helper functions for bip32 derivations

lib/bitcoin.py

@@ -540,18 +540,17 @@ def get_pubkeys_from_secret(secret):
 # However, if n is positive, the resulting private key's corresponding
 #  public key can be determined without the master private key.
 def CKD_priv(k, c, n):
+    is_prime = n & BIP32_PRIME
+    return _CKD_priv(k, c, rev_hex(int_to_hex(n,4)).decode('hex'), is_prime)
+
+def _CKD_priv(k, c, s, is_prime):
     import hmac
     from ecdsa.util import string_to_number, number_to_string
     order = generator_secp256k1.order()
     keypair = EC_KEY(k)
-    K = GetPubKey(keypair.pubkey,True)
-
-    if n & BIP32_PRIME: # We want to make a "secret" address that can't be determined from K
-        data = chr(0) + k + rev_hex(int_to_hex(n,4)).decode('hex')
-        I = hmac.new(c, data, hashlib.sha512).digest()
-    else: # We want a "non-secret" address that can be determined from K
-        I = hmac.new(c, K + rev_hex(int_to_hex(n,4)).decode('hex'), hashlib.sha512).digest()
-        
+    cK = GetPubKey(keypair.pubkey,True)
+    data = chr(0) + k + s if is_prime else cK + s
+    I = hmac.new(c, data, hashlib.sha512).digest()
     k_n = number_to_string( (string_to_number(I[0:32]) + string_to_number(k)) % order , order )
     c_n = I[32:]
     return k_n, c_n
@@ -563,17 +562,20 @@ def CKD_priv(k, c, n):
 # This function allows us to find the nth public key, as long as n is 
 #  non-negative. If n is negative, we need the master private key to find it.
 def CKD_pub(cK, c, n):
+    if n & BIP32_PRIME: raise
+    return _CKD_pub(cK, c, rev_hex(int_to_hex(n,4)).decode('hex'))
+
+# helper function, callable with arbitrary string
+def _CKD_pub(cK, c, s):
     import hmac
     from ecdsa.util import string_to_number, number_to_string
     order = generator_secp256k1.order()
-    if n & BIP32_PRIME: raise
-    I = hmac.new(c, cK + rev_hex(int_to_hex(n,4)).decode('hex'), hashlib.sha512).digest()
+    I = hmac.new(c, cK + s, hashlib.sha512).digest()
     curve = SECP256k1
     pubkey_point = string_to_number(I[0:32])*curve.generator + ser_to_point(cK)
     public_key = ecdsa.VerifyingKey.from_public_point( pubkey_point, curve = SECP256k1 )
     c_n = I[32:]
     cK_n = GetPubKey(public_key.pubkey,True)
-
     return cK_n, c_n