feat: add PKCE option to generic OAuth2 / OIDC identity providers (#9373)

# Which Problems Are Solved

Some OAuth2 and OIDC providers require the use of PKCE for all their
clients. While ZITADEL already recommended the same for its clients, it
did not yet support the option on the IdP configuration.

# How the Problems Are Solved

- A new boolean `use_pkce` is added to the add/update generic OAuth/OIDC
endpoints.
- A new checkbox is added to the generic OAuth and OIDC provider
templates.
- The `rp.WithPKCE` option is added to the provider if the use of PKCE
has been set.
- The `rp.WithCodeChallenge` and `rp.WithCodeVerifier` options are added
to the OIDC/Auth BeginAuth and CodeExchange function.
- Store verifier or any other persistent argument in the intent or auth
request.
- Create corresponding session object before creating the intent, to be
able to store the information.
- (refactored session structs to use a constructor for unified creation
and better overview of actual usage)

Here's a screenshot showing the URI including the PKCE params:


![use_pkce_in_url](https://github.com/zitadel/zitadel/assets/30386061/eaeab123-a5da-4826-b001-2ae9efa35169)

# Additional Changes

None.

# Additional Context

- Closes #6449
- This PR replaces the existing PR (#8228) of @doncicuto. The base he
did was cherry picked. Thank you very much for that!

---------

Co-authored-by: Miguel Cabrerizo <doncicuto@gmail.com>
Co-authored-by: Stefan Benz <46600784+stebenz@users.noreply.github.com>

internal/idp/providers/oidc/oidc.go

@@ -24,6 +24,7 @@ type Provider struct {
 	useIDToken        bool
 	userInfoMapper    func(info *oidc.UserInfo) idp.User
 	authOptions       []func(bool) rp.AuthURLOpt
+	generateVerifier  func() string
 }
 
 type ProviderOpts func(provider *Provider)
@@ -102,8 +103,9 @@ var DefaultMapper UserInfoMapper = func(info *oidc.UserInfo) idp.User {
 // New creates a generic OIDC provider
 func New(name, issuer, clientID, clientSecret, redirectURI string, scopes []string, userInfoMapper UserInfoMapper, options ...ProviderOpts) (provider *Provider, err error) {
 	provider = &Provider{
-		name:           name,
-		userInfoMapper: userInfoMapper,
+		name:             name,
+		userInfoMapper:   userInfoMapper,
+		generateVerifier: oauth2.GenerateVerifier,
 	}
 	for _, option := range options {
 		option(provider)
@@ -150,8 +152,15 @@ func (p *Provider) BeginAuth(ctx context.Context, state string, params ...idp.Pa
 			opts = append(opts, opt)
 		}
 	}
+
+	var codeVerifier string
+	if p.RelyingParty.IsPKCE() {
+		codeVerifier = p.generateVerifier()
+		opts = append(opts, rp.WithCodeChallenge(oidc.NewSHACodeChallenge(codeVerifier)))
+	}
+
 	url := rp.AuthURL(state, p.RelyingParty, opts...)
-	return &Session{AuthURL: url, Provider: p}, nil
+	return &Session{AuthURL: url, Provider: p, CodeVerifier: codeVerifier}, nil
 }
 
 func loginHint(hint string) rp.AuthURLOpt {

internal/idp/providers/oidc/oidc_test.go

@@ -31,7 +31,7 @@ func TestProvider_BeginAuth(t *testing.T) {
 		want   idp.Session
 	}{
 		{
-			name: "successful auth",
+			name: "successful auth without PKCE",
 			fields: fields{
 				name:         "oidc",
 				issuer:       "https://issuer.com",
@@ -55,6 +55,31 @@ func TestProvider_BeginAuth(t *testing.T) {
 			},
 			want: &Session{AuthURL: "https://issuer.com/authorize?client_id=clientID&prompt=select_account&redirect_uri=redirectURI&response_type=code&scope=openid&state=testState"},
 		},
+		{
+			name: "successful auth with PKCE",
+			fields: fields{
+				name:         "oidc",
+				issuer:       "https://issuer.com",
+				clientID:     "clientID",
+				clientSecret: "clientSecret",
+				redirectURI:  "redirectURI",
+				scopes:       []string{"openid"},
+				userMapper:   DefaultMapper,
+				httpMock: func(issuer string) {
+					gock.New(issuer).
+						Get(oidc.DiscoveryEndpoint).
+						Reply(200).
+						JSON(&oidc.DiscoveryConfiguration{
+							Issuer:                issuer,
+							AuthorizationEndpoint: issuer + "/authorize",
+							TokenEndpoint:         issuer + "/token",
+							UserinfoEndpoint:      issuer + "/userinfo",
+						})
+				},
+				opts: []ProviderOpts{WithSelectAccount(), WithRelyingPartyOption(rp.WithPKCE(nil))},
+			},
+			want: &Session{AuthURL: "https://issuer.com/authorize?client_id=clientID&code_challenge=2ZoH_a01aprzLkwVbjlPsBo4m8mJ_zOKkaDqYM7Oh5w&code_challenge_method=S256&prompt=select_account&redirect_uri=redirectURI&response_type=code&scope=openid&state=testState"},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -65,6 +90,9 @@ func TestProvider_BeginAuth(t *testing.T) {
 
 			provider, err := New(tt.fields.name, tt.fields.issuer, tt.fields.clientID, tt.fields.clientSecret, tt.fields.redirectURI, tt.fields.scopes, tt.fields.userMapper, tt.fields.opts...)
 			r.NoError(err)
+			provider.generateVerifier = func() string {
+				return "pkceOAuthVerifier"
+			}
 
 			ctx := context.Background()
 			session, err := provider.BeginAuth(ctx, "testState")

internal/idp/providers/oidc/session.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/zitadel/zitadel/internal/domain"
 	"github.com/zitadel/zitadel/internal/idp"
+	"github.com/zitadel/zitadel/internal/idp/providers/oauth"
 )
 
 var ErrCodeMissing = errors.New("no auth code provided")
@@ -18,10 +19,16 @@ var _ idp.Session = (*Session)(nil)
 
 // Session is the [idp.Session] implementation for the OIDC provider.
 type Session struct {
-	Provider *Provider
-	AuthURL  string
-	Code     string
-	Tokens   *oidc.Tokens[*oidc.IDTokenClaims]
+	Provider     *Provider
+	AuthURL      string
+	CodeVerifier string
+	Code         string
+	Tokens       *oidc.Tokens[*oidc.IDTokenClaims]
+}
+
+func NewSession(provider *Provider, code string, idpArguments map[string]any) *Session {
+	verifier, _ := idpArguments[oauth.CodeVerifier].(string)
+	return &Session{Provider: provider, Code: code, CodeVerifier: verifier}
 }
 
 // GetAuth implements the [idp.Session] interface.
@@ -29,6 +36,14 @@ func (s *Session) GetAuth(ctx context.Context) (string, bool) {
 	return idp.Redirect(s.AuthURL)
 }
 
+// PersistentParameters implements the [idp.Session] interface.
+func (s *Session) PersistentParameters() map[string]any {
+	if s.CodeVerifier == "" {
+		return nil
+	}
+	return map[string]any{oauth.CodeVerifier: s.CodeVerifier}
+}
+
 // FetchUser implements the [idp.Session] interface.
 // It will execute an OIDC code exchange if needed to retrieve the tokens,
 // call the userinfo endpoint and map the received information into an [idp.User].
@@ -61,7 +76,11 @@ func (s *Session) Authorize(ctx context.Context) (err error) {
 	if s.Code == "" {
 		return ErrCodeMissing
 	}
-	s.Tokens, err = rp.CodeExchange[*oidc.IDTokenClaims](ctx, s.Code, s.Provider.RelyingParty)
+	var opts []rp.CodeExchangeOpt
+	if s.CodeVerifier != "" {
+		opts = append(opts, rp.WithCodeVerifier(s.CodeVerifier))
+	}
+	s.Tokens, err = rp.CodeExchange[*oidc.IDTokenClaims](ctx, s.Code, s.Provider.RelyingParty, opts...)
 	return err
 }