicKey crypto v c)
54 instance
55 ( Reifies v Version
56 , ToJSON (G crypto c)
57 ) => ToJSON (TrusteePublicKey crypto v c) where
58 toJSON TrusteePublicKey{..} =
59 JSON.object
60 [ "pok" .= trustee_SecretKeyProof
61 , "public_key" .= trustee_PublicKey
62 ]
63 toEncoding TrusteePublicKey{..} =
64 JSON.pairs
65 ( "pok" .= trustee_SecretKeyProof
66 <> "public_key" .= trustee_PublicKey
67 )
68 instance
69 ( Reifies v Version
70 , CryptoParams crypto c
71 ) => FromJSON (TrusteePublicKey crypto v c) where
72 parseJSON = JSON.withObject "TrusteePublicKey" $ \o -> do
73 trustee_PublicKey <- o .: "public_key"
74 trustee_SecretKeyProof <- o .: "pok"
75 return TrusteePublicKey{..}
76
77 -- ** Generating a 'TrusteePublicKey'
78
79 -- | @('proveIndispensableTrusteePublicKey' trustSecKey)@
80 -- returns the 'PublicKey' associated to 'trustSecKey'
81 -- and a 'Proof' of its knowledge.
82 proveIndispensableTrusteePublicKey ::
83 Reifies v Version =>
84 CryptoParams crypto c =>
85 Key crypto =>
86 Monad m => RandomGen r =>
87 SecretKey crypto c -> S.StateT r m (TrusteePublicKey crypto v c)
88 proveIndispensableTrusteePublicKey trustSecKey = do
89 let trustee_PublicKey = publicKey trustSecKey
90 trustee_SecretKeyProof <-
91 prove trustSecKey [groupGen] $
92 hash (indispensableTrusteePublicKeyStatement trustee_PublicKey)
93 return TrusteePublicKey{..}
94
95 -- ** Checking a 'TrusteePublicKey' before incorporating it into the 'Election''s 'PublicKey'
96
97 -- | @('verifyIndispensableTrusteePublicKey' trustPubKey)@
98 -- returns 'True' iif. the given 'trustee_SecretKeyProof'
99 -- does 'prove' that the 'SecretKey' associated with
100 -- the given 'trustee_PublicKey' is known by the trustee.
101 verifyIndispensableTrusteePublicKey ::
102 Reifies v Version =>
103 CryptoParams crypto c =>
104 Monad m =>
105 TrusteePublicKey crypto v c ->
106 ExceptT ErrorTrusteePublicKey m ()
107 verifyIndispensableTrusteePublicKey TrusteePublicKey{..} =
108 unless (
109 proof_challenge trustee_SecretKeyProof == hash
110 (indispensableTrusteePublicKeyStatement trustee_PublicKey)
111 [commit trustee_SecretKeyProof groupGen trustee_PublicKey]
112 ) $
113 throwE ErrorTrusteePublicKey_WrongProof
114
115 -- ** Type 'ErrorTrusteePublicKey'
116 data ErrorTrusteePublicKey
117 = ErrorTrusteePublicKey_WrongProof
118 -- ^ The 'trustee_SecretKeyProof' is wrong.
119 deriving (Eq,Show)
120
121 -- ** Hashing
122 indispensableTrusteePublicKeyStatement ::
123 CryptoParams crypto c =>
124 PublicKey crypto c -> BS.ByteString
125 indispensableTrusteePublicKeyStatement trustPubKey =
126 "pok|"<>bytesNat trustPubKey<>"|"
127
128 -- * 'Election''s 'PublicKey'
129
130 -- ** Generating an 'Election''s 'PublicKey' from multiple 'TrusteePublicKey's.
131
132 combineIndispensableTrusteePublicKeys ::
133 CryptoParams crypto c =>
134 [TrusteePublicKey crypto v c] -> PublicKey crypto c
135 combineIndispensableTrusteePublicKeys =
136 List.foldr (\TrusteePublicKey{..} -> (trustee_PublicKey *)) one
137
138 -- ** Checking the trustee's 'DecryptionShare's before decrypting an 'EncryptedTally'.
139
140 verifyIndispensableDecryptionShareByTrustee ::
141 Reifies v Version =>
142 CryptoParams crypto c =>
143 Monad m =>
144 EncryptedTally crypto v c -> [PublicKey crypto c] -> [DecryptionShare crypto v c] ->
145 ExceptT ErrorTally m ()
146 verifyIndispensableDecryptionShareByTrustee encByChoiceByQuest =
147 isoZipWithM_ (throwE $ ErrorTally_NumberOfTrustees)
148 (verifyDecryptionShare encByChoiceByQuest)
149
150 -- ** Decrypting an 'EncryptedTally' from multiple 'TrusteePublicKey's.
151
152 -- | @('combineDecryptionShares' pubKeyByTrustee decShareByTrustee)@
153 -- returns the 'DecryptionFactor's by choice by 'Question'
154 combineIndispensableDecryptionShares ::
155 Reifies v Version =>
156 CryptoParams crypto c =>
157 [PublicKey crypto c] -> DecryptionShareCombinator crypto v c
158 combineIndispensableDecryptionShares
159 pubKeyByTrustee
160 encByChoiceByQuest
161 decByChoiceByQuestByTrustee = do
162 verifyIndispensableDecryptionShareByTrustee
163 encByChoiceByQuest
164 pubKeyByTrustee
165 decByChoiceByQuestByTrustee
166 (DecryptionShare dec0,decs) <-
167 maybe (throwE ErrorTally_NumberOfTrustees) return $
168 List.uncons decByChoiceByQuestByTrustee
169 foldM (isoZipWithM (throwE ErrorTally_NumberOfQuestions)
170 (maybe (throwE ErrorTally_NumberOfChoices) return `o2`
171 isoZipWith (\a (decFactor, _proof) -> a * decFactor)))
172 ((fst <$>) <$> dec0) (unDecryptionShare <$> decs)