3DES 加解密，对长度不限制

#region 3DES
        /// <summary>
        /// 3DES加密
        /// </summary>
        /// <param name="strString">需加密的字符串</param>
        /// <param name="strKey">密匙</param>
        /// <returns></returns>
        public static string DES3Encrypt(string strString, string strKey)
        {
            strString = strString + "".PadLeft(8);
            byte[] bMsg = Encoding.GetEncoding("GBK").GetBytes(strString);

            int l = (bMsg.Length / 16 + 1) * 16;
            byte[] btMsg = new byte[l];
            Array.Copy(bMsg, btMsg, bMsg.Length);

            byte[] digestOfPassword = Encoding.Default.GetBytes(strKey);
            byte[] keyBytes = new byte[24];
            Array.Copy(digestOfPassword, keyBytes, digestOfPassword.Length);
            for (int j = 0, k = 16; j < 8; )
            {
                keyBytes[k++] = keyBytes[j++];
            }
            TripleDESCryptoServiceProvider DES = new TripleDESCryptoServiceProvider();
            DES.Mode = CipherMode.ECB;
            ICryptoTransform DESEncrypt = DES.CreateEncryptor(keyBytes, keyBytes);
            var dd = DESEncrypt.TransformFinalBlock(btMsg, 0, btMsg.Length);
            return byte2hex(dd).ToString().Substring(0, (bMsg.Length / 8 + 1) * 16);
        }

        public static String getAdd(int length, String strKey)
        {
            byte[] btMsg = new byte[length / 2];

            byte[] digestOfPassword = Encoding.UTF8.GetBytes(strKey);
            //byte[] digestOfPassword = Encoding.Default.GetBytes(strKey);
            byte[] keyBytes = new byte[24];
            Array.Copy(digestOfPassword, keyBytes, digestOfPassword.Length);
            for (int j = 0, k = 16; j < 8; )
            {
                keyBytes[k++] = keyBytes[j++];
            }

            TripleDESCryptoServiceProvider DES = new TripleDESCryptoServiceProvider();
            DES.Mode = CipherMode.ECB;
            ICryptoTransform DESEncrypt = DES.CreateEncryptor(keyBytes, keyBytes);
            var dd = DESEncrypt.TransformFinalBlock(btMsg, 0, btMsg.Length);
            String rtn = byte2hex(dd);
            return rtn.Substring(length);
        }

        /// <summary> 
        /// 字节数组转16进制字符串 
        /// </summary> 
        /// <param name="bytes"></param> 
        /// <returns></returns> 
        private static string byte2hex(byte[] dd)
        {
            StringBuilder ret = new StringBuilder();
            foreach (var item in dd)
            {
                ret.AppendFormat("{0:X2}", item);
            }
            return ret.ToString();
        }
        /// <summary> 
        /// 字符串转16进制字节数组 
        /// </summary> 
        /// <param name="hexString"></param> 
        /// <returns></returns> 
        private static byte[] strToToHexByte(string hexString)
        {
            hexString = hexString.Replace(" ", "");
            if ((hexString.Length % 2) != 0)
                hexString += " ";
            byte[] returnBytes = new byte[hexString.Length / 2];
            for (int i = 0; i < returnBytes.Length; i++)
                returnBytes[i] = Convert.ToByte(hexString.Substring(i * 2, 2), 16);
            return returnBytes;
        }
        /// <summary>
        /// 3DES解密
        /// </summary>
        /// <param name="strString">需解密的字符串</param>
        /// <param name="strKey">密匙</param>
        /// <returns></returns>
        public static string DES3Decrypt(string strString, string strKey)
        {
            strString += getAdd(strString.Length, strKey);

            byte[] digestOfPassword = Encoding.Default.GetBytes(strKey);
            byte[] keyBytes = new byte[24];
            Array.Copy(digestOfPassword, keyBytes, digestOfPassword.Length);
            for (int j = 0, k = 16; j < 8; )
            {
                keyBytes[k++] = keyBytes[j++];
            }
            TripleDESCryptoServiceProvider DES = new TripleDESCryptoServiceProvider();
            DES.Mode = CipherMode.ECB;
            ICryptoTransform DESDecrypt = DES.CreateDecryptor(keyBytes, keyBytes);
            string result = "";
            try
            {
                byte[] Buffer = strToToHexByte(strString);
                byte[] bb = DESDecrypt.TransformFinalBlock(Buffer, 0, Buffer.Length);
                result = Encoding.GetEncoding("GBK").GetString(bb);
            }
            catch (Exception e)
            {
                throw e;
            }
            return result;
        }
        #endregion

 2015-1-13 添加

在我传递key时，出现

查资料，原来是key的长度大于24了，（IV则是8位，但我没有传递）

修改后的3DES算法

#region 3DES
        /// <summary>
        /// 3DES加密
        /// </summary>
        /// <param name="strString">需加密的字符串</param>
        /// <param name="strKey">密匙</param>
        /// <returns></returns>
        public static string DES3Encrypt(string strString, string strKey)
        {
            byte[] bMsg = Encoding.UTF8.GetBytes(strString);
            TripleDESCryptoServiceProvider DES = new TripleDESCryptoServiceProvider();
            DES.Mode = CipherMode.ECB;

            string key = strKey;
            while (key.Length < 24)
            {
                key += strKey;
            }
            byte[] digestOfPassword = Encoding.UTF8.GetBytes(key);
            byte[] keyBytes = new byte[24];
            Array.Copy(digestOfPassword.Take(24).ToArray(), keyBytes, digestOfPassword.Take(24).ToArray().Length);

            DES.Key = keyBytes;
            ICryptoTransform DESEncrypt = DES.CreateEncryptor();
            var dd = DESEncrypt.TransformFinalBlock(bMsg, 0, bMsg.Length);
            return Convert.ToBase64String(dd);
        }

        /// <summary>
        /// 3DES解密
        /// </summary>
        /// <param name="strString">需解密的字符串</param>
        /// <param name="strKey">密匙</param>
        /// <returns></returns>
        public static string DES3Decrypt(string strString, string strKey)
        {
            TripleDESCryptoServiceProvider DES = new TripleDESCryptoServiceProvider();
            DES.Mode = CipherMode.ECB;
            string key = strKey;
            while (key.Length < 24)
            {
                key += strKey;
            }
            byte[] digestOfPassword = Encoding.UTF8.GetBytes(key);
            byte[] keyBytes = new byte[24];
            Array.Copy(digestOfPassword.Take(24).ToArray(), keyBytes, digestOfPassword.Take(24).ToArray().Length);
            DES.Key = keyBytes;
            ICryptoTransform DESDecrypt = DES.CreateDecryptor();
            string result = "";
            try
            {
                byte[] Buffer = Convert.FromBase64String(strString);
                byte[] bb = DESDecrypt.TransformFinalBlock(Buffer, 0, Buffer.Length);
                result = Encoding.UTF8.GetString(bb);
            }
            catch (Exception e)
            {
                throw e;
            }
            return result;
        }
        #endregion