public class base64
    {
        char[] source;
        int length, length2, length3;
        int blockCount;
        int paddingCount;
        public static base64 Decoder = new base64();
        public base64()
        { }
        private void init(char[] input)
        {
            int temp = 0;
            source = input;
            length = input.Length;

            for (int x = 0; x < 2; x++)
            {
                if (input[length - x - 1] == '=')
                    temp++;
            }
            paddingCount = temp;

            blockCount = length / 4;
            length2 = blockCount * 3;
        }
        /// <summary>
        /// 加密后的字符串解密
        /// </summary>
        /// <param name="strInput"></param>
        /// <returns></returns>
        public byte[] GetDecoded(string strInput)
        {
            //初始化
            init(strInput.ToCharArray());

            byte[] buffer = new byte[length];
            byte[] buffer2 = new byte[length2];

            for (int x = 0; x < length; x++)
            {
                buffer[x] = char2sixbit(source[x]);
            }

            byte b, b1, b2, b3;
            byte temp1, temp2, temp3, temp4;

            for (int x = 0; x < blockCount; x++)
            {
                temp1 = buffer[x * 4];
                temp2 = buffer[x * 4 + 1];
                temp3 = buffer[x * 4 + 2];
                temp4 = buffer[x * 4 + 3];

                b = (byte)(temp1 << 2);
                b1 = (byte)((temp2 & 48) >> 4);
                b1 += b;

                b = (byte)((temp2 & 15) << 4);
                b2 = (byte)((temp3 & 60) >> 2);
                b2 += b;

                b = (byte)((temp3 & 3) << 6);
                b3 = temp4;
                b3 += b;

                buffer2[x * 3] = b1;
                buffer2[x * 3 + 1] = b2;
                buffer2[x * 3 + 2] = b3;
            }

            length3 = length2 - paddingCount;
            byte[] result = new byte[length3];

            for (int x = 0; x < length3; x++)
            {
                result[x] = buffer2[x];
            }

            return result;
        }

        private byte char2sixbit(char c)
        {
            char[] lookupTable = new char[64]{  
                 'A','B','C','D','E','F','G','H','I','J','K','L','M','N',
                 'O','P','Q','R','S','T','U','V','W','X','Y', 'Z',
                 'a','b','c','d','e','f','g','h','i','j','k','l','m','n',
                 'o','p','q','r','s','t','u','v','w','x','y','z',
                 '0','1','2','3','4','5','6','7','8','9','+','/'};
            if (c == '=')
                return 0;
            else
            {
                for (int x = 0; x < 64; x++)
                {
                    if (lookupTable[x] == c)
                        return (byte)x;
                }

                return 0;
            }

        }

        /// <summary>
        /// Base64解密，采用utf8编码方式解密
        /// </summary>
        /// <param name="result">待解密的密文</param>
        /// <returns>解密后的字符串</returns>
        public static string DecodeBase64(string result)
        {
            return DecodeBase64(Encoding.UTF8, result);
        }
        /// <summary>
        /// Base64解密
        /// </summary>
        /// <param name="codeName">解密采用的编码方式，注意和加密时采用的方式一致</param>
        /// <param name="result">待解密的密文</param>
        /// <returns>解密后的字符串</returns>
        public static string DecodeBase64(Encoding encode, string result)
        {
            string decode = "";
            byte[] bytes = Convert.FromBase64String(result);
            try
            {
                decode = encode.GetString(bytes);
            }
            catch
            {
                decode = result;
            }
            return decode;
        }
        /// <summary>
        /// Base64加密
        /// </summary>
        /// <param name="codeName">加密采用的编码方式</param>
        /// <param name="source">待加密的明文</param>
        /// <returns></returns>
        public static string EncodeBase64(Encoding encode, string source)
        {
            string decode = "";
            byte[] bytes = encode.GetBytes(source);
            try
            {
                decode = Convert.ToBase64String(bytes);
            }
            catch
            {
                decode = source;
            }
            return decode;
        }

        /// <summary>
        /// Base64加密，采用utf8编码方式加密
        /// </summary>
        /// <param name="source">待加密的明文</param>
        /// <returns>加密后的字符串</returns>
        public static string EncodeBase64(string source)
        {
            return EncodeBase64(Encoding.UTF8, source);
        }
    }