Fix issues with overflows (#73)

* Fix issues with overflows

* Minor: PEP8

* Test that we actually fixed #62

* Get rid of a bunch of code by using the standard library...

* Minor: unused imports

* Minor: helpful comment

Author: nbgl

phe/encoding.py

@@ -1,3 +1,4 @@
+import fractions
 import math
 import sys
 
@@ -186,7 +187,9 @@ def encode(cls, public_key, scalar, precision=None, max_exponent=None):
         else:
             exponent = min(max_exponent, prec_exponent)
 
-        int_rep = int(round(scalar * pow(cls.BASE, -exponent)))
+        # Use rationals instead of floats to avoid overflow.
+        int_rep = round(fractions.Fraction(scalar)
+                        * fractions.Fraction(cls.BASE) ** -exponent)
 
         if abs(int_rep) > public_key.max_int:
             raise ValueError('Integer needs to be within +/- %d but got %d'
@@ -217,7 +220,17 @@ def decode(self):
         else:
             raise OverflowError('Overflow detected in decrypted number')
 
-        return mantissa * pow(self.BASE, self.exponent)
+        if self.exponent >= 0:
+            # Integer multiplication. This is exact.
+            return mantissa * self.BASE ** self.exponent
+        else:
+            # BASE ** -e is an integer, so below is a division of ints.
+            # Not coercing mantissa to float prevents some overflows.
+            try:
+                return mantissa / self.BASE ** -self.exponent
+            except OverflowError as e:
+                raise OverflowError(
+                    'decoded result too large for a float') from e
 
     def decrease_exponent_to(self, new_exp):
         """Return an `EncodedNumber` with same value but lower exponent.
@@ -249,4 +262,4 @@ def decrease_exponent_to(self, new_exp):
                              'old exponent %i' % (new_exp, self.exponent))
         factor = pow(self.BASE, self.exponent - new_exp)
         new_enc = self.encoding * factor % self.public_key.n
-        return self.__class__(self.public_key, new_enc, new_exp)
+        return self.__class__(self.public_key, new_enc, new_exp)

phe/tests/paillier_test.py

@@ -1085,6 +1085,16 @@ def testKeyring(self):
         self.assertRaises(KeyError, keyring1.decrypt, ciphertext1)
 
 
+class TestIssue62(unittest.TestCase):
+    def testIssue62(self):
+        pub, priv = paillier.generate_paillier_keypair()
+        a = pub.encrypt(445)
+        # Force big exponent.
+        b = pub.encrypt(0.16413409062205825) + pub.encrypt(2 ** -965)
+        # This will raise OverflowError without bugfix #73.
+        priv.decrypt(a + b)
+
+
 def main():
     unittest.main()