password_challenge.rb

require 'thread'
require 'socket'
require 'concurrent' # gem install concurrent-ruby

PASSWORD = 'GS2W_{P4ssw0RD_T1MinG_4TT4Ck!}'
ALPHABET = ('a'..'z').to_a + ('A'..'Z').to_a + ('0'..'9').to_a
PORT = 9999
THROTTLE = Concurrent::Semaphore.new 5

def generate_id
    ALPHABET.sample(8).join
end

def validate_password(password)
    time_to_validade = 0
    s_end = ''
    valid = true
    password.split('').each_with_index do |val, idx|
        if val != PASSWORD[idx]
            valid = false
            break
        end
        time_to_validade += 0.05
        sleep(0.05)
    end
    if password != PASSWORD
        # Wrong length
        return "false #{time_to_validade.round(2)}"
    end
    if valid
        s_end = " The password is the flag."
    end
    "#{valid} #{time_to_validade.round(2)}" + s_end
end

def password_challenge(client)
    id = generate_id
    puts "client at #{client.remote_address.ip_address} id=#{id}"

    unless THROTTLE.try_acquire(1, 1)
        client.puts "Too many requests at the same time. Try again later!"
        return
    end

    begin
        client.write "Please input the password: "
        password = client.readpartial(31).chomp
        response = validate_password password
        puts "id=#{id} password=#{password} response='#{response}'"
        client.puts response
    ensure
        THROTTLE.release(1)
    end
end


server = TCPServer.new PORT
loop do
    Thread.new(server.accept) do |client|
        password_challenge client
        client.close
    end
end