diff --git a/shadowsocks/crypto/aead.py b/shadowsocks/crypto/aead.py
index 7c52843..c7240b3 100644
--- a/shadowsocks/crypto/aead.py
+++ b/shadowsocks/crypto/aead.py
@@ -155,9 +155,6 @@ class AeadCryptoBase(object):
         # n, n > 0, waiting data
         self._chunk = {'mlen': AEAD_MSG_LEN_UNKNOWN, 'data': b''}
 
-        self.encrypt_once = self.aead_encrypt
-        self.decrypt_once = self.aead_decrypt
-
         # load libsodium for nonce increment
         if not sodium_loaded:
             crypto_path = dict(crypto_path) if crypto_path else dict()
diff --git a/shadowsocks/crypto/openssl.py b/shadowsocks/crypto/openssl.py
index f2948ae..f034bd1 100644
--- a/shadowsocks/crypto/openssl.py
+++ b/shadowsocks/crypto/openssl.py
@@ -107,8 +107,11 @@ class OpenSSLCryptoBase(object):
         if not self._ctx:
             raise Exception('can not create cipher context')
 
-        self.encrypt_once = self.update
-        self.decrypt_once = self.update
+    def encrypt_once(self, data):
+        return self.update(data)
+
+    def decrypt_once(self, data):
+        return self.update(data)
 
     def update(self, data):
         """
@@ -136,6 +139,7 @@ class OpenSSLCryptoBase(object):
         if self._ctx:
             ctx_cleanup(self._ctx)
             libcrypto.EVP_CIPHER_CTX_free(self._ctx)
+            self._ctx = None
 
 
 class OpenSSLAeadCrypto(OpenSSLCryptoBase, AeadCryptoBase):
@@ -267,6 +271,12 @@ class OpenSSLAeadCrypto(OpenSSLCryptoBase, AeadCryptoBase):
         self.cipher_ctx_init()
         return plaintext
 
+    def encrypt_once(self, data):
+        return self.aead_encrypt(data)
+
+    def decrypt_once(self, data):
+        return self.aead_decrypt(data)
+
 
 class OpenSSLStreamCrypto(OpenSSLCryptoBase):
     """
@@ -281,8 +291,12 @@ class OpenSSLStreamCrypto(OpenSSLCryptoBase):
         if not r:
             self.clean()
             raise Exception('can not initialize cipher context')
-        self.encrypt = self.update
-        self.decrypt = self.update
+
+    def encrypt(self, data):
+        return self.update(data)
+
+    def decrypt(self, data):
+        return self.update(data)
 
 
 ciphers = {
diff --git a/shadowsocks/crypto/sodium.py b/shadowsocks/crypto/sodium.py
index fce65c0..981321e 100644
--- a/shadowsocks/crypto/sodium.py
+++ b/shadowsocks/crypto/sodium.py
@@ -192,10 +192,18 @@ class SodiumCrypto(object):
             raise Exception('Unknown cipher')
         # byte counter, not block counter
         self.counter = 0
-        self.encrypt = self.update
-        self.decrypt = self.update
-        self.encrypt_once = self.update
-        self.decrypt_once = self.update
+
+    def encrypt(self, data):
+        return self.update(data)
+
+    def decrypt(self, data):
+        return self.update(data)
+
+    def encrypt_once(self, data):
+        return self.update(data)
+
+    def decrypt_once(self, data):
+        return self.update(data)
 
     def update(self, data):
         global buf_size, buf
@@ -300,6 +308,12 @@ class SodiumAeadCrypto(AeadCryptoBase):
         self.cipher_ctx_init()
         return buf.raw[:cipher_out_len.value]
 
+    def encrypt_once(self, data):
+        return self.aead_encrypt(data)
+
+    def decrypt_once(self, data):
+        return self.aead_decrypt(data)
+
 
 ciphers = {
     'salsa20': (32, 8, SodiumCrypto),