passes gang tests. ship it?

aux/ristretto.sage

@@ -308,6 +308,7 @@ class Decaf_1_1_Point(QuotientEdwardsPoint):
         x,y,z,t = self.xyzt()
         
         if self.cofactor == 8:
+            # Cofactor 8 version
             num = (z+y)*(z-y)
             den = x*y
             tmp = isqrt(num*(a-d)*den^2)
@@ -327,6 +328,7 @@ class Decaf_1_1_Point(QuotientEdwardsPoint):
             s = tmp*den*yisr*(tiisr*z - 1)
         
         else:
+            # Much simpler cofactor 4 version
             num = (x+t)*(x-t)
             isr = isqrt(num*(a-d)*x^2)
             ratio = isr*num
@@ -339,8 +341,55 @@ class Decaf_1_1_Point(QuotientEdwardsPoint):
     @optimized_version_of("decodeSpec")
     def decode(cls,s):
         """Decode, optimized version"""
-        return cls.decodeSpec(s) # TODO
+        a,d = cls.a,cls.d
+        s = cls.bytesToGf(s,mustBePositive=True)
+        
+        if s==0: return cls()
+        s2 = s^2
+        den = 1+a*s2
+        num = den^2 - 4*d*s2
+        isr = isqrt(num*den^2)
+        altx = 2*s*isr*den*cls.isoMagic
+        if negative(altx): isr = -isr
+        x = 2*s *isr^2*den*num
+        y = (1-a*s^2) * isr*den
+        
+        if cls.cofactor==8 and (negative(x*y*cls.isoMagic) or y==0):
+            raise InvalidEncodingException("x*y is invalid: %d, %d" % (x,y))
+        
+        return cls(x,y)
 
+    @classmethod     
+    def fromJacobiQuartic(cls,s,t,sgn=1):
+        """Convert point from its Jacobi Quartic representation"""
+        a,d = cls.a,cls.d
+        if s==0: return cls()
+        altx = 2*s*cls.isoMagic / t
+        if negative(altx): t = -t
+        x = 2*s / (1+a*s^2)
+        y = (1-a*s^2) / t
+        return cls(x,sgn*y)
+            
+    @classmethod
+    def elligatorSpec(cls,r0):
+        a,d = cls.a,cls.d
+        r = cls.qnr * cls.bytesToGf(r0)^2
+        
+        den = (d*r-(d-a))*((d-a)*r-d)
+        n1 = (r+1)*(a-2*d)/den
+        n2 = r*n1
+        if is_square(n1):
+            sgn,s,t = 1,xsqrt(n1), -(r-1)*(a-2*d)^2 / den - 1
+        else:
+            sgn,s,t = -1,xsqrt(n2), r*(r-1)*(a-2*d)^2 / den - 1
+        
+        return cls.fromJacobiQuartic(s,t,sgn)
+            
+    @classmethod
+    @optimized_version_of("elligatorSpec")
+    def elligator(cls,r0):
+        return cls.elligatorSpec(r0)
+            
 class Ed25519Point(RistrettoPoint):
     F = GF(2^255-19)
     d = F(-121665/121666)
@@ -427,6 +476,7 @@ class IsoEd25519Point(Decaf_1_1_Point):
 class TestFailedException(Exception): pass
 
 def test(cls,n):
+    print "Testing curve %s" % cls.__name__
     # TODO: test corner cases like 0,1,i
     P = cls.base()
     Q = cls()
@@ -451,30 +501,39 @@ def test(cls,n):
         Q2 = Q0*(r+1)
         if Q1 + Q0 != Q2: raise TestFailedException("Scalarmul doesn't work")
         Q = Q1
+
 test(Ed25519Point,100)
 test(IsoEd25519Point,100)
 test(IsoEd448Point,100)
 test(TwistedEd448GoldilocksPoint,100)
 test(Ed448GoldilocksPoint,100)
+        
+   
+def testElligator(cls,n):
+    print "Testing elligator on %s" % cls.__name__
+    for i in xrange(n):
+        cls.elligator(randombytes(cls.encLen))
+testElligator(Ed25519Point,100)
+testElligator(IsoEd448Point,100)
+testElligator(Ed448GoldilocksPoint,100)
+testElligator(TwistedEd448GoldilocksPoint,100)
 
 def gangtest(classes,n):
+    print "Gang test",[cls.__name__ for cls in classes]
     for i in xrange(n):
         rets = [bytes((cls.base()*i).encode()) for cls in classes]
         if len(set(rets)) != 1:
-            print "Divergence at %d" % i
+            print "Divergence in encode at %d" % i
+            for c,ret in zip(classes,rets):
+                print c,binascii.hexlify(ret)
+            print
+        
+        r0 = randombytes(classes[0].encLen)
+        rets = [bytes((cls.elligator(r0)*i).encode()) for cls in classes]
+        if len(set(rets)) != 1:
+            print "Divergence in elligator at %d" % i
             for c,ret in zip(classes,rets):
                 print c,binascii.hexlify(ret)
             print
 gangtest([IsoEd448Point,TwistedEd448GoldilocksPoint,Ed448GoldilocksPoint],100)
 gangtest([Ed25519Point,IsoEd25519Point],100)
-        
-    
-    
-
-def testElligator(cls,n):
-    for i in xrange(n):
-        cls.elligator(randombytes(cls.encLen))
-testElligator(Ed25519Point,100)
-testElligator(IsoEd448Point,100)
-# testElligator(Ed448GoldilocksPoint,100)
-# testElligator(TwistedEd448GoldilocksPoint,100)