diff --git a/shadowsocks/encrypt_salsa20.py b/shadowsocks/encrypt_salsa20.py
index e061c7d..85b30ff 100644
--- a/shadowsocks/encrypt_salsa20.py
+++ b/shadowsocks/encrypt_salsa20.py
@@ -4,6 +4,7 @@ import time
 import struct
 import logging
 import sys
+import encrypt
 
 slow_xor = False
 imported = False
@@ -72,14 +73,17 @@ class Salsa20Cipher(object):
             cur_data = data[:remain]
             cur_data_len = len(cur_data)
             cur_stream = self._stream[self._pos:self._pos + cur_data_len]
-            self._pos = (self._pos + cur_data_len) % BLOCK_SIZE
+            self._pos = self._pos + cur_data_len
             data = data[remain:]
 
             results.append(numpy_xor(cur_data, cur_stream))
 
+            if self._pos >= BLOCK_SIZE:
+                self._next_stream()
+                self._pos -= BLOCK_SIZE
+                assert self._pos == 0
             if not data:
                 break
-            self._next_stream()
         return ''.join(results)
 
 
@@ -87,8 +91,16 @@ def test():
     from os import urandom
     import random
 
-    rounds = 1 * 10
+    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 = Salsa20Cipher('salsa20-ctr', 'k' * 32, 'i' * 8, 1)
     decipher = Salsa20Cipher('salsa20-ctr', 'k' * 32, 'i' * 8, 1)
     results = []
@@ -96,13 +108,20 @@ def test():
     print 'start'
     start = time.time()
     while pos < len(plain):
-        l = random.randint(10000, 32768)
+        l = random.randint(100, 16384)
         c = cipher.update(plain[pos:pos + l])
-        results.append(decipher.update(c))
+        results.append(c)
+        pos += l
+    pos = 0
+    c = ''.join(results)
+    results = []
+    while pos < len(plain):
+        l = random.randint(100, 16384)
+        results.append(decipher.update(c[pos:pos + l]))
         pos += l
-    assert ''.join(results) == plain
     end = time.time()
     print BLOCK_SIZE * rounds / (end - start)
+    assert ''.join(results) == plain
 
 
 if __name__ == '__main__':