feat(oidc): use web keys for token signing and verification (#8449)

# Which Problems Are Solved

Use web keys, managed by the `resources/v3alpha/web_keys` API, for OIDC
token signing and verification,
as well as serving the public web keys on the jwks / keys endpoint.
Response header on the keys endpoint now allows caching of the response.
This is now "safe" to do since keys can be created ahead of time and
caches have sufficient time to pickup the change before keys get
enabled.

# How the Problems Are Solved

- The web key format is used in the `getSignerOnce` function in the
`api/oidc` package.
- The public key cache is changed to get and store web keys.
- The jwks / keys endpoint returns the combined set of valid "legacy"
public keys and all available web keys.
- Cache-Control max-age default to 5 minutes and is configured in
`defaults.yaml`.

When the web keys feature is enabled, fallback mechanisms are in place
to obtain and convert "legacy" `query.PublicKey` as web keys when
needed. This allows transitioning to the feature without invalidating
existing tokens. A small performance overhead may be noticed on the keys
endpoint, because 2 queries need to be run sequentially. This will
disappear once the feature is stable and the legacy code gets cleaned
up.

# Additional Changes

- Extend legacy key lifetimes so that tests can be run on an existing
database with more than 6 hours apart.
- Discovery endpoint returns all supported algorithms when the Web Key
feature is enabled.

# Additional Context

- Closes https://github.com/zitadel/zitadel/issues/8031
- Part of https://github.com/zitadel/zitadel/issues/7809
- After https://github.com/zitadel/oidc/pull/637
- After https://github.com/zitadel/oidc/pull/638

internal/api/oidc/key.go

@@ -3,16 +3,19 @@ package oidc
 import (
 	"context"
 	"fmt"
+	"slices"
 	"sync"
 	"sync/atomic"
 	"time"
 
 	"github.com/go-jose/go-jose/v4"
 	"github.com/jonboulle/clockwork"
+	"github.com/muhlemmer/gu"
 	"github.com/zitadel/logging"
 	"github.com/zitadel/oidc/v3/pkg/op"
 
 	"github.com/zitadel/zitadel/internal/api/authz"
+	http_util "github.com/zitadel/zitadel/internal/api/http"
 	"github.com/zitadel/zitadel/internal/crypto"
 	"github.com/zitadel/zitadel/internal/eventstore"
 	"github.com/zitadel/zitadel/internal/query"
@@ -22,14 +25,33 @@ import (
 	"github.com/zitadel/zitadel/internal/zerrors"
 )
 
-type cachedPublicKey struct {
-	lastUse atomic.Int64 // unix micro time.
-	query.PublicKey
+var supportedWebKeyAlgs = []string{
+	string(jose.EdDSA),
+	string(jose.RS256),
+	string(jose.RS384),
+	string(jose.RS512),
+	string(jose.ES256),
+	string(jose.ES384),
+	string(jose.ES512),
 }
 
-func newCachedPublicKey(key query.PublicKey, now time.Time) *cachedPublicKey {
+func supportedSigningAlgs(ctx context.Context) []string {
+	if authz.GetFeatures(ctx).WebKey {
+		return supportedWebKeyAlgs
+	}
+	return []string{string(jose.RS256)}
+}
+
+type cachedPublicKey struct {
+	lastUse atomic.Int64 // unix micro time.
+	expiry  *time.Time   // expiry may be nil if the key does not expire.
+	webKey  *jose.JSONWebKey
+}
+
+func newCachedPublicKey(key *jose.JSONWebKey, expiry *time.Time, now time.Time) *cachedPublicKey {
 	cachedKey := &cachedPublicKey{
-		PublicKey: key,
+		expiry: expiry,
+		webKey: key,
 	}
 	cachedKey.setLastUse(now)
 	return cachedKey
@@ -53,14 +75,17 @@ func (c *cachedPublicKey) expired(now time.Time, validity time.Duration) bool {
 type publicKeyCache struct {
 	mtx          sync.RWMutex
 	instanceKeys map[string]map[string]*cachedPublicKey
-	queryKey     func(ctx context.Context, keyID string) (query.PublicKey, error)
-	clock        clockwork.Clock
+
+	// queryKey returns a public web key.
+	// If the key does not have expiry, Time may be nil.
+	queryKey func(ctx context.Context, keyID string) (*jose.JSONWebKey, *time.Time, error)
+	clock    clockwork.Clock
 }
 
 // newPublicKeyCache initializes a keySetCache starts a purging Go routine.
 // The purge routine deletes all public keys that are older than maxAge.
 // When the passed context is done, the purge routine will terminate.
-func newPublicKeyCache(background context.Context, maxAge time.Duration, queryKey func(ctx context.Context, keyID string) (query.PublicKey, error)) *publicKeyCache {
+func newPublicKeyCache(background context.Context, maxAge time.Duration, queryKey func(ctx context.Context, keyID string) (*jose.JSONWebKey, *time.Time, error)) *publicKeyCache {
 	k := &publicKeyCache{
 		instanceKeys: make(map[string]map[string]*cachedPublicKey),
 		queryKey:     queryKey,
@@ -119,11 +144,11 @@ func (k *publicKeyCache) getKey(ctx context.Context, keyID string) (_ *cachedPub
 	if ok {
 		key.setLastUse(k.clock.Now())
 	} else {
-		newKey, err := k.queryKey(ctx, keyID)
+		newKey, expiry, err := k.queryKey(ctx, keyID)
 		if err != nil {
 			return nil, err
 		}
-		key = newCachedPublicKey(newKey, k.clock.Now())
+		key = newCachedPublicKey(newKey, expiry, k.clock.Now())
 		k.setKey(instanceID, keyID, key)
 	}
 
@@ -144,10 +169,10 @@ func (k *publicKeyCache) verifySignature(ctx context.Context, jws *jose.JSONWebS
 	if err != nil {
 		return nil, err
 	}
-	if checkKeyExpiry && key.Expiry().Before(k.clock.Now()) {
+	if checkKeyExpiry && key.expiry != nil && key.expiry.Before(k.clock.Now()) {
 		return nil, zerrors.ThrowInvalidArgument(err, "QUERY-ciF4k", "Errors.Key.ExpireBeforeNow")
 	}
-	return jws.Verify(jsonWebkey(key))
+	return jws.Verify(key.webKey)
 }
 
 type oidcKeySet struct {
@@ -423,3 +448,68 @@ func retry(retryable func() error) (err error) {
 	}
 	return err
 }
+
+func (s *Server) Keys(ctx context.Context, r *op.Request[struct{}]) (_ *op.Response, err error) {
+	ctx, span := tracing.NewSpan(ctx)
+	defer func() { span.EndWithError(err) }()
+
+	if !authz.GetFeatures(ctx).WebKey {
+		return s.LegacyServer.Keys(ctx, r)
+	}
+
+	keyset, err := s.query.GetWebKeySet(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	// Return legacy keys, so we do not invalidate all tokens
+	// once the feature flag is enabled.
+	legacyKeys, err := s.query.ActivePublicKeys(ctx, time.Now())
+	logging.OnError(err).Error("oidc server: active public keys (legacy)")
+	appendPublicKeysToWebKeySet(keyset, legacyKeys)
+
+	resp := op.NewResponse(keyset)
+	if s.jwksCacheControlMaxAge != 0 {
+		resp.Header.Set(http_util.CacheControl,
+			fmt.Sprintf("max-age=%d, must-revalidate", int(s.jwksCacheControlMaxAge/time.Second)),
+		)
+	}
+
+	return resp, nil
+}
+
+func appendPublicKeysToWebKeySet(keyset *jose.JSONWebKeySet, pubkeys *query.PublicKeys) {
+	if pubkeys == nil || len(pubkeys.Keys) == 0 {
+		return
+	}
+	keyset.Keys = slices.Grow(keyset.Keys, len(pubkeys.Keys))
+
+	for _, key := range pubkeys.Keys {
+		keyset.Keys = append(keyset.Keys, jose.JSONWebKey{
+			Key:       key.Key(),
+			KeyID:     key.ID(),
+			Algorithm: key.Algorithm(),
+			Use:       key.Use().String(),
+		})
+	}
+}
+
+func queryKeyFunc(q *query.Queries) func(ctx context.Context, keyID string) (*jose.JSONWebKey, *time.Time, error) {
+	return func(ctx context.Context, keyID string) (*jose.JSONWebKey, *time.Time, error) {
+		if authz.GetFeatures(ctx).WebKey {
+			webKey, err := q.GetPublicWebKeyByID(ctx, keyID)
+			if err == nil {
+				return webKey, nil, nil
+			}
+			if !zerrors.IsNotFound(err) {
+				return nil, nil, err
+			}
+		}
+
+		pubKey, err := q.GetPublicKeyByID(ctx, keyID)
+		if err != nil {
+			return nil, nil, err
+		}
+		return jsonWebkey(pubKey), gu.Ptr(pubKey.Expiry()), nil
+	}
+}