decrypt-v2.js

/*************************

Use: "Wy0xLjc4MjM2NDU0OTY3NTIyNjNlLTM0LC0 ... YyNzg2LDk3LjAwMDAwMDAwMDAwMDExXQ==".decrypt(6) // String is much longer in fact
=>  "attack"

************************/

function polynomial() {
	const args = [...arguments].reverse()
	let buffer = "";
	for (let i = 0; i < args.length; i++) {
		buffer += `${args[i]} * x**${i} ${i == args.length -1 ? '': '+ '}`
	}
	return {
		type: "polynomial",
		v: "x",
		f: buffer,
		values: [...arguments],
		core: x => {
			let regex = new RegExp("x")
			let newStr = buffer.replace(regex, `(${x})`)
			return eval(newStr)
		}
	}
}
String.prototype.fromB64 = function() {
    let s = this // creates a mutable value
    const base64chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    // remove/ignore any characters not in the base64 characters list
    //  or the pad character -- particularly newlines
    s = s.replace(new RegExp(`[^${base64chars}=]`, "g"), "");

    // replace any incoming padding with a zero pad (the "A" character is zero)
    const p = (s.charAt(s.length - 1) == "=" ?
        (s.charAt(s.length - 2) == "=" ? "AA" : "A") : "");
    let r = "";
    s = s.substr(0, s.length - p.length) + p;

    // increment over the length of this encoded string, four characters at a time
    for (let c = 0; c < s.length; c += 4) {

        // each of these four characters represents a 6-bit index in the base64 characters list
        //  which, when concatenated, will give the 24-bit number for the original 3 characters
        const n = (base64chars.indexOf(s.charAt(c)) << 18) + (base64chars.indexOf(s.charAt(c + 1)) << 12) +
            (base64chars.indexOf(s.charAt(c + 2)) << 6) + base64chars.indexOf(s.charAt(c + 3));

        // split the 24-bit number into the original three 8-bit (ASCII) characters
        r += String.fromCharCode((n >>> 16) & 255, (n >>> 8) & 255, n & 255);
    }
    // remove any zero pad that was added to make this a multiple of 24 bits
    return r.substring(0, r.length - p.length);
}

String.prototype.decrypt = function(length) {
    let v = this.fromB64()
    let f = polynomial(...JSON.parse(v))
    let arr = []
    for (let i = 0; i < length; i++) {
        arr.push(Math.round(f.core(i)))
    }
    return String.fromCharCode(...arr)
}