rc4.js

/*
 * RC4 symmetric cipher encryption/decryption
 *
 * @license Public Domain
 * @param string key - secret key for encryption/decryption
 * @param string str - string to be encrypted/decrypted
 * @return string
 */
function rc4(key, str) {
	var s = [], j = 0, x, res = '';
	for (var i = 0; i < 256; i++) {
		s[i] = i;
	}
	for (i = 0; i < 256; i++) {
		j = (j + s[i] + key.charCodeAt(i % key.length)) % 256;
		x = s[i];
		s[i] = s[j];
		s[j] = x;
	}
	i = 0;
	j = 0;
	for (var y = 0; y < str.length; y++) {
		i = (i + 1) % 256;
		j = (j + s[i]) % 256;
		x = s[i];
		s[i] = s[j];
		s[j] = x;
		res += String.fromCharCode(str.charCodeAt(y) ^ s[(s[i] + s[j]) % 256]);
	}
	return res;
}

rc4.py

def rc4(key, data):
	"""
	Decrypt/encrypt the passed data using RC4 and the given key.
	https://github.com/EmpireProject/Empire/blob/73358262acc8ed3c34ffc87fa593655295b81434/data/agent/stagers/dropbox.py
	"""
	S, j, out = range(256), 0, []
	for i in range(256):
		j = (j + S[i] + ord(key[i % len(key)])) % 256
		S[i], S[j] = S[j], S[i]
	i = j = 0
	for char in data:
		i = (i + 1) % 256
		j = (j + S[i]) % 256
		S[i], S[j] = S[j], S[i]
		out.append(chr(ord(char) ^ S[(S[i] + S[j]) % 256]))
	return ''.join(out)