#!/usr/bin/env ruby

require 'openssl'
require 'securerandom'
require 'sinatra'

Users = {
  "Athena" => "Passw0rd!"
}

Services = {
  "_TGS" => "eiqu@a5ahs8mooqu9Eng",
  "Mail" => "{FvM<kgG}VpHxKJO;6Zo"
}

def encrypt(obj, key)
  cipher = OpenSSL::Cipher::AES.new(256, :CBC).encrypt
  cipher.key = Digest::SHA2.digest(key)
  s = cipher.update(obj) + cipher.final
  s.unpack('H*')[0].upcase
end

def decrypt(obj, key)
  ticket = [obj].pack("H*").unpack("C*").pack("c*")
  cipher = OpenSSL::Cipher::AES.new(256, :CBC).decrypt
  cipher.key = Digest::SHA2.digest(key)
  p = cipher.update(ticket) + cipher.final
  p.split(?\0)
end

def ticket(username, ws_address, service)
  encrypt([username, ws_address, service].join(?\0), Services[service])
end

def noleak(msg, ul, ws)
  puts "Error: #{msg}, returning nonsense to avoid leakage."
  ticket(SecureRandom.alphanumeric(ul), ws, SecureRandom.alphanumeric(rand(24) + 8))
end

post '/login' do
  request.body.rewind
  data = JSON.parse(request.body.read)
  next "Invalid request\n" unless data.keys == %w(username)
  un = data["username"]
  ul = un.length
  ws = request.ip
  next noleak("Invalid username", ul, ws) unless Users.keys.include?(un)
  tgt = ticket(un, ws, "_TGS")
  encrypt(tgt, Users[un])
end

post '/ticket' do
  request.body.rewind
  data = JSON.parse(request.body.read)
  next "Invalid request\n" unless data.keys.sort == %w(service ticket username)
  un, ws, sn = decrypt(data["ticket"], Services["_TGS"])
  next "Invalid ticket\n" unless sn == "_TGS"
  next "Invalid ticket\n" unless un == data["username"]
  next "Invalid ticket\n" unless ws == request.ip
  ticket(un, ws, data["service"])
end