Fetch key from JWKS URI if available

In non-standard OpenID Connect providers, such as Azure B2C, discovery
does not work because the discovery URL does not match the issuer field.
If a JWKS URI is provided when discovery is disabled, we should make an
HTTP request for the keys and use the response.

Closes #72

lib/omniauth/strategies/openid_connect.rb

@@ -9,7 +9,7 @@
 
 module OmniAuth
   module Strategies
-    class OpenIDConnect
+    class OpenIDConnect # rubocop:disable Metrics/ClassLength
       include OmniAuth::Strategy
       extend Forwardable
 
@@ -198,9 +198,13 @@ def authorize_uri
       end
 
       def public_key
-        return config.jwks if options.discovery
-
-        key_or_secret || config.jwks
+        @public_key ||= if options.discovery
+                          config.jwks
+                        elsif key_or_secret
+                          key_or_secret
+                        elsif client_options.jwks_uri
+                          fetch_key
+                        end
       end
 
       def pkce_authorize_params(verifier)
@@ -213,6 +217,10 @@ def pkce_authorize_params(verifier)
 
       private
 
+      def fetch_key
+        @fetch_key ||= parse_jwk_key(::OpenIDConnect.http_client.get_content(client_options.jwks_uri))
+      end
+
       def issuer
         resource = "#{ client_options.scheme }://#{ client_options.host }"
         resource = "#{ resource }:#{ client_options.port }" if client_options.port
@@ -301,16 +309,17 @@ def session
       end
 
       def key_or_secret
-        case options.client_signing_alg
-        when :HS256, :HS384, :HS512
-          client_options.secret
-        when :RS256, :RS384, :RS512
-          if options.client_jwk_signing_key
-            parse_jwk_key(options.client_jwk_signing_key)
-          elsif options.client_x509_signing_key
-            parse_x509_key(options.client_x509_signing_key)
+        @key_or_secret ||=
+          case options.client_signing_alg&.to_sym
+          when :HS256, :HS384, :HS512
+            client_options.secret
+          when :RS256, :RS384, :RS512
+            if options.client_jwk_signing_key
+              parse_jwk_key(options.client_jwk_signing_key)
+            elsif options.client_x509_signing_key
+              parse_x509_key(options.client_x509_signing_key)
+            end
           end
-        end
       end
 
       def parse_x509_key(key)

test/lib/omniauth/strategies/openid_connect_test.rb

@@ -335,6 +335,39 @@ def test_callback_phase_with_invalid_state_without_state_verification
         strategy.callback_phase
       end
 
+      def test_callback_phase_with_jwks_uri
+        id_token = jwt.to_s
+        state = SecureRandom.hex(16)
+        request.stubs(:params).returns('id_token' => id_token, 'state' => state)
+        request.stubs(:path_info).returns('')
+
+        strategy.options.issuer = 'example.com'
+        strategy.options.client_options.jwks_uri = 'https://jwks.example.com'
+        strategy.options.response_type = 'id_token'
+
+        HTTPClient
+          .any_instance.stubs(:get_content)
+          .with(strategy.options.client_options.jwks_uri)
+          .returns(jwks.to_json)
+
+        strategy.unstub(:user_info)
+        access_token = stub('OpenIDConnect::AccessToken')
+        access_token.stubs(:access_token)
+        access_token.stubs(:refresh_token)
+        access_token.stubs(:expires_in)
+        access_token.stubs(:scope)
+        access_token.stubs(:id_token).returns(id_token)
+
+        id_token = stub('OpenIDConnect::ResponseObject::IdToken')
+        id_token.stubs(:raw_attributes).returns('sub' => 'sub', 'name' => 'name', 'email' => 'email')
+        id_token.stubs(:verify!).with(issuer: strategy.options.issuer, client_id: @identifier, nonce: nonce).returns(true)
+        ::OpenIDConnect::ResponseObject::IdToken.stubs(:decode).returns(id_token)
+        id_token.expects(:verify!)
+
+        strategy.call!('rack.session' => { 'omniauth.state' => state, 'omniauth.nonce' => nonce })
+        strategy.callback_phase
+      end
+
       def test_callback_phase_with_error
         state = SecureRandom.hex(16)
         request.stubs(:params).returns('error' => 'invalid_request')