Check PartialDecryption's decryption_proofs

index.html

@@ -24,6 +24,8 @@ <h3>Ballots</h3>
           <div id="ballots"></div>
           <h3>EncryptedTally</h3>
           <div id="encryptedTally"></div>
+          <h3>PartialDecryptions</h3>
+          <div id="partialDecryptions"></div>
           <h3></h3>
         </div>
     </div>

src/checkPartialDecryptions.js

@@ -1,8 +1,41 @@
-import { check } from './utils.js'
-import { rev, one } from './math.js'
+import { assert, check } from './utils.js'
+import { rev, one, g, l, erem } from './math.js'
 import { ed25519 } from '@noble/curves/ed25519'
+import sjcl from 'sjcl'
 
 export default function (state) {
+  const et = state.encryptedTally.payload.encrypted_tally
+
+  for (let k = 0; k < state.partialDecryptions.length; k++)
+  {
+    const partialDecryption = state.partialDecryptions[k]
+    const trusteeIdx = partialDecryption.payload.owner - 1
+    assert(state.setup.payload.trustees[trusteeIdx][0] === 'Single')
+    const publicKey = ed25519.ExtendedPoint.fromHex(
+      rev(state.setup.payload.trustees[trusteeIdx][1].public_key))
+    const df = partialDecryption.payload.payload.decryption_factors
+    const dp = partialDecryption.payload.payload.decryption_proofs
+
+    for (let i = 0; i < et.length; i++) {
+      for (let j = 0; j < et[i].length; j++) {
+        const alpha  = ed25519.ExtendedPoint.fromHex(rev(et[i][j].alpha))
+        const factor = ed25519.ExtendedPoint.fromHex(rev(df[i][j]))
+        const challenge = BigInt(dp[i][j].challenge)
+        const response  = BigInt(dp[i][j].response)
+
+        const A = g.multiply(response).add(publicKey.multiply(challenge))
+        const B = alpha.multiply(response).add(factor.multiply(challenge))
+
+        const verificationHash = sjcl.codec.hex.fromBits(
+          sjcl.hash.sha256.hash(`decrypt|${state.setup.fingerprint}|${rev(publicKey.toHex())}|${rev(A.toHex())},${rev(B.toHex())}`))
+        const hexReducedVerificationHash = erem(BigInt('0x' + verificationHash), l).toString(16)
+
+        check('partialDecryptions', 'Valid decryption proof',
+          challenge.toString(16) === hexReducedVerificationHash)
+      }
+    }
+  }
+
   // TODO: Check that there is a partial decryption for every trustee
   // (Maybe in checkResult)
 }