RC4 encoder/decoder

rc4.js

function rc4(msg, key) {
    // If you need to encode strings instead of `Uint8Array`s you have to
    // convert them with `new TextEncoder().encode('your message')` before
    // calling this function.
    console.assert(msg instanceof Uint8Array);
    console.assert(key instanceof Uint8Array);

    // setup S-box from key
    let S = [...new Uint8Array(256).keys()];
    let j = 0;
    for (let i = 0; i < 256; ++i) {
        j = (j + S[i] + key[i % key.length]) % 256;
        [S[i], S[j]] = [S[j], S[i]];
    }

    // encode/decode
    let i = 0;
    j = 0;
    let res = new Uint8Array(msg.length);
    for (let k = 0; k < msg.length; ++k) {
        i = (i + 1) % 256;
        j = (j + S[i]) % 256;
        [S[i], S[j]] = [S[j], S[i]];
        res[k] = msg[k] ^ S[(S[i] + S[j]) % 256];
    }
    return res;
}