feat: Add send_state parameter to disable sending of state

This reverts #181 and adds a `send_state` parameter instead to address #174.

According to https://openid.net/specs/openid-connect-core-1_0.html#rfc.section.3.1.2.1,
`state` is recommended but not required:

```
state
    RECOMMENDED. Opaque value used to maintain state between the
    request and the callback. Typically, Cross-Site Request Forgery
    (CSRF, XSRF) mitigation is done by cryptographically binding the
    value of this parameter with a browser cookie.
```

In #181 we
attempted to make `require_state` skip the `state` verification if
it were `true`, but this was reverted for two reasons:

1. If identity providers make direct requests to the callback phase
with a valid token, no `state` is passed in the request, so
`require_state` to fail the request would break the existing flow.

2. If `state` isn't used, it shouldn't be sent in the first place.

`send_state` will now disable the sending of a `state` in the
authorize phase.

README.md

@@ -79,7 +79,8 @@ end
 | scope                        | Which OpenID scopes to include (:openid is always required)                                                                                                              | no       | Array<sym> [:openid]          | [:openid, :profile, :email]                         |
 | response_type                | Which OAuth2 response type to use with the authorization request                                                                                                         | no       | String: code                  | one of: 'code', 'id_token'                          |
 | state                        | A value to be used for the OAuth2 state parameter on the authorization request. Can be a proc that generates a string.                                                   | no       | Random 16 character string    | Proc.new { SecureRandom.hex(32) }                   |
-| require_state                | Should state param be verified - this is recommended, not required by the OIDC specification                                                                             | no       | true                          | false                                               |
+| require_state                | Should the callback phase require that a state is present. If `sent_state` is true, then the callback state must match the authorize state. This is recommended, not required by the OIDC specification. | no       | true                          | false                                               |
+| send_state                   | Should the authorize phase send a `state` parameter - this is recommended, not required by the OIDC specification                                                        | no       | true                          | false                                               |
 | response_mode                | The response mode per [spec](https://openid.net/specs/oauth-v2-form-post-response-mode-1_0.html)                                                                         | no       | nil                           | one of: :query, :fragment, :form_post, :web_message |
 | display                      | An optional parameter to the authorization request to determine how the authorization and consent page                                                                   | no       | nil                           | one of: :page, :popup, :touch, :wap                 |
 | prompt                       | An optional parameter to the authrization request to determine what pages the user will be shown                                                                         | no       | nil                           | one of: :none, :login, :consent, :select_account    |

lib/omniauth/strategies/openid_connect.rb

@@ -42,6 +42,7 @@ class OpenIDConnect # rubocop:disable Metrics/ClassLength
       option :client_x509_signing_key
       option :scope, [:openid]
       option :response_type, 'code' # ['code', 'id_token']
+      option :send_state, true
       option :require_state, true
       option :state
       option :response_mode # [:query, :fragment, :form_post, :web_message]
@@ -120,7 +121,12 @@ def request_phase
       def callback_phase
         error = params['error_reason'] || params['error']
         error_description = params['error_description'] || params['error_reason']
-        invalid_state = (options.require_state && params['state'].to_s.empty?) || params['state'] != stored_state
+        invalid_state =
+          if options.send_state
+            (options.require_state && params['state'].to_s.empty?) || params['state'] != stored_state
+          else
+            false
+          end
 
         raise CallbackError, error: params['error'], reason: error_description, uri: params['error_uri'] if error
         raise CallbackError, error: :csrf_detected, reason: "Invalid 'state' parameter" if invalid_state
@@ -169,13 +175,12 @@ def end_session_uri
         end_session_uri.to_s
       end
 
-      def authorize_uri
+      def authorize_uri # rubocop:disable Metrics/AbcSize
         client.redirect_uri = redirect_uri
         opts = {
           response_type: options.response_type,
           response_mode: options.response_mode,
           scope: options.scope,
-          state: new_state,
           login_hint: params['login_hint'],
           ui_locales: params['ui_locales'],
           claims_locales: params['claims_locales'],
@@ -185,6 +190,7 @@ def authorize_uri
           acr_values: options.acr_values,
         }
 
+        opts[:state] = new_state if options.send_state
         opts.merge!(options.extra_authorize_params) unless options.extra_authorize_params.empty?
 
         options.allow_authorize_params.each do |key|

test/lib/omniauth/strategies/openid_connect_test.rb

@@ -453,6 +453,50 @@ def test_callback_phase_with_discovery # rubocop:disable Metrics/AbcSize
         strategy.callback_phase
       end
 
+      def test_callback_phase_with_send_state_disabled # rubocop:disable Metrics/AbcSize
+        code = SecureRandom.hex(16)
+
+        strategy.options.client_options.host = 'example.com'
+        strategy.options.require_state = true
+        strategy.options.send_state = false
+        strategy.options.discovery = true
+        refute_match(/state/, strategy.authorize_uri, 'URI must not contain state')
+
+        request.stubs(:params).returns('code' => code)
+        request.stubs(:path).returns('')
+
+        issuer = stub('OpenIDConnect::Discovery::Issuer')
+        issuer.stubs(:issuer).returns('https://example.com/')
+        ::OpenIDConnect::Discovery::Provider.stubs(:discover!).returns(issuer)
+
+        config = stub('OpenIDConnect::Discovery::Provder::Config')
+        config.stubs(:authorization_endpoint).returns('https://example.com/authorization')
+        config.stubs(:token_endpoint).returns('https://example.com/token')
+        config.stubs(:userinfo_endpoint).returns('https://example.com/userinfo')
+        config.stubs(:jwks_uri).returns('https://example.com/jwks')
+        config.stubs(:jwks).returns(JSON::JWK::Set.new(jwks['keys']))
+
+        ::OpenIDConnect::Discovery::Provider::Config.stubs(:discover!).with('https://example.com/').returns(config)
+
+        id_token = stub('OpenIDConnect::ResponseObject::IdToken')
+        id_token.stubs(:raw_attributes).returns('sub' => 'sub', 'name' => 'name', 'email' => 'email')
+        id_token.stubs(:verify!).with(issuer: 'https://example.com/', client_id: @identifier, nonce: nonce).returns(true)
+        ::OpenIDConnect::ResponseObject::IdToken.stubs(:decode).returns(id_token)
+
+        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(jwt.to_s)
+        client.expects(:access_token!).at_least_once.returns(access_token)
+        access_token.expects(:userinfo!).returns(user_info)
+
+        strategy.call!('rack.session' => { 'omniauth.nonce' => nonce })
+        strategy.callback_phase
+      end
+
       def test_callback_phase_with_no_state_without_state_verification # rubocop:disable Metrics/AbcSize
         code = SecureRandom.hex(16)