Toy class demonstrating encryption and decryption with aes-256-gcm

cryptor.rb

class Cryptor
  # Usage:
  # cipher = Cryptor.new(key, auth_data: '1234').encrypt(clear)
  # clear = Cryptor.new(key, auth_data: '12344').decrypt(cipher)
  attr_accessor :key, :auth_data

  # The auth_data should be something different for each text, perhaps a database row id?
  def initialize(key, auth_data: 'aad')
    self.mode = mode
    self.key = key
    self.auth_data = auth_data
  end

  def encrypt(text)
    aes = OpenSSL::Cipher.new(mode)
    aes.encrypt
    aes.key = key
    iv = aes.random_iv
    aes.auth_data = auth_data
    iv + aes.update(text) + aes.final + aes.auth_tag
  end

  def decrypt(cypher)
    aes = OpenSSL::Cipher.new(mode)
    aes.decrypt
    aes.key = key
    aes.iv = cypher[0..iv_length-1]
    aes.auth_data = auth_data
    aes.auth_tag = cypher[-(tag_length)..-1]
    aes.update(cypher[iv_length..-(tag_length+1)]) + aes.final
  end

private
  # The two lengths are a function of the mode.
  def mode
    'aes-256-gcm'
  end
  def iv_length
    12
  end
  def tag_length
    16
  end
end