diff --git a/lib/resty/aes.lua b/lib/resty/aes.lua
index 377476f..4a61aeb 100644
--- a/lib/resty/aes.lua
+++ b/lib/resty/aes.lua
@@ -58,6 +58,7 @@ const EVP_CIPHER *EVP_aes_256_cfb1(void);
 const EVP_CIPHER *EVP_aes_256_cfb8(void);
 const EVP_CIPHER *EVP_aes_256_cfb128(void);
 const EVP_CIPHER *EVP_aes_256_ofb(void);
+const EVP_CIPHER *EVP_aes_256_ctr(void);
 const EVP_CIPHER *EVP_aes_128_gcm(void);
 const EVP_CIPHER *EVP_aes_192_gcm(void);
 const EVP_CIPHER *EVP_aes_256_gcm(void);
@@ -103,11 +104,14 @@ hash = {
 _M.hash = hash
 
 local EVP_MAX_BLOCK_LENGTH = 32
+local shared_out_len_ptr = ffi_new("int[1]")
+local shared_tmp_len_ptr = ffi_new("int[1]")
+local shared_tag_buf_ptr = ffi_new("unsigned char[?]", 16)
 
 local cipher
 cipher = function (size, _cipher)
     local _size = size or 128
-    local _cipher = _cipher or "cbc"
+    _cipher = _cipher or "cbc"
     local func = "EVP_aes_" .. _size .. "_" .. _cipher
     if C[func] then
         return { size=_size, cipher=_cipher, method=C[func]()}
@@ -132,12 +136,12 @@ function _M.new(self, key, salt, _cipher, _hash, hash_rounds, iv_len, enable_pad
 
     ffi_gc(decrypt_ctx, C.EVP_CIPHER_CTX_free)
 
-    local _cipher = _cipher or cipher()
-    local _hash = _hash or hash.md5
-    local hash_rounds = hash_rounds or 1
+    _cipher = _cipher or cipher()
+    _hash = _hash or hash.md5
+    hash_rounds = hash_rounds or 1
     local _cipherLength = _cipher.size/8
-    local gen_key = ffi_new("unsigned char[?]",_cipherLength)
-    local gen_iv = ffi_new("unsigned char[?]",_cipherLength)
+    local gen_key = ffi_new("unsigned char[?]", _cipherLength)
+    local gen_iv = ffi_new("unsigned char[?]", _cipherLength)
     iv_len = iv_len or _cipherLength
     -- enable padding by default
     local padding = (enable_padding == nil or enable_padding) and 1 or 0
@@ -223,6 +227,21 @@ function _M.new(self, key, salt, _cipher, _hash, hash_rounds, iv_len, enable_pad
       }, mt)
 end
 
+local function alloc_buf(max_len)
+    local buf = ffi_new("unsigned char[?]", max_len)
+    return buf, max_len
+end
+
+do
+    local ok, str_buf_mod = pcall(require, "string.buffer")
+    if ok then
+        local str_buf = str_buf_mod.new(4096)
+        function alloc_buf(max_len)
+            local buf, sz = str_buf:reset():reserve(max_len)
+            return buf, sz
+        end
+    end
+end
 
 function _M.encrypt(self, s, aad)
     local typ = type(self)
@@ -231,10 +250,9 @@ function _M.encrypt(self, s, aad)
     end
 
     local s_len = #s
-    local max_len = s_len + 2 * EVP_MAX_BLOCK_LENGTH
-    local buf = ffi_new("unsigned char[?]", max_len)
-    local out_len = ffi_new("int[1]")
-    local tmp_len = ffi_new("int[1]")
+    local buf = alloc_buf(s_len + 2 * EVP_MAX_BLOCK_LENGTH)
+    local out_len = shared_out_len_ptr
+    local tmp_len = shared_tmp_len_ptr
     local ctx = self._encrypt_ctx
 
     if C.EVP_EncryptInit_ex(ctx, nil, nil, self._key, self._iv) == 0 then
@@ -254,7 +272,7 @@ function _M.encrypt(self, s, aad)
     if self._cipher == "gcm" then
         local encrypt_data = ffi_str(buf, out_len[0])
         if C.EVP_EncryptFinal_ex(ctx, buf, out_len) == 0 then
-            return nil, "EVP_DecryptFinal_ex failed"
+            return nil, "EVP_EncryptFinal_ex failed"
         end
 
         -- FIXME: For OCB mode the taglen must either be 16
@@ -280,14 +298,13 @@ function _M.decrypt(self, s, tag, aad)
     end
 
     local s_len = #s
-    local max_len = s_len + 2 * EVP_MAX_BLOCK_LENGTH
-    local buf = ffi_new("unsigned char[?]", max_len)
-    local out_len = ffi_new("int[1]")
-    local tmp_len = ffi_new("int[1]")
+    local buf = alloc_buf(s_len + 2 * EVP_MAX_BLOCK_LENGTH)
+    local out_len = shared_out_len_ptr
+    local tmp_len = shared_tmp_len_ptr
     local ctx = self._decrypt_ctx
 
     if C.EVP_DecryptInit_ex(ctx, nil, nil, self._key, self._iv) == 0 then
-      return nil, "EVP_DecryptInit_ex failed"
+        return nil, "EVP_DecryptInit_ex failed"
     end
 
     if self._cipher == "gcm" and aad ~= nil then
@@ -297,14 +314,14 @@ function _M.decrypt(self, s, tag, aad)
     end
 
     if C.EVP_DecryptUpdate(ctx, buf, out_len, s, s_len) == 0 then
-      return nil, "EVP_DecryptUpdate failed"
+        return nil, "EVP_DecryptUpdate failed"
     end
 
     if self._cipher == "gcm" then
         local plain_txt = ffi_str(buf, out_len[0])
         if tag ~= nil then
-            local tag_buf = ffi_new("unsigned char[?]", 16)
-            ffi.copy(tag_buf, tag, 16)
+            local tag_buf = shared_tag_buf_ptr
+            ffi_copy(tag_buf, tag, 16)
             C.EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_TAG, 16, tag_buf);
         end