function base64_decode(str){
        var c1, c2, c3, c4;
        var base64DecodeChars = new Array(
            -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
        );
        var i=0, len = str.length, string = '';

        while (i < len){
            do{
                c1 = base64DecodeChars[str.charCodeAt(i++) & 0xff]
            } while (
                i < len && c1 == -1
            );

            if (c1 == -1) break;

            do{
                c2 = base64DecodeChars[str.charCodeAt(i++) & 0xff]
            } while (
                i < len && c2 == -1
            );

            if (c2 == -1) break;

            string += String.fromCharCode((c1 << 2) | ((c2 & 0x30) >> 4));

            do{
                c3 = str.charCodeAt(i++) & 0xff;
                if (c3 == 61)
                    return string;

                c3 = base64DecodeChars[c3]
            } while (
                i < len && c3 == -1
            );

            if (c3 == -1) break;

            string += String.fromCharCode(((c2 & 0XF) << 4) | ((c3 & 0x3C) >> 2));

            do{
                c4 = str.charCodeAt(i++) & 0xff;
                if (c4 == 61) return string;
                c4 = base64DecodeChars[c4]
            } while (
                i < len && c4 == -1
            );

            if (c4 == -1) break;

            string += String.fromCharCode(((c3 & 0x03) << 6) | c4)
        }
        return string;
    }