Make registered subtree root generation closer to the spec.

Co-authored-by: Jack Grigg <[email protected]>

Author: daira

zcash_test_vectors/zip_0032.py

@@ -42,44 +42,40 @@ def CKDh(Context, sk_par, c_par, i, lead, tag):
 class RegisteredKey(object):
     Registered = HardenedOnlyContext(b'ZIPRegistered_KD', b'\xAC')
 
-    def __init__(self, IKM, path, sk, chaincode, full_width=None):
+    def __init__(self, IKM, subpath, sk, chaincode, full_width=None):
         self.IKM = IKM
-        self.path = path
+        self.subpath = subpath
         self.sk = sk
         self.chaincode = chaincode
         self.full_width = full_width  # the full-width cryptovalue at this path
 
     @classmethod
-    def master(cls, ContextString, S):
+    def subtree_root(cls, ContextString, S, ZipNumber):
         length_ContextString = len(ContextString)
         length_S = len(S)
 
         assert length_ContextString <= 252
         assert 32 <= length_S <= 252
 
         IKM = bytes([length_ContextString]) + ContextString + bytes([length_S]) + S
-        (sk, chaincode) = MKGh(cls.Registered, IKM)
+        (sk_m, c_m) = MKGh(cls.Registered, IKM)
+        (sk, chaincode) = CKDh(cls.Registered, sk_m, c_m, hardened(ZipNumber), 0, b"")
         return cls(IKM, [], sk, chaincode)
 
-    def child(self, i):
-        (sk_child, c_child) = CKDh(self.Registered, self.sk, self.chaincode, i, 0, b"")
-        return self.__class__(None, self.path + [i], sk_child, c_child)
-
-    def child_with_tag(self, i, tag):
+    def child(self, i, tag):
         (sk_child, c_child) = CKDh(self.Registered, self.sk, self.chaincode, i, 0, tag)
-        (left, right) = CKDh(self.Registered, self.sk, self.chaincode, i, 1, tag)
-        return self.__class__(None, self.path + [(i, tag)], sk_child, c_child, left + right)
+        (I_L, I_R) = CKDh(self.Registered, self.sk, self.chaincode, i, 1, tag)
+        return self.__class__(None, self.subpath + [(i, tag)], sk_child, c_child, I_L + I_R)
 
 
 def registered_key_derivation_tvs():
     args = render_args()
 
     context_string = b'Zcash test vectors'
     seed = bytes(range(32))
-    m = RegisteredKey.master(context_string, seed)
-    m_1h = m.child(hardened(1))
-    m_1h_2h = m_1h.child_with_tag(hardened(2), b"trans rights are human rights")
-    m_1h_2h_3h = m_1h_2h.child_with_tag(hardened(3), b"")
+    m_1h = RegisteredKey.subtree_root(context_string, seed, 1)
+    m_1h_2h = m_1h.child(hardened(2), b"trans rights are human rights")
+    m_1h_2h_3h = m_1h_2h.child(hardened(3), b"")
 
     keys = [m_1h, m_1h_2h, m_1h_2h_3h]
 
@@ -88,7 +84,7 @@ def registered_key_derivation_tvs():
             'context_string': context_string,
             'seed':       seed,
             'zip_number': 1,
-            'subpath':    k.path[1:],
+            'subpath':    k.subpath,
             'sk':         k.sk,
             'c':          k.chaincode,
             'full_width': k.full_width,