feat: allow request uuid to be stored

Introduces a :store_request_uuid option for later comparison with InResponseTo

By default it saves the request uuid in the session as "saml_transaction_id",
but also accepts a proc that will then be called with the uuid for custom storage.

README.md

@@ -155,6 +155,10 @@ be present.
 
 * `:uid_attribute` - Attribute that uniquely identifies the user. If unset, the name identifier returned by the IdP is used.
 
+* `:store_request_uuid` - Used to store the request's UUID for later verification of InReponseTo.
+  By default it saves the request uuid in the session as "saml_transaction_id",
+  but also accepts a proc that will then be called with the uuid for custom storage.
+
 * See the `OneLogin::RubySaml::Settings` class in the [Ruby SAML gem](https://github.com/onelogin/ruby-saml) for additional supported options.
 
 ## IdP Metadata

lib/omniauth/strategies/saml.rb

@@ -30,15 +30,26 @@ def self.inherited(subclass)
       option :slo_default_relay_state
       option :uid_attribute
       option :idp_slo_session_destroy, proc { |_env, session| session.clear }
+      option :store_request_uuid
 
       def request_phase
         authn_request = OneLogin::RubySaml::Authrequest.new
 
+        store_request_uuid(authn_request.uuid)
+
         with_settings do |settings|
           redirect(authn_request.create(settings, additional_params_for_authn_request))
         end
       end
 
+      def store_request_uuid(uuid)
+        if options.store_request_uuid.respond_to?(:call)
+          options.store_request_uuid.call(uuid)
+        elsif options.store_request_uuid
+          session["saml_transaction_id"] = uuid
+        end
+      end
+
       def callback_phase
         raise OmniAuth::Strategies::SAML::ValidationError.new("SAML response missing") unless request.params["SAMLResponse"]
 

spec/omniauth/strategies/saml_spec.rb

@@ -115,6 +115,29 @@ def post_xml(xml=:example_response, opts = {})
         expect(query['SigAlg']).to eq XMLSecurity::Document::RSA_SHA256
       end
     end
+
+    context 'with store_request_uuid set' do
+      let(:store_request_uuid) { true }
+      let(:uuid_regex) { /_\w{8}-\w{4}-\w{4}-\w{4}-\w{11}/ }
+
+      before do
+        saml_options[:store_request_uuid] = store_request_uuid
+
+        get '/auth/saml'
+      end
+
+      it 'stores uuid as saml_transaction_id' do
+        expect(session['saml_transaction_id']).to match(uuid_regex)
+      end
+
+      context 'using a proc' do
+        let(:store_request_uuid) { Proc.new { |uuid| @uuid_stored = uuid } }
+
+        it 'allows customized storage of request uuid' do
+          expect(@uuid_stored).to match(uuid_regex)
+        end
+      end
+    end
   end
 
   describe 'POST /auth/saml/callback' do