module HexaPDF::Encryption::AES

def self.prepended(klass) # :nodoc:

:nodoc:
Automatically extends the klass with the necessary class level methods. 
def self.prepended(klass) # :nodoc:
  klass.extend(ClassMethods)
end

def initialize(key, iv, mode)

just performs basic checks.
Classes prepending this module have to have their own initialization method as this method

The mode must either be :encrypt or :decrypt.

Creates a new AES object using the given encryption key and initialization vector. 
def initialize(key, iv, mode)
  unless VALID_KEY_LENGTH.include?(key.length)
    raise HexaPDF::EncryptionError, "AES key length must be 128, 192 or 256 bit"
  end
  unless iv.length == BLOCK_SIZE
    raise HexaPDF::EncryptionError, "AES initialization vector length must be 128 bit"
  end
  mode = mode.intern
  super
end