Warden - how to setup custom encryptor

I`m writting sinatra app and i would like to setup encryption in Warden
like i did it in devise when i used Rails.

  module Devise
    module Encryptors
      class Md5 < Base
        def self.digest(password, stretches, salt, pepper)
          str = [password].flatten.compact.join
          Digest::MD5.hexdigest(Digest::MD5.hexdigest(str))
        end
      end
    end
  end

Where I can setup this module ??
How to set Warden to use MD5 encryption?