Use a ValueError exception to represent out-of-range field element representations.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

sapling_jubjub.py

@@ -13,7 +13,9 @@ assert (q_j - 1) // 2 == qm1d2
 #
 
 class FieldElement(object):
-    def __init__(self, t, s, modulus):
+    def __init__(self, t, s, modulus, strict=False):
+        if strict and not (0 <= s and s < modulus):
+            raise ValueError
         self.t = t
         self.s = s % modulus
         self.m = modulus
@@ -47,7 +49,6 @@ class FieldElement(object):
         return i2lebsp(l, self.s)
 
     def __bytes__(self):
-        # TODO: Check length
         return i2leosp(256, self.s)
 
     def __eq__(self, a):
@@ -58,10 +59,10 @@ class FieldElement(object):
 class Fq(FieldElement):
     @staticmethod
     def from_bytes(buf):
-        return Fq(leos2ip(buf))
+        return Fq(leos2ip(buf), strict=True)
 
-    def __init__(self, s):
-        FieldElement.__init__(self, Fq, s, q_j)
+    def __init__(self, s, strict=False):
+        FieldElement.__init__(self, Fq, s, q_j, strict=strict)
 
     def __str__(self):
         return 'Fq(%s)' % self.s
@@ -106,8 +107,8 @@ class Fq(FieldElement):
 
 
 class Fr(FieldElement):
-    def __init__(self, s):
-        FieldElement.__init__(self, Fr, s, r_j)
+    def __init__(self, s, strict=False):
+        FieldElement.__init__(self, Fr, s, r_j, strict=strict)
 
     def __str__(self):
         return 'Fr(%s)' % self.s
@@ -141,10 +142,12 @@ JUBJUB_COFACTOR = Fr(8)
 class Point(object):
     @staticmethod
     def from_bytes(buf):
+        assert len(buf) == 32
         u_sign = buf[31] >> 7
         buf = buf[:31] + bytes([buf[31] & 0b01111111])
-        v = Fq.from_bytes(buf)
-        if bytes(v) != buf:
+        try:
+            v = Fq.from_bytes(buf)
+        except ValueError:
             return None
 
         vv = v * v