jkws: refactor RemoteKeySet cache to a map

this refactors RemoteKeySet to cache keys using a map so that keys can
be looked up by keyID in constant-time.

oidc/jwks.go

@@ -46,14 +46,14 @@ type RemoteKeySet struct {
 	inflight *inflight
 
 	// A set of cached keys.
-	cachedKeys []jose.JSONWebKey
+	cachedKeys map[string]jose.JSONWebKey
 }
 
 // inflight is used to wait on some in-flight request from multiple goroutines.
 type inflight struct {
 	doneCh chan struct{}
 
-	keys []jose.JSONWebKey
+	keys map[string]jose.JSONWebKey
 	err  error
 }
 
@@ -70,14 +70,14 @@ func (i *inflight) wait() <-chan struct{} {
 // done can only be called by a single goroutine. It records the result of the
 // inflight request and signals other goroutines that the result is safe to
 // inspect.
-func (i *inflight) done(keys []jose.JSONWebKey, err error) {
+func (i *inflight) done(keys map[string]jose.JSONWebKey, err error) {
 	i.keys = keys
 	i.err = err
 	close(i.doneCh)
 }
 
 // result cannot be called until the wait() channel has returned a value.
-func (i *inflight) result() ([]jose.JSONWebKey, error) {
+func (i *inflight) result() (map[string]jose.JSONWebKey, error) {
 	return i.keys, i.err
 }
 
@@ -102,43 +102,53 @@ func (r *RemoteKeySet) verify(ctx context.Context, jws *jose.JSONWebSignature) (
 		break
 	}
 
-	keys := r.keysFromCache()
-	for _, key := range keys {
-		if keyID == "" || key.KeyID == keyID {
-			if payload, err := jws.Verify(&key); err == nil {
-				return payload, nil
-			}
-		}
+	if payload, ok := r.verifyWithKey(keyID, jws); ok {
+		return payload, nil
 	}
-
 	// If the kid doesn't match, check for new keys from the remote. This is the
 	// strategy recommended by the spec.
 	//
 	// https://openid.net/specs/openid-connect-core-1_0.html#RotateSigKeys
-	keys, err := r.keysFromRemote(ctx)
+	_, err := r.keysFromRemote(ctx)
 	if err != nil {
 		return nil, fmt.Errorf("fetching keys %v", err)
 	}
 
-	for _, key := range keys {
-		if keyID == "" || key.KeyID == keyID {
+	if payload, ok := r.verifyWithKey(keyID, jws); ok {
+		return payload, nil
+	}
+
+	return nil, errors.New("failed to verify id token signature")
+}
+
+// verifyWithKey attempts to verify the jws using the key with keyID from the cache
+// if keyID is the empty string, it tries each key in the cache
+func (r *RemoteKeySet) verifyWithKey(keyID string, jws *jose.JSONWebSignature) (payload []byte, ok bool) {
+	if keyID == "" {
+		for _, key := range r.keysFromCache() {
 			if payload, err := jws.Verify(&key); err == nil {
-				return payload, nil
+				return payload, true
+			}
+		}
+	} else {
+		if key, ok := r.keysFromCache()[keyID]; ok {
+			if payload, err := jws.Verify(&key); err == nil {
+				return payload, true
 			}
 		}
 	}
-	return nil, errors.New("failed to verify id token signature")
+	return nil, false
 }
 
-func (r *RemoteKeySet) keysFromCache() (keys []jose.JSONWebKey) {
+func (r *RemoteKeySet) keysFromCache() (keys map[string]jose.JSONWebKey) {
 	r.mu.Lock()
 	defer r.mu.Unlock()
 	return r.cachedKeys
 }
 
 // keysFromRemote syncs the key set from the remote set, records the values in the
 // cache, and returns the key set.
-func (r *RemoteKeySet) keysFromRemote(ctx context.Context) ([]jose.JSONWebKey, error) {
+func (r *RemoteKeySet) keysFromRemote(ctx context.Context) (map[string]jose.JSONWebKey, error) {
 	// Need to lock to inspect the inflight request field.
 	r.mu.Lock()
 	// If there's not a current inflight request, create one.
@@ -178,7 +188,7 @@ func (r *RemoteKeySet) keysFromRemote(ctx context.Context) ([]jose.JSONWebKey, e
 	}
 }
 
-func (r *RemoteKeySet) updateKeys() ([]jose.JSONWebKey, error) {
+func (r *RemoteKeySet) updateKeys() (map[string]jose.JSONWebKey, error) {
 	req, err := http.NewRequest("GET", r.jwksURL, nil)
 	if err != nil {
 		return nil, fmt.Errorf("oidc: can't create request: %v", err)
@@ -204,5 +214,9 @@ func (r *RemoteKeySet) updateKeys() ([]jose.JSONWebKey, error) {
 	if err != nil {
 		return nil, fmt.Errorf("oidc: failed to decode keys: %v %s", err, body)
 	}
-	return keySet.Keys, nil
+	keys := make(map[string]jose.JSONWebKey)
+	for _, key := range keySet.Keys {
+		keys[key.KeyID] = key
+	}
+	return keys, nil
 }