Cifra de cesar.

cesarjs.js

let nome = "vini! ¨¨%A%S&¨SA¨&SA ¨ vini";

function mod(n, p) {
    if (n < 0)
        n = p - Math.abs(n) % p;

    return n % p;
}

function encriptar(chave, str) {
    let alpha = "abcdefghijklmnopqrstuvwxyz";
    let saida = '';

    if (chave <= 0) return '';
    if (!str) return '';

    for (let i = 0; i < str.length; i++) {
        if (str[i].match(/[A-Za-z]/gi))
            saida += alpha[mod(alpha.indexOf(str[i]) + chave, 26)];
        else
            saida += str[i];
    }

    return saida;
}

function desencriptar(chave, str) {
    let alpha = "abcdefghijklmnopqrstuvwxyz";
    let saida = '';

    if (chave <= 0) return '';
    if (!str) return '';

    for (let i = 0; i < str.length; i++) {
        if (str[i].match(/[A-Za-z]/gi))
            saida += alpha[mod(alpha.indexOf(str[i]) - chave, 26)];
        else
            saida += str[i];
    }

    return saida;
}


let r = encriptar(8, nome);
alert(r);
alert(desencriptar(8, r));

Veja funcionando no JSFddle.