JavaScript Base64 解密方法

var END_OF_INPUT = -1;
    var base64Chars = new Array("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", "+", "/");
    var reverseBase64Chars = new Array();
    for (var i = 0; i < base64Chars.length; i++) {
        reverseBase64Chars[base64Chars[i]] = i
    }
    var base64Str;
    var base64Count;
    function setBase64Str(a) {
        base64Str = a;
        base64Count = 0
    }
    function readReverseBase64() {
        if (!base64Str) {
            return END_OF_INPUT
        }
        while (true) {
            if (base64Count >= base64Str.length) {
                return END_OF_INPUT
            }
            var a = base64Str.charAt(base64Count);
            base64Count++;
            if (reverseBase64Chars[a]) {
                return reverseBase64Chars[a]
            }
            if (a == "A") {
                return 0
            }
        }
        return END_OF_INPUT
    }
    function ntos(a) {
        a = a.toString(16);
        if (a.length == 1) {
            a = "0" + a
        }
        a = "%" + a;
        return unescape(a)
    }
    function decodeBase64(d) {
        setBase64Str(d);
        var a = "";
        var c = new Array(4);
        var b = false;
        while (!b && (c[0] = readReverseBase64()) != END_OF_INPUT && (c[1] = readReverseBase64()) != END_OF_INPUT) {
            c[2] = readReverseBase64();
            c[3] = readReverseBase64();
            a += ntos((((c[0] << 2) & 255) | c[1] >> 4));
            if (c[2] != END_OF_INPUT) {
                a += ntos((((c[1] << 4) & 255) | c[2] >> 2));
                if (c[3] != END_OF_INPUT) {
                    a += ntos((((c[2] << 6) & 255) | c[3]))
                } else {
                    b = true
                }
            } else {
                b = true
            }
        }
        return a
    }