Implement verification of election results

eos/base/election.py

@@ -27,7 +27,7 @@ class NullEncryptedAnswer(EncryptedAnswer):
 	answer = EmbeddedObjectField()
 	
 	def decrypt(self):
-		return self.answer
+		return None, self.answer
 
 class Ballot(EmbeddedObject):
 	#_id = UUIDField()
@@ -123,6 +123,9 @@ class PreferentialAnswer(Answer):
 	choices = ListField(IntField())
 
 class RawResult(Result):
+	_ver = StringField(default='0.2')
+	
+	plaintexts = ListField(EmbeddedObjectListField())
 	answers = EmbeddedObjectListField()
 	
 	def count(self):
@@ -143,3 +146,16 @@ class Election(TopLevelObject):
 	voters = EmbeddedObjectListField(is_hashed=False)
 	questions = EmbeddedObjectListField()
 	results = EmbeddedObjectListField(is_hashed=False)
+	
+	def verify(self):
+		#__pragma__('skip')
+		from eos.core.hashing import SHA256
+		#__pragma__('noskip')
+		election_hash = SHA256().update_obj(self).hash_as_b64()
+		
+		for voter in self.voters:
+			for vote in voter.votes:
+				if vote.ballot.election_id != self._id:
+					raise Exception('Invalid election ID on ballot')
+				if vote.ballot.election_hash != election_hash:
+					raise Exception('Invalid election hash on ballot')

eos/base/workflow.py

@@ -142,7 +142,8 @@ class TaskDecryptVotes(WorkflowTask):
 				vote = voter.votes[-1]
 				ballot = vote.ballot
 				for i in range(len(ballot.encrypted_answers)):
-					answer = ballot.encrypted_answers[i].decrypt()
+					plaintexts, answer = ballot.encrypted_answers[i].decrypt()
+					election.results[i].plaintexts.append(plaintexts)
 					election.results[i].answers.append(answer)
 		
 		self.exit()

eos/core/objects/__init__.py

@@ -111,9 +111,13 @@ class EmbeddedObjectListField(Field):
 	
 	def serialise(self, value, for_hash=False, should_protect=False):
 		# TNYI: Doesn't know how to deal with iterators like EosList
+		if value is None:
+			return None
 		return [EosObject.serialise_and_wrap(x, self.object_type, for_hash, should_protect) for x in (value.impl if isinstance(value, EosList) else value)]
 	
 	def deserialise(self, value):
+		if value is None:
+			return None
 		return EosList([EosObject.deserialise_and_unwrap(x, self.object_type) for x in value])
 
 if is_python:

eos/psr/bitstream.py

@@ -137,8 +137,8 @@ class BitStream(EosObject):
 	@classmethod
 	def unmap(cls, value, func, block_size):
 		bs = cls()
-		for x in value:
-			bs.write(func(x), block_size)
+		for i in range(len(value)):
+			bs.write(func(value[i]), block_size)
 		bs.seek(0)
 		return bs
 	

eos/psr/crypto.py

@@ -132,6 +132,24 @@ class EGPrivateKey(EmbeddedObject):
 		# Undo the encryption mapping
 		return self.public_key.m0_to_message(pt)
 	
+	def decrypt_and_prove(self, ciphertext):
+		result = EGProvedPlaintext()
+		result.ciphertext = ciphertext
+		
+		result.message = self.decrypt(ciphertext)
+		
+		# Adida 2008
+		
+		w = BigInt.crypto_random(ZERO, self.public_key.group.q - ONE) # random element in Z_q
+		result.commitmentA = pow(self.public_key.group.g, w, self.public_key.group.p)
+		result.commitmentB = pow(ciphertext.gamma, w, self.public_key.group.p)
+		
+		result.challenge = SHA256().update_obj(ciphertext).update_obj(result.commitmentA).update_obj(result.commitmentB).hash_as_bigint()
+		
+		result.response = w + self.x * result.challenge
+		
+		return result
+
 class EGCiphertext(EmbeddedObject):
 	public_key = EmbeddedObjectField(EGPublicKey)
 	gamma = EmbeddedObjectField(BigInt) # G^k
@@ -156,6 +174,31 @@ class EGCiphertext(EmbeddedObject):
 		ct = self.public_key._encrypt(self.m0, self.randomness)
 		return ct.gamma == self.gamma and ct.delta == self.delta
 
+class EGProvedPlaintext(EmbeddedObject):
+	message = EmbeddedObjectField(BigInt)
+	
+	ciphertext = EmbeddedObjectField()
+	
+	commitmentA = EmbeddedObjectField(BigInt)
+	commitmentB = EmbeddedObjectField(BigInt)
+	challenge = EmbeddedObjectField(BigInt)
+	response = EmbeddedObjectField(BigInt)
+	
+	def is_proof_valid(self):
+		gt = pow(self.ciphertext.public_key.group.g, self.response, self.ciphertext.public_key.group.p)
+		Ayc = (self.commitmentA * pow(self.ciphertext.public_key.X, self.challenge, self.ciphertext.public_key.group.p)) % self.ciphertext.public_key.group.p
+		if gt != Ayc:
+			return False
+		
+		at = pow(self.ciphertext.gamma, self.response, self.ciphertext.public_key.group.p)
+		m0 = self.ciphertext.public_key.message_to_m0(self.message)
+		m_inv = pow(m0, self.ciphertext.public_key.group.p - TWO, self.ciphertext.public_key.group.p)
+		Bbmc = (self.commitmentB * pow(self.ciphertext.delta * m_inv, self.challenge, self.ciphertext.public_key.group.p)) % self.ciphertext.public_key.group.p
+		if at != Bbmc:
+			return False
+		
+		return True
+
 # Signed ElGamal per Schnorr & Jakobssen
 class SEGPublicKey(EGPublicKey):
 	def _encrypt(self, message, randomness=None):

eos/psr/election.py

@@ -40,11 +40,13 @@ class BlockEncryptedAnswer(EncryptedAnswer):
 		if sk is None:
 			sk = self.recurse_parents(PSRElection).sk
 		
-		bs = BitStream.unmap(self.blocks, sk.decrypt, sk.public_key.nbits())
+		plaintexts = EosList([sk.decrypt_and_prove(self.blocks[i]) for i in range(len(self.blocks))])
+		
+		bs = BitStream.unmap(plaintexts, lambda plaintext: plaintext.message, sk.public_key.nbits())
 		m = bs.read_string()
 		obj = EosObject.deserialise_and_unwrap(EosObject.from_json(m))
 		
-		return obj
+		return plaintexts, obj
 	
 	def deaudit(self):
 		blocks_deaudit = EosList()
@@ -227,3 +229,39 @@ class PSRElection(Election):
 	
 	public_key = EmbeddedObjectField(SEGPublicKey)
 	mixing_trustees = EmbeddedObjectListField()
+	
+	def verify(self):
+		# Verify ballots
+		super().verify()
+		
+		# Verify mixes
+		for i in range(len(self.questions)):
+			for j in range(len(self.mixing_trustees)):
+				self.mixing_trustees[j].verify(i)
+		
+		# Verify decryption proofs
+		for q_num in range(len(self.questions)):
+			raw_result = self.results[q_num]
+			for answer_num in range(len(raw_result.plaintexts)):
+				# Input and output blocks:
+				plaintexts = raw_result.plaintexts[answer_num]
+				ciphertexts = self.mixing_trustees[-1].mixed_questions[q_num][answer_num].blocks
+				
+				# Verify ciphertexts
+				if len(plaintexts) != len(ciphertexts):
+					raise Exception('Different number of plaintexts and ciphertexts')
+				for i in range(len(ciphertexts)):
+					if ciphertexts[i] != plaintexts[i].ciphertext:
+						raise Exception('Ciphertext does not match mixnet output')
+				
+				# Verify decryption
+				for plaintext in plaintexts:
+					if not plaintext.is_proof_valid():
+						raise Exception('Proof of decryption is not valid')
+				
+				# Verify block combination
+				bs = BitStream.unmap(plaintexts, lambda plaintext: plaintext.message, self.public_key.nbits())
+				m = bs.read_string()
+				answer = EosObject.deserialise_and_unwrap(EosObject.from_json(m))
+				if answer != raw_result.answers[answer_num]:
+					raise Exception('Result does not match claimed decryption')

eos/psr/tests.py

@@ -69,9 +69,13 @@ class EGTestCase(EosTestCase):
 		pt = DEFAULT_GROUP.random_Zq_element()
 		sk = EGPrivateKey.generate()
 		ct = sk.public_key.encrypt(pt)
-		m = sk.decrypt(ct)
+		proved_pt = sk.decrypt_and_prove(ct)
+		
+		m = proved_pt.message
 		self.assertEqualJSON(pt, m)
 		
+		self.assertTrue(proved_pt.is_proof_valid())
+
 class SEGTestCase(EosTestCase):
 	def test_eg(self):
 		pt = DEFAULT_GROUP.random_Zq_element()
@@ -148,7 +152,7 @@ class BlockEGTestCase(EosTestCase):
 		obj = self.Person(name='John Smith')
 		
 		ct = BlockEncryptedAnswer.encrypt(self.sk.public_key, obj)
-		m = ct.decrypt(self.sk)
+		_, m = ct.decrypt(self.sk)
 		
 		self.assertEqualJSON(obj, m)
 
@@ -310,6 +314,9 @@ class ElectionTestCase(EosTestCase):
 		# Check the hash hasn't changed during that
 		self.assertEqual(SHA256().update_obj(election).hash_as_b64(), election_hash)
 		
+		# Check the election verifies
+		election.verify()
+
 class PVSSTestCase(EosTestCase):
 	@py_only
 	def test_basic(self):

eos/psr/workflow.py

@@ -68,7 +68,8 @@ class TaskDecryptVotes(eos.base.workflow.TaskDecryptVotes):
 		
 		for i in range(len(election.mixing_trustees[-1].mixed_questions)):
 			for encrypted_answer in election.mixing_trustees[-1].mixed_questions[i]:
-				answer = encrypted_answer.decrypt()
+				plaintexts, answer = encrypted_answer.decrypt()
+				election.results[i].plaintexts.append(plaintexts)
 				election.results[i].answers.append(answer)
 		
 		self.exit()

eosweb/core/main.py

@@ -166,6 +166,17 @@ def count_election(electionid):
 	
 	election.save()
 
+@app.cli.command('verify_election')
+@click.option('--electionid', default=None)
+def verify_election(electionid):
+	if electionid is None:
+		election = Election.get_all()[0]
+	else:
+		election = Election.get_by_id(electionid)
+	
+	election.verify()
+	print('The election has passed validation')
+
 @app.context_processor
 def inject_globals():
 	return {'eos': eos, 'eosweb': eosweb, 'SHA256': eos.core.hashing.SHA256}