diff --git a/shadowsocks/crypto/ctypes_openssl.py b/shadowsocks/crypto/ctypes_openssl.py
index 807c54f..7707639 100644
--- a/shadowsocks/crypto/ctypes_openssl.py
+++ b/shadowsocks/crypto/ctypes_openssl.py
@@ -163,8 +163,8 @@ def test():
     # 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 = CtypesCrypto('aes-128-cfb', 'k' * 32, 'i' * 16, 1)
-    decipher = CtypesCrypto('aes-128-cfb', 'k' * 32, 'i' * 16, 0)
+    cipher = CtypesCrypto('aes-128-cfb', b'k' * 32, b'i' * 16, 1)
+    decipher = CtypesCrypto('aes-128-cfb', b'k' * 32, b'i' * 16, 0)
 
     # cipher = Salsa20Cipher('salsa20-ctr', 'k' * 32, 'i' * 8, 1)
     # decipher = Salsa20Cipher('salsa20-ctr', 'k' * 32, 'i' * 8, 1)
diff --git a/shadowsocks/crypto/salsa20_ctr.py b/shadowsocks/crypto/salsa20_ctr.py
index 0f5bfb8..e6c561c 100644
--- a/shadowsocks/crypto/salsa20_ctr.py
+++ b/shadowsocks/crypto/salsa20_ctr.py
@@ -72,9 +72,16 @@ def numpy_xor(a, b):
 
 def py_xor_str(a, b):
     c = []
-    for i in range(0, len(a)):
-        c.append(chr(ord(a[i]) ^ ord(b[i])))
-    return ''.join(c)
+    if bytes == str:
+        for i in range(0, len(a)):
+            c.append(chr(ord(a[i]) ^ ord(b[i])))
+    else:
+        for i in range(0, len(a)):
+            c.append(a[i] ^ b[i])
+    if bytes == str:
+        return ''.join(c)
+    else:
+        return bytes(c)
 
 
 class Salsa20Cipher(object):
@@ -83,7 +90,7 @@ class Salsa20Cipher(object):
     def __init__(self, alg, key, iv, op, key_as_bytes=0, d=None, salt=None,
                  i=1, padding=1):
         run_imports()
-        if alg != 'salsa20-ctr':
+        if alg != b'salsa20-ctr':
             raise Exception('unknown algorithm')
         self._key = key
         self._nonce = struct.unpack('<Q', iv)[0]
@@ -115,7 +122,7 @@ class Salsa20Cipher(object):
                 self._pos = 0
             if not data:
                 break
-        return ''.join(results)
+        return b''.join(results)
 
 
 ciphers = {
@@ -137,8 +144,8 @@ def test():
     #                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)
+    cipher = Salsa20Cipher(b'salsa20-ctr', b'k' * 32, b'i' * 8, 1)
+    decipher = Salsa20Cipher(b'salsa20-ctr', b'k' * 32, b'i' * 8, 1)
     results = []
     pos = 0
     print('salsa20 test start')
@@ -149,7 +156,7 @@ def test():
         results.append(c)
         pos += l
     pos = 0
-    c = ''.join(results)
+    c = b''.join(results)
     results = []
     while pos < len(plain):
         l = random.randint(100, 32768)
@@ -157,7 +164,7 @@ def test():
         pos += l
     end = time.time()
     print('speed: %d bytes/s' % (BLOCK_SIZE * rounds / (end - start)))
-    assert ''.join(results) == plain
+    assert b''.join(results) == plain
 
 
 if __name__ == '__main__':