feat: extend token exchange response (#567)

* feat: extend token exchange response

This change adds fields to the token exchange and token claims types.

The `act` claim has been added to describe the actor in case of impersonation or delegation. An actor can be nested in case an obtained token is used as actor token to obtain impersonation or delegation. This allows creating a chain of actors. See [RFC 8693, section 4.1](https://www.rfc-editor.org/rfc/rfc8693#name-act-actor-claim).

The `id_token` field has been added to the Token Exchange response  so an ID Token can be returned along with an access token. This is not specified in RFC 8693, but it allows us be consistent with OpenID responses when the scope `openid` is set, while the requested token type may remain access token.

* allow jwt profile for token exchange client

* add invalid target error

pkg/client/tokenexchange/tokenexchange.go

@@ -4,7 +4,9 @@ import (
 	"context"
 	"errors"
 	"net/http"
+	"time"
 
+	"github.com/go-jose/go-jose/v3"
 	"github.com/zitadel/oidc/v3/pkg/client"
 	httphelper "github.com/zitadel/oidc/v3/pkg/http"
 	"github.com/zitadel/oidc/v3/pkg/oidc"
@@ -33,6 +35,17 @@ func NewTokenExchangerClientCredentials(ctx context.Context, issuer, clientID, c
 	return newOAuthTokenExchange(ctx, issuer, authorizer, options...)
 }
 
+func NewTokenExchangerJWTProfile(ctx context.Context, issuer, clientID string, signer jose.Signer, options ...func(source *OAuthTokenExchange)) (TokenExchanger, error) {
+	authorizer := func() (any, error) {
+		assertion, err := client.SignedJWTProfileAssertion(clientID, []string{issuer}, time.Hour, signer)
+		if err != nil {
+			return nil, err
+		}
+		return client.ClientAssertionFormAuthorization(assertion), nil
+	}
+	return newOAuthTokenExchange(ctx, issuer, authorizer, options...)
+}
+
 func newOAuthTokenExchange(ctx context.Context, issuer string, authorizer func() (any, error), options ...func(source *OAuthTokenExchange)) (*OAuthTokenExchange, error) {
 	te := &OAuthTokenExchange{
 		httpClient: httphelper.DefaultHTTPClient,

pkg/oidc/error.go

@@ -27,6 +27,11 @@ const (
 	SlowDown             errorType = "slow_down"
 	AccessDenied         errorType = "access_denied"
 	ExpiredToken         errorType = "expired_token"
+
+	// InvalidTarget error is returned by Token Exchange if
+	// the requested target or audience is invalid.
+	// [RFC 8693, Section 2.2.2: Error Response](https://www.rfc-editor.org/rfc/rfc8693#section-2.2.2)
+	InvalidTarget errorType = "invalid_target"
 )
 
 var (
@@ -112,6 +117,14 @@ var (
 			Description: "The \"device_code\" has expired.",
 		}
 	}
+
+	// Token exchange error
+	ErrInvalidTarget = func() *Error {
+		return &Error{
+			ErrorType:   InvalidTarget,
+			Description: "The requested audience or target is invalid.",
+		}
+	}
 )
 
 type Error struct {

pkg/oidc/token.go

@@ -47,6 +47,7 @@ type TokenClaims struct {
 	AuthorizedParty                     string       `json:"azp,omitempty"`
 	ClientID                            string       `json:"client_id,omitempty"`
 	JWTID                               string       `json:"jti,omitempty"`
+	Actor                               *ActorClaims `json:"act,omitempty"`
 
 	// Additional information set by this framework
 	SignatureAlg jose.SignatureAlgorithm `json:"-"`
@@ -204,6 +205,28 @@ func (i *IDTokenClaims) UnmarshalJSON(data []byte) error {
 	return unmarshalJSONMulti(data, (*itcAlias)(i), &i.Claims)
 }
 
+// ActorClaims provides the `act` claims used for impersonation or delegation Token Exchange.
+//
+// An actor can be nested in case an obtained token is used as actor token to obtain impersonation or delegation.
+// This allows creating a chain of actors.
+// See [RFC 8693, section 4.1](https://www.rfc-editor.org/rfc/rfc8693#name-act-actor-claim).
+type ActorClaims struct {
+	Actor   *ActorClaims   `json:"act,omitempty"`
+	Issuer  string         `json:"iss,omitempty"`
+	Subject string         `json:"sub,omitempty"`
+	Claims  map[string]any `json:"-"`
+}
+
+type acAlias ActorClaims
+
+func (c *ActorClaims) MarshalJSON() ([]byte, error) {
+	return mergeAndMarshalClaims((*acAlias)(c), c.Claims)
+}
+
+func (c *ActorClaims) UnmarshalJSON(data []byte) error {
+	return unmarshalJSONMulti(data, (*acAlias)(c), &c.Claims)
+}
+
 type AccessTokenResponse struct {
 	AccessToken  string `json:"access_token,omitempty" schema:"access_token,omitempty"`
 	TokenType    string `json:"token_type,omitempty" schema:"token_type,omitempty"`
@@ -352,4 +375,8 @@ type TokenExchangeResponse struct {
 	ExpiresIn       uint64              `json:"expires_in,omitempty"`
 	Scopes          SpaceDelimitedArray `json:"scope,omitempty"`
 	RefreshToken    string              `json:"refresh_token,omitempty"`
+
+	// IDToken field allows returning an additional ID token
+	// if the requested_token_type was Access Token and scope contained openid.
+	IDToken string `json:"id_token,omitempty"`
 }

pkg/oidc/token_request.go

@@ -3,6 +3,7 @@ package oidc
 import (
 	"encoding/json"
 	"fmt"
+	"slices"
 	"time"
 
 	jose "github.com/go-jose/go-jose/v3"
@@ -57,13 +58,7 @@ var AllTokenTypes = []TokenType{
 type TokenType string
 
 func (t TokenType) IsSupported() bool {
-	for _, tt := range AllTokenTypes {
+	return slices.Contains(AllTokenTypes, t)
-		if t == tt {
-			return true
-		}
-	}
-
-	return false
 }
 
 type TokenRequest interface {