diff --git a/shadowsocks/encrypt_salsa20.py b/shadowsocks/encrypt_salsa20.py
index 85b30ff..d69010f 100644
--- a/shadowsocks/encrypt_salsa20.py
+++ b/shadowsocks/encrypt_salsa20.py
@@ -4,7 +4,6 @@ import time
 import struct
 import logging
 import sys
-import encrypt
 
 slow_xor = False
 imported = False
@@ -32,8 +31,14 @@ def run_imports():
 def numpy_xor(a, b):
     if slow_xor:
         return py_xor_str(a, b)
-    ab = numpy.frombuffer(a, dtype=numpy.byte)
-    bb = numpy.frombuffer(b, dtype=numpy.byte)
+    dtype = numpy.byte
+    if len(a) % 4 == 0:
+        dtype = numpy.uint32
+    elif len(a) % 2 == 0:
+        dtype = numpy.uint16
+
+    ab = numpy.frombuffer(a, dtype=dtype)
+    bb = numpy.frombuffer(b, dtype=dtype)
     c = numpy.bitwise_xor(ab, bb)
     r = c.tostring()
     return r
@@ -80,8 +85,7 @@ class Salsa20Cipher(object):
 
             if self._pos >= BLOCK_SIZE:
                 self._next_stream()
-                self._pos -= BLOCK_SIZE
-                assert self._pos == 0
+                self._pos = 0
             if not data:
                 break
         return ''.join(results)
@@ -94,12 +98,12 @@ def test():
     rounds = 1 * 1024
     plain = urandom(BLOCK_SIZE * rounds)
     import M2Crypto.EVP
-    cipher = M2Crypto.EVP.Cipher('aes_128_cfb', 'k' * 32, 'i' * 16, 1,
-                   key_as_bytes=0, d='md5', salt=None, i=1,
-                   padding=1)
-    decipher = M2Crypto.EVP.Cipher('aes_128_cfb', 'k' * 32, 'i' * 16, 0,
-                   key_as_bytes=0, d='md5', salt=None, i=1,
-                   padding=1)
+    # cipher = M2Crypto.EVP.Cipher('aes_128_cfb', 'k' * 32, 'i' * 16, 1,
+    #                key_as_bytes=0, d='md5', salt=None, i=1,
+    #                padding=1)
+    # decipher = M2Crypto.EVP.Cipher('aes_128_cfb', 'k' * 32, 'i' * 16, 0,
+    #                key_as_bytes=0, d='md5', salt=None, i=1,
+    #                padding=1)
 
     cipher = Salsa20Cipher('salsa20-ctr', 'k' * 32, 'i' * 8, 1)
     decipher = Salsa20Cipher('salsa20-ctr', 'k' * 32, 'i' * 8, 1)
@@ -108,7 +112,7 @@ def test():
     print 'start'
     start = time.time()
     while pos < len(plain):
-        l = random.randint(100, 16384)
+        l = random.randint(100, 32768)
         c = cipher.update(plain[pos:pos + l])
         results.append(c)
         pos += l
@@ -116,7 +120,7 @@ def test():
     c = ''.join(results)
     results = []
     while pos < len(plain):
-        l = random.randint(100, 16384)
+        l = random.randint(100, 32768)
         results.append(decipher.update(c[pos:pos + l]))
         pos += l
     end = time.time()