Add functions for encoding and decoding to bytes

Source: https://github.com/sipa/writeups/tree/main/elligator-square-for-bn

Author: stratospher

test/functional/test_framework/ellsq.py

@@ -141,6 +141,41 @@ def reverse_map(x, y, i):
     [(fe(0x3498662504b73c7c8cecb6c33cd493bdfc190e0f87d913d7ff9ad42e222bfe95), fe(0x245b3a61b8d46997f14f2fea2874899691eb32542b9907d65eb9d21d42454021)), [fe(0x7f556282c3dd9d263390d6bbddada698ab8fd7c7d1a06498f42b30437c8361ad), None                                                                  , None                                                                  , None                                                                  ]]
 ]
 
+def encode(P):
+    while True:
+        u = fe(random.randrange(1, SECP256K1_ORDER))
+        ge = forward_map(u)
+        # convert ge to jacobian form for EC operations
+        ge = (ge[0].val, ge[1].val, 1)
+        T = SECP256K1.negate(ge)
+        Q = SECP256K1.add(T, SECP256K1.affine(P))
+        if SECP256K1.is_infinity(Q):
+            Q = T
+        j = random.randrange(4)
+        Q = SECP256K1.affine(Q)
+        v = reverse_map(fe(Q[0]), fe(Q[1]), j)
+        if v is not None:
+            return u, v
+
+def decode(u, v):
+    ge1 = forward_map(u)
+    ge2 = forward_map(v)
+    # convert ge1 and ge2 to jacobian form for EC operations
+    T = ge1[0].val, ge1[1].val, 1
+    S = ge2[0].val, ge2[1].val, 1
+    P = SECP256K1.add(T, S)
+    if SECP256K1.is_infinity(P):
+        P = T
+    P = SECP256K1.affine(P)
+    return fe(P[0]), fe(P[1])
+
+def encode_bytes(P):
+    u, v = encode(P)
+    return u.to_bytes() + v.to_bytes()
+
+def decode_bytes(enc):
+    return decode(fe.from_bytes(enc[:32]), fe.from_bytes(enc[32:]))
+
 class TestFrameworkEllsq(unittest.TestCase):
     def test_fe_to_ge_to_fe(self):
         for i in range(100):