Enforce a consistent message length for encrypted ballots

Closes #6

eos/base/election.py

@@ -35,7 +35,7 @@ class Ballot(EmbeddedObject):
 	election_id = UUIDField()
 	election_hash = StringField()
 	
-	answers = EmbeddedObjectListField(is_hashed=False)
+	answers = EmbeddedObjectListField(is_hashed=False) # Used for ballots to be audited
 	
 	def deaudit(self):
 		encrypted_answers_deaudit = EosList()
@@ -119,18 +119,22 @@ class ListChoiceQuestion(Question):
 			return '(blank votes)'
 		return ', '.join([self.choices[choice] for choice in answer.choices])
 	
-class ApprovalQuestion(ListChoiceQuestion):
-	pass
+	def max_bits(self):
+		answer = self.answer_type(choices=list(range(len(self.choices))))
+		return len(EosObject.to_json(EosObject.serialise_and_wrap(answer))) * 8
 
 class ApprovalAnswer(Answer):
 	choices = ListField(IntField())
 
-class PreferentialQuestion(ListChoiceQuestion):
-	pass
+class ApprovalQuestion(ListChoiceQuestion):
+	answer_type = ApprovalAnswer
 
 class PreferentialAnswer(Answer):
 	choices = ListField(IntField())
 
+class PreferentialQuestion(ListChoiceQuestion):
+	answer_type = PreferentialAnswer
+
 class RawResult(Result):
 	plaintexts = ListField(EmbeddedObjectListField())
 	answers = EmbeddedObjectListField()

eos/core/objects/__init__.py

@@ -60,6 +60,7 @@ class DBInfo:
 dbinfo = DBInfo()
 
 def db_connect(db_name, db_uri='mongodb://localhost:27017/', db_type='mongodb'):
+	if is_python:
 		dbinfo.provider = eos.core.db.db_providers[db_type](db_name, db_uri)
 		dbinfo.provider.connect()
 

eos/psr/bitstream.py

@@ -111,18 +111,29 @@ class BitStream(EosObject):
 		
 		return self
 	
+	def pad_by(self, padding_size, pad_at_end=False):
+		if padding_size > 0:
+			if pad_at_end:
+				# Suitable for structured data
+				self.seek(self.nbits)
+				self.write(ZERO, padding_size)
+			else:
+				# Suitable for raw numbers
+				self.nbits += padding_size
+		return self # For convenient chaining
+	
+	def pad_to(self, target_size, pad_at_end=False):
+		if self.nbits < target_size:
+			diff = target_size - self.nbits
+			self.pad_by(diff, pad_at_end)
+		return self
+	
 	# Make the size of this BitStream a multiple of the block_size
 	def multiple_of(self, block_size, pad_at_end=False):
 		if self.nbits % block_size != 0:
 			diff = block_size - (self.nbits % block_size)
-			if pad_at_end:
-				# Suitable for structured data
-				self.seek(self.nbits)
-				self.write(ZERO, diff)
-			else:
-				# Suitable for raw numbers
-				self.nbits += diff
-		return self # For convenient chaining
+			self.pad_by(diff, pad_at_end)
+		return self
 	
 	def map(self, func, block_size):
 		if self.nbits % block_size != 0:

eos/psr/crypto.py

@@ -41,7 +41,7 @@ class CyclicGroup(EmbeddedObject):
 		crypto_method = BigInt.crypto_random if crypto_random else BigInt.noncrypto_random
 		return crypto_method(ZERO, self.q - ONE)
 
-# RFC 3526
+# RFC 3526, 2048-bit MODP Group
 DEFAULT_GROUP = CyclicGroup(
 	p=BigInt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
 	g=TWO

eos/psr/election.py

@@ -27,10 +27,14 @@ class BlockEncryptedAnswer(EncryptedAnswer):
 	blocks = EmbeddedObjectListField()
 	
 	@classmethod
-	def encrypt(cls, pk, obj):
+	def encrypt(cls, pk, obj, nbits=None):
 		pt = EosObject.to_json(EosObject.serialise_and_wrap(obj))
 		bs = BitStream()
 		bs.write_string(pt)
+		if nbits is not None:
+			if bs.nbits > nbits:
+				raise Exception('Message is too big')
+			bs.pad_to(nbits, True)
 		bs.multiple_of(pk.nbits(), True)
 		ct = bs.map(pk.encrypt, pk.nbits())
 		

eos/psr/tests.py

@@ -153,7 +153,12 @@ class BlockEGTestCase(EosTestCase):
 		
 		ct = BlockEncryptedAnswer.encrypt(self.sk.public_key, obj)
 		_, m = ct.decrypt(self.sk)
+		self.assertEqualJSON(obj, m)
 		
+		# Force another block
+		ct2 = BlockEncryptedAnswer.encrypt(self.sk.public_key, obj, (len(ct.blocks) * self.sk.public_key.nbits()) + 1)
+		self.assertEqual(len(ct2.blocks), len(ct.blocks) + 1)
+		_, m = ct2.decrypt(self.sk)
 		self.assertEqualJSON(obj, m)
 
 class MixnetTestCase(EosTestCase):
@@ -177,7 +182,7 @@ class MixnetTestCase(EosTestCase):
 		
 		def do_mixnet(mix_order):
 			# Set up mixnet
-			mixnet = RPCMixnet(mix_order)
+			mixnet = RPCMixnet(mix_order=mix_order)
 			
 			# Mix answers
 			shuffled_answers, commitments = mixnet.shuffle(answers)

eosweb/core/static/js/booth_worker.js

@@ -21,9 +21,10 @@ isLibrariesLoaded = false;
 
 function generateEncryptedVote(election, answers) {
 	encrypted_answers = [];
-	for (var answer_json of answers) {
+	for (var q_num = 0; q_num < answers.length; q_num++) {
+		answer_json = answers[q_num];
 		answer = eosjs.eos.core.objects.__all__.EosObject.deserialise_and_unwrap(answer_json, null);
-		encrypted_answer = eosjs.eos.psr.election.__all__.BlockEncryptedAnswer.encrypt(election.public_key, answer);
+		encrypted_answer = eosjs.eos.psr.election.__all__.BlockEncryptedAnswer.encrypt(election.public_key, answer, election.questions.__getitem__(q_num).max_bits());
 		encrypted_answers.push(eosjs.eos.core.objects.__all__.EosObject.serialise_and_wrap(encrypted_answer, null));
 	}