# File lib/mongo/auth/scram/conversation.rb, line 225
        def initialize(user, mechanism)
          unless [:scram, :scram256].include?(mechanism)
            raise InvalidMechanism.new(mechanism)
          end

          @user = user
          @nonce = SecureRandom.base64
          @client_key = user.send(:client_key)
          @mechanism = mechanism
        end