diff --git a/monero/ed25519.py b/monero/ed25519.py
index 6c9bbe8..4513426 100644
--- a/monero/ed25519.py
+++ b/monero/ed25519.py
@@ -1,7 +1,6 @@
 import binascii
-import six
-
 import nacl.bindings
+import six
 
 
 def bit(h, i):
@@ -21,12 +20,13 @@ def decodeint(s):
 
 edwards_add = nacl.bindings.crypto_core_ed25519_add
 inv = nacl.bindings.crypto_core_ed25519_scalar_invert
+scalar_add = nacl.bindings.crypto_core_ed25519_scalar_add
 scalarmult_B = nacl.bindings.crypto_scalarmult_ed25519_base_noclamp
 scalarmult = nacl.bindings.crypto_scalarmult_ed25519_noclamp
 
 
 def scalar_reduce(v):
-    return nacl.bindings.crypto_core_ed25519_scalar_reduce(v + 32 * b"\0")
+    return nacl.bindings.crypto_core_ed25519_scalar_reduce(v + (64 - len(v)) * b"\0")
 
 
 def public_from_secret_hex(hk):
diff --git a/monero/transaction/__init__.py b/monero/transaction/__init__.py
index 3671b2c..fe5f780 100644
--- a/monero/transaction/__init__.py
+++ b/monero/transaction/__init__.py
@@ -142,11 +142,14 @@ class Transaction(object):
 
         def _scan_pubkeys(svk, psk, stealth_address, amount, encamount):
             for keyidx, tx_key in enumerate(self.pubkeys):
+                # precompute
+                svk_2 = ed25519.scalar_add(svk, svk)
+                svk_4 = ed25519.scalar_add(svk_2, svk_2)
+                svk_8 = ed25519.scalar_add(svk_4, svk_4)
+                #
                 hsdata = b"".join(
                     [
-                        ed25519.scalarmult(
-                            ed25519.encodeint(ed25519.decodeint(svk) * 8), tx_key
-                        ),
+                        ed25519.scalarmult(svk_8, tx_key),
                         varint.encode(idx),
                     ]
                 )