update hash_to_curve

cashu/core/b_dhke.py

@@ -6,7 +6,7 @@ Alice:
 A = a*G
 return A
 Bob:
-Y = hash_to_point(secret_message)
+Y = hash_to_curve(secret_message)
 r = random blinding factor
 B'= Y + r*G
 return B'
@@ -20,7 +20,7 @@ C = C' - r*A
  (= a*Y)
 return C, secret_message
 Alice:
-Y = hash_to_point(secret_message)
+Y = hash_to_curve(secret_message)
 C == a*Y
 If true, C must have originated from Alice
 """
@@ -30,28 +30,22 @@ 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."""
+def hash_to_curve(message: bytes):
+    """Generates a point from the message hash and checks if the point lies on the curve.
+    If it does not, it tries computing a new point from the hash."""
     point = None
-    msg = secret_msg
+    msg_to_hash = message
     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)
+            _hash = hashlib.sha256(msg_to_hash).digest()
+            point = PublicKey(b"\x02" + _hash, raw=True)
         except:
-            msg = _hash
-
+            msg_to_hash = _hash
     return point
 
 
-def step1_alice(secret_msg):
-    secret_msg = secret_msg.encode("utf-8")
-    Y = hash_to_point(secret_msg)
+def step1_alice(secret_msg: str):
+    Y = hash_to_curve(secret_msg.encode("utf-8"))
     r = PrivateKey()
     B_ = Y + r.pubkey
     return B_, r
@@ -68,7 +62,7 @@ def step3_alice(C_, r, A):
 
 
 def verify(a, C, secret_msg):
-    Y = hash_to_point(secret_msg.encode("utf-8"))
+    Y = hash_to_curve(secret_msg.encode("utf-8"))
     return C == Y.mult(a)
 
 

cashu/core/base.py

@@ -247,7 +247,7 @@ class MintKeyset:
         first_seen=None,
         active=None,
         seed: Union[None, str] = None,
-        derivation_path: str = "0",
+        derivation_path: str = None,
     ):
         self.derivation_path = derivation_path
         self.id = id

cashu/core/crypto.py

@@ -6,6 +6,20 @@ from cashu.core.secp import PrivateKey, PublicKey
 from cashu.core.settings import MAX_ORDER
 
 
+def derivation_path_newer_than(d1: str, d2: str):
+    """
+    Returns whether derivation path d1 is newer (deeper down the derivation tree)
+    than d2. Used for treating tokens with old keysets differently.
+    """
+    # NOTE: Derivation paths are still just simple integers for now. This function needs
+    # to change once we upgrade to BIP32.
+
+    # the very first derivation path was "" (empty string) so anything is newer than itself
+    if d1 == "" and d2 != "":
+        return False
+    return int(d1) > int(d2)
+
+
 def derive_keys(master_key: str, derivation_path: str = ""):
     """
     Deterministic derivation of keys for 2^n values.

cashu/mint/__init__.py

@@ -1,4 +1,4 @@
 from cashu.core.settings import MINT_PRIVATE_KEY
 from cashu.mint.ledger import Ledger
 
-ledger = Ledger(MINT_PRIVATE_KEY, "data/mint")
+ledger = Ledger(MINT_PRIVATE_KEY, "data/mint", derivation_path="0")

cashu/mint/ledger.py

@@ -112,6 +112,7 @@ class Ledger:
             secret_key = self.keysets.keysets[proof.id].private_keys[proof.amount]
 
         C = PublicKey(bytes.fromhex(proof.C), raw=True)
+
         return b_dhke.verify(secret_key, C, proof.secret)
 
     def _verify_script(self, idx: int, proof: Proof):

docs/README.md

@@ -17,9 +17,9 @@ Mint: `Bob`
 
 # Blind Diffie-Hellmann key exchange (BDH)
 -   Mint `Bob` publishes `K = kG` 
--   `Alice` picks secret `x` and computes `Y = hash_to_point(x)` 
+-   `Alice` picks secret `x` and computes `Y = hash_to_curve(x)` 
 -   `Alice` sends to `Bob`: `T = Y + rG` with `r` being a random nonce
 -   `Bob` sends back to `Alice` blinded key: `Q = kT` (these two steps are the DH key exchange)
 -   `Alice` can calculate the unblinded key as `Q - rK = kY + krG - krG = kY = Z`
 -   Alice can take the pair `(x, Z)` as a token and can send it to `Carol`.
--   `Carol` can send `(x, Z)` to `Bob` who then checks that `k*hash_to_point(x) == Z`, and if so treats it as a valid spend of a token, adding `x`  to the list of spent secrets.
+-   `Carol` can send `(x, Z)` to `Bob` who then checks that `k*hash_to_curve(x) == Z`, and if so treats it as a valid spend of a token, adding `x`  to the list of spent secrets.

docs/specs/cashu_client_spec.md

@@ -17,12 +17,12 @@ Mint: `Bob`
 
 # Blind Diffie-Hellmann key exchange (BDH)
 -   Mint `Bob` publishes `K = kG` 
--   `Alice` picks secret `x` and computes `Y = hash_to_point(x)` 
+-   `Alice` picks secret `x` and computes `Y = hash_to_curve(x)` 
 -   `Alice` sends to `Bob`: `T = Y + rG` with `r` being a random nonce
 -   `Bob` sends back to `Alice` blinded key: `Q = kT` (these two steps are the DH key exchange)
 -   `Alice` can calculate the unblinded key as `Q - rK = kY + krG - krG = kY = Z`
 -   Alice can take the pair `(x, Z)` as a token and can send it to `Carol`.
--   `Carol` can send `(x, Z)` to `Bob` who then checks that `k*hash_to_point(x) == Z`, and if so treats it as a valid spend of a token, adding `x`  to the list of spent secrets.
+-   `Carol` can send `(x, Z)` to `Bob` who then checks that `k*hash_to_curve(x) == Z`, and if so treats it as a valid spend of a token, adding `x`  to the list of spent secrets.
 
 # Cashu client protocol