c# Base64解密加密

  private static string base64encodechars = "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789+/";

        private static int[] base64decodechars ={
            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,
            52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1,
            -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
            15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,
            -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
            41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1 };

    /// <summary>
        /// base64加密
        /// </summary>
        /// <param name="str"></param>
        /// <returns></returns>
        public static string base64encode(string str)
        {
            string res = "";
            int i = 0, len;
            int c1, c2, c3;
            len = str.length;
            i = 0;
            res = "";
            while (i < len)
            {
                c1 = convert.tobyte(str[i++]) & 0xff;
                if (i == len)
                {
                    res += base64encodechars.charat(c1 >> 2);
                    res += base64encodechars.charat((c1 & 0x3) << 4);
                    res += "==";
                    break;
                }
                c2 = convert.tobyte(str[i++]);
                if (i == len)
                {
                    res += base64encodechars.charat(c1 >> 2);
                    res += base64encodechars.charat(((c1 & 0x3) << 4) | ((c2 & 0xf0) >> 4));
                    res += base64encodechars.charat((c2 & 0xf) << 2);
                    res += "=";
                    break;
                }
                c3 = convert.tobyte(str[i++]);
                res += base64encodechars.charat(c1 >> 2);
                res += base64encodechars.charat(((c1 & 0x3) << 4) | ((c2 & 0xf0) >> 4));
                res += base64encodechars.charat(((c2 & 0xf) << 2) | ((c3 & 0xc0) >> 6));
                res += base64encodechars.charat(c3 & 0x3f);
            }
            return res;
        }

    /// <summary>
        /// base64加解密
        /// </summary>
        /// <param name="str"></param>
        /// <returns></returns>
        public static string base64decode(string str)
        {
            string res = "";
            int i = 0, len;
            int c1, c2, c3, c4;
            len = str.length;
            len = str.length;
            i = 0;
            res = "";
            while (i < len)
            {
                /* c1 */
                do
                {
                    c1 = base64decodechars[convert.tobyte(str[i++]) & 0xff];
                } while (i < len && c1 == -1);
                if (c1 == -1)
                    break;
                /* c2 */
                do
                {
                    c2 = base64decodechars[convert.tobyte(str[i++]) & 0xff];
                } while (i < len && c2 == -1);
                if (c2 == -1)
                    break;
                res += convert.tochar((c1 << 2) | ((c2 & 0x30) >> 4));
                /* c3 */
                do
                {
                    c3 = convert.tobyte(str[i++]) & 0xff;
                    if (c3 == 61)
                        return res;
                    c3 = base64decodechars[c3];
                } while (i < len && c3 == -1);
                if (c3 == -1)
                    break;
                res += convert.tochar(((c2 & 0xf) << 4) | ((c3 & 0x3c) >> 2));
                /* c4 */
                do
                {
                    c4 = convert.tobyte(str[i++]) & 0xff;
                    if (c4 == 61)
                        return res;
                    c4 = base64decodechars[c4];
                } while (i < len && c4 == -1);
                if (c4 == -1)
                    break;
                res += convert.tochar(((c3 & 0x03) << 6) | c4);
            }
            return res;
        }