/// <summary>
        /// Aes解密
        /// </summary>
        /// <param name="str">需要解密的字符串</param>
        /// <param name="key">密钥,长度不够时空格补齐,超过时从左截取</param>
        /// <param name="iv">偏移量,长度不够时空格补齐,超过时从左截取</param>
        /// <param name="keyLenth">秘钥长度,16 24 32</param>
        /// <param name="aesMode">解密模式</param>
        /// <param name="aesPadding">填充方式</param>
        /// <returns></returns>
        public static string AesDecode(string str, string key, string iv, int keyLenth = 16, CipherMode aesMode = CipherMode.CBC, PaddingMode aesPadding = PaddingMode.PKCS7)
        {
　　　　　　　if (!new List<int> { 16, 24, 32 }.Contains(keyLenth))
            {
                return null;//密钥的长度，16位密钥 = 128位，24位密钥 = 192位，32位密钥 = 256位。
            }
            var oldBytes = Convert.FromBase64String(str);
            var bKey = new Byte[keyLenth];
            Array.Copy(Convert.FromBase64String(key.PadRight(keyLenth)), bKey, keyLenth);
            var bIv = new Byte[16];
            Array.Copy(Convert.FromBase64String(iv.PadRight(16)), bIv, 16);

            var rijalg = new RijndaelManaged
            {
                Mode = aesMode,
                Padding = aesPadding,
                Key = bKey,
                IV = bIv,
            };
            var decryptor = rijalg.CreateDecryptor(rijalg.Key, rijalg.IV);
            var rtByte = decryptor.TransformFinalBlock(oldBytes, 0, oldBytes.Length);
            return Encoding.UTF8.GetString(rtByte);
        }