refactor

cashu/core/b_dhke.py

@@ -0,0 +1,93 @@
+# Don't trust me with cryptography.
+
+"""
+Implementation of https://gist.github.com/RubenSomsen/be7a4760dd4596d06963d67baf140406
+Alice:
+A = a*G
+return A
+Bob:
+Y = hash_to_point(secret_message)
+r = random blinding factor
+B'= Y + r*G
+return B'
+Alice:
+C' = a*B'
+  (= a*Y + a*r*G)
+return C'
+Bob:
+C = C' - r*A
+ (= C' - a*r*G)
+ (= a*Y)
+return C, secret_message
+Alice:
+Y = hash_to_point(secret_message)
+C == a*Y
+If true, C must have originated from Alice
+"""
+
+import hashlib
+
+from secp256k1 import PrivateKey, PublicKey
+
+
+def hash_to_point(secret_msg):
+    """Generates x coordinate from the message hash and checks if the point lies on the curve.
+    If it does not, it tries computing again a new x coordinate from the hash of the coordinate."""
+    point = None
+    msg = secret_msg
+    while point is None:
+        _hash = hashlib.sha256(msg).hexdigest().encode("utf-8")
+        try:
+            # We construct compressed pub which has x coordinate encoded with even y
+            _hash = list(_hash[:33])  # take the 33 bytes and get a list of bytes
+            _hash[0] = 0x02  # set first byte to represent even y coord
+            _hash = bytes(_hash)
+            point = PublicKey(_hash, raw=True)
+        except:
+            msg = _hash
+
+    return point
+
+
+def step1_alice(secret_msg):
+    secret_msg = secret_msg.encode("utf-8")
+    Y = hash_to_point(secret_msg)
+    r = PrivateKey()
+    B_ = Y + r.pubkey
+    return B_, r
+
+
+def step2_bob(B_, a):
+    C_ = B_.mult(a)
+    return C_
+
+
+def step3_alice(C_, r, A):
+    C = C_ - A.mult(r)
+    return C
+
+
+def verify(a, C, secret_msg):
+    Y = hash_to_point(secret_msg.encode("utf-8"))
+    return C == Y.mult(a)
+
+
+### Below is a test of a simple positive and negative case
+
+# # Alice's keys
+# a = PrivateKey()
+# A = a.pubkey
+# secret_msg = "test"
+# B_, r = step1_alice(secret_msg)
+# C_ = step2_bob(B_, a)
+# C = step3_alice(C_, r, A)
+# print("C:{}, secret_msg:{}".format(C, secret_msg))
+# assert verify(a, C, secret_msg)
+# assert verify(a, C + C, secret_msg) == False  # adding C twice shouldn't pass
+# assert verify(a, A, secret_msg) == False  # A shouldn't pass
+
+# # Test operations
+# b = PrivateKey()
+# B = b.pubkey
+# assert -A -A + A == -A  # neg
+# assert B.mult(a) == A.mult(b)  # a*B = A*b