Add JWKS endpoint for one login

app/controllers/one_login_jwks_controller.rb

@@ -0,0 +1,6 @@
+class OneLoginJwksController < ApplicationController
+  def show
+    jwk = Rails.application.config.x.one_login.public_key_jwk
+    render json: JWT::JWK::Set.new(jwk).export
+  end
+end

config/application.rb

@@ -89,5 +89,7 @@ class Application < Rails::Application
 
     I18n.available_locales = %i[en cy]
     I18n.default_locale = :en
+
+    JWT.configuration.jwk.kid_generator_type = :rfc7638_thumbprint
   end
 end

config/initializers/omniauth.rb

@@ -7,6 +7,9 @@
   private_key_pem = private_key_pem.gsub('\n', "\n")
 
   private_key = OpenSSL::PKey::RSA.new(private_key_pem)
+
+  public_key_jwk = JWT::JWK.new(private_key.public_key, use: "sig")
+  Rails.application.config.x.one_login.public_key_jwk = public_key_jwk
 end
 
 Rails.application.config.middleware.use OmniAuth::Builder do
@@ -16,7 +19,7 @@
     idp_base_url: Settings.govuk_one_login.base_url,
     private_key: private_key,
     redirect_uri: "/auth/govuk_one_login/callback",
-    private_key_kid: "", # TODO: we'll need to set this when we switch to using a JWKS endpoint
+    private_key_kid: public_key_jwk&.kid,
     signing_algorithm: "ES256",
     scope: "openid email",
     ui_locales: "en cy",

config/routes.rb

@@ -19,6 +19,8 @@
   get "/submission" => "submission_status#status", as: :status
   get "/.well-known/security.txt" => redirect("https://vulnerability-reporting.service.security.gov.uk/.well-known/security.txt")
 
+  get "/govuk-one-login-jwks", to: "one_login_jwks#show", as: :one_login_jwks
+
   form_id_constraints = { form_id: UrlPatterns::FORM_ID_REGEX }
   form_constraints = {
     **form_id_constraints,

spec/initializers/omniauth_spec.rb

@@ -0,0 +1,60 @@
+require "rails_helper"
+
+RSpec.describe "omniauth initializer" do
+  let(:initializer_path) { Rails.root.join("config/initializers/omniauth.rb") }
+
+  let(:one_login_settings) do
+    double(
+      private_key: private_key_setting,
+      client_id: "test-client-id",
+      base_url: "https://oidc.integration.account.gov.uk",
+    )
+  end
+
+  before do
+    allow(Settings).to receive(:govuk_one_login).and_return(one_login_settings)
+    allow(Rails.application.config.middleware).to receive(:use)
+    allow(OmniAuth::GovukOneLogin::IdpConfiguration).to receive(:new).and_return(instance_double(OmniAuth::GovukOneLogin::IdpConfiguration))
+  end
+
+  around do |example|
+    original_public_key_jwk = Rails.application.config.x.one_login.public_key_jwk
+    original_idp_configuration = Rails.application.config.x.one_login.idp_configuration
+    example.run
+  ensure
+    Rails.application.config.x.one_login.public_key_jwk = original_public_key_jwk
+    Rails.application.config.x.one_login.idp_configuration = original_idp_configuration
+  end
+
+  context "when the private key is present" do
+    let(:rsa_private_key) { OpenSSL::PKey::RSA.generate(2048) }
+    let(:private_key_setting) { Base64.encode64(rsa_private_key.to_pem) }
+
+    before { load initializer_path }
+
+    it "sets Rails.application.config.x.one_login.public_key_jwk" do
+      expect(Rails.application.config.x.one_login.public_key_jwk).to be_a(JWT::JWK::RSA)
+    end
+
+    it "sets the JWK use to 'sig'" do
+      expect(Rails.application.config.x.one_login.public_key_jwk[:use]).to eq("sig")
+    end
+
+    it "sets the JWK kid" do
+      expect(Rails.application.config.x.one_login.public_key_jwk.kid).to be_a(String)
+    end
+  end
+
+  context "when the private key is absent" do
+    let(:private_key_setting) { nil }
+
+    before do
+      Rails.application.config.x.one_login.public_key_jwk = nil
+      load initializer_path
+    end
+
+    it "does not set Rails.application.config.x.one_login.public_key_jwk" do
+      expect(Rails.application.config.x.one_login.public_key_jwk).to be_nil
+    end
+  end
+end

spec/requests/one_login_jwks_controller_spec.rb

@@ -0,0 +1,34 @@
+require "rails_helper"
+require "base64"
+
+RSpec.describe OneLoginJwksController, type: :request do
+  describe "GET #show" do
+    before do
+      public_key = OpenSSL::PKey::RSA.generate(2048).public_key
+      public_key_jwk = JWT::JWK.new(public_key, use: "sig")
+
+      one_login_config = ActiveSupport::OrderedOptions.new
+      one_login_config.public_key_jwk = public_key_jwk
+
+      allow(Rails.application.config.x).to receive(:one_login).and_return(one_login_config)
+    end
+
+    it "returns the JWKS JSON" do
+      get one_login_jwks_path
+
+      expect(response).to have_http_status(:ok)
+      expect(response.content_type).to eq("application/json; charset=utf-8")
+      expect(response.parsed_body).to match({
+        "keys" => [
+          {
+            "e" => "AQAB",
+            "kty" => "RSA",
+            "use" => "sig",
+            "kid" => a_kind_of(String),
+            "n" => a_kind_of(String),
+          },
+        ],
+      })
+    end
+  end
+end