diff --git a/test/test_mailer_url_for.rb b/test/test_mailer_url_for.rb
new file mode 100644
index 0000000..22f06b8
--- /dev/null
+++ b/test/test_mailer_url_for.rb
@@ -0,0 +1,86 @@
+require 'minitest/autorun'
+require 'prism'
+require 'tmpdir'
+require 'fileutils'
+
+class MailerUrlForTest < Minitest::Test
+  def setup
+    @dir = Dir.mktmpdir('prism-mailer-url-for')
+    FileUtils.mkdir_p(File.join(@dir, 'url_for'))
+    write('url_for/welcome.text.erb', "Click here: <%= url_for(:confirmation, id: user_id) %>\n")
+
+    @router = Prism::Router.new
+    @router.add('get', '/confirmations/:id', name: :confirmation)
+
+    mailer_dir = @dir
+    @mailer_class = Class.new(Prism::Mailer::Base) do
+      self.template_roots = [mailer_dir]
+
+      def welcome(user_id)
+        @user_id = user_id
+        mail(to: 'someone@example.com', subject: 'Welcome!')
+      end
+    end
+    Object.const_set(:UrlForMailer, @mailer_class)
+  end
+
+  def teardown
+    Object.send(:remove_const, :UrlForMailer) if Object.const_defined?(:UrlForMailer)
+    FileUtils.remove_entry(@dir)
+    Prism::Mailer.router = nil
+    Prism::Mailer.default_url_options = nil
+  end
+
+  def write(relative, source)
+    File.write(File.join(@dir, relative), source)
+  end
+
+  def test_url_for_raises_without_a_configured_router
+    Prism::Mailer.router = nil
+    error = assert_raises(RuntimeError) { UrlForMailer.welcome(42) }
+    assert_match(/no router configured/, error.message)
+  end
+
+  def test_url_for_raises_without_a_host
+    Prism::Mailer.router = @router
+    error = assert_raises(ArgumentError) { UrlForMailer.welcome(42) }
+    assert_match(/:host/, error.message)
+  end
+
+  def test_url_for_builds_an_absolute_url_from_global_default_url_options
+    Prism::Mailer.router = @router
+    Prism::Mailer.default_url_options = { host: 'example.com' }
+
+    body = UrlForMailer.welcome(42).message.text_body
+    assert_equal "Click here: http://example.com/confirmations/42\n", body
+  end
+
+  def test_per_mailer_class_default_url_options_win_over_the_global_ones
+    Prism::Mailer.router = @router
+    Prism::Mailer.default_url_options = { host: 'example.com', protocol: 'http' }
+    UrlForMailer.default_url_options(host: 'mail.example.com', protocol: 'https')
+
+    body = UrlForMailer.welcome(42).message.text_body
+    assert_equal "Click here: https://mail.example.com/confirmations/42\n", body
+  end
+
+  def test_port_is_included_when_given
+    Prism::Mailer.router = @router
+    Prism::Mailer.default_url_options = { host: 'example.com', port: 3000 }
+
+    body = UrlForMailer.welcome(42).message.text_body
+    assert_equal "Click here: http://example.com:3000/confirmations/42\n", body
+  end
+
+  def test_application_enable_mailer_wires_router_and_default_url_options
+    app_class = Class.new(Prism::Application)
+    app_class.router.add('get', '/confirmations/:id', name: :confirmation)
+    app_class.enable_mailer(host: 'app.example.com', protocol: 'https')
+
+    assert_same app_class.router, Prism::Mailer.router
+    assert_equal({ host: 'app.example.com', protocol: 'https' }, Prism::Mailer.default_url_options)
+
+    body = UrlForMailer.welcome(7).message.text_body
+    assert_equal "Click here: https://app.example.com/confirmations/7\n", body
+  end
+end
diff --git a/test/test_message_encryptor.rb b/test/test_message_encryptor.rb
new file mode 100644
index 0000000..89dfa0e
--- /dev/null
+++ b/test/test_message_encryptor.rb
@@ -0,0 +1,59 @@
+require 'minitest/autorun'
+require 'prism'
+
+class MessageEncryptorTest < Minitest::Test
+  def setup
+    @key = Prism::MessageEncryptor.derive_key('super-secret-app-key')
+    @encryptor = Prism::MessageEncryptor.new(@key)
+  end
+
+  def test_derived_key_has_correct_length
+    assert_equal 32, @key.bytesize
+  end
+
+  def test_derive_key_is_deterministic_for_same_secret_and_salt
+    assert_equal @key, Prism::MessageEncryptor.derive_key('super-secret-app-key')
+  end
+
+  def test_derive_key_differs_per_salt
+    other = Prism::MessageEncryptor.derive_key('super-secret-app-key', salt: 'other-salt')
+    refute_equal @key, other
+  end
+
+  def test_round_trips_a_hash
+    token = @encryptor.encrypt_and_sign('admin' => true, 'user_id' => 7)
+    assert_equal({ 'admin' => true, 'user_id' => 7 }, @encryptor.decrypt_and_verify(token))
+  end
+
+  def test_ciphertext_does_not_contain_plaintext
+    token = @encryptor.encrypt_and_sign('super-secret-value-xyz')
+    refute_includes token, 'super-secret-value-xyz'
+  end
+
+  def test_two_encryptions_of_same_value_differ
+    first = @encryptor.encrypt_and_sign('same value')
+    second = @encryptor.encrypt_and_sign('same value')
+    refute_equal first, second
+  end
+
+  def test_tampered_ciphertext_fails_to_decrypt
+    token = @encryptor.encrypt_and_sign('user_id' => 1)
+    encrypted, iv, tag = token.split('--', 3)
+    tampered = "#{encrypted}AA--#{iv}--#{tag}"
+    assert_raises(Prism::MessageEncryptor::InvalidMessage) { @encryptor.decrypt_and_verify(tampered) }
+  end
+
+  def test_wrong_key_cannot_decrypt
+    token = @encryptor.encrypt_and_sign('user_id' => 1)
+    other = Prism::MessageEncryptor.new(Prism::MessageEncryptor.derive_key('a-completely-different-key'))
+    assert_raises(Prism::MessageEncryptor::InvalidMessage) { other.decrypt_and_verify(token) }
+  end
+
+  def test_malformed_message_raises
+    assert_raises(Prism::MessageEncryptor::InvalidMessage) { @encryptor.decrypt_and_verify('garbage') }
+  end
+
+  def test_rejects_key_of_wrong_length
+    assert_raises(ArgumentError) { Prism::MessageEncryptor.new('too-short') }
+  end
+end