ridiculous_not_optmizied_untested_monkey_patch.rb

class String
  def encrypt(options = {})
    crypt :encrypt, options
  end

  def decrypt(options = {})
    crypt :decrypt, options
  end

  private

  def crypt(method, options)
    raise "You must provide a key" unless options[:key]
    options[:with] ||= "AES-128-CBC"

    cipher = OpenSSL::Cipher.new(options[:with])
    cipher.send method
    cipher.pkcs5_keyivgen(options[:key])

    cipher.update(self) + cipher.final
  end
end

encrypted_thing = "EU VOU SER ENCRYPTADO YAY".encrypt :key => "LOL"
decrypted_thing = encrypted_thing.decrypt :key => "LOL"

simple_encrypter_decrypter.rb

require "openssl"

encrypter = OpenSSL::Cipher.new("AES-128-CBC")
encrypter.encrypt
encrypter.pkcs5_keyivgen("LOL WUT WTF OMG")

encrypted_thing = encrypter.update("EU VOU SER ENCRYPTADO YAY")
encrypted_thing << encrypter.final

decrypter = OpenSSL::Cipher.new("AES-128-CBC")
decrypter.decrypt
decrypter.pkcs5_keyivgen("LOL WUT WTF OMG")

i_am_back = decrypter.update(encrypted_thing)
i_am_back << decrypter.final