[backend] Refetch user keys when HTTP Signature validation fails

If a user has had a key rotation, and nobody on this server follows
that user, we will not receive the Update activity with the new key

Therefore, when we encounter key validation errors we should check
for an up-to-date key.

References (other implementations):

 * [Mastodon](fc9ab61448/app/controllers/concerns/signature_verification.rb (L96))
 * [Akkoma](https://akkoma.dev/AkkomaGang/http_signatures/src/branch/main/lib/http_signatures/http_signatures.ex#L46)

packages/backend/src/queue/processors/inbox.ts

@@ -95,11 +95,25 @@ export default async (job: Bull.Job<InboxJobData>): Promise<string> => {
 	}
 
 	// HTTP-Signatureの検証
-	const httpSignatureValidated = httpSignature.verifySignature(
+	let httpSignatureValidated = httpSignature.verifySignature(
 		signature,
 		authUser.key.keyPem,
 	);
 
+	// If signature validation failed, try refetching the actor
+	if (!httpSignatureValidated) {
+		authUser.key = await dbResolver.refetchPublicKeyForApId(authUser.user);
+
+		if (authUser.key == null) {
+			return "skip: failed to re-resolve user publicKey";
+		}
+
+		httpSignatureValidated = httpSignature.verifySignature(
+			signature,
+			authUser.key.keyPem,
+		);
+	}
+
 	// また、signatureのsignerは、activity.actorと一致する必要がある
 	if (!httpSignatureValidated || authUser.user.uri !== activity.actor) {
 		// 一致しなくても、でもLD-Signatureがありそうならそっちも見る

packages/backend/src/remote/activitypub/check-fetch.ts

@@ -87,11 +87,25 @@ export async function checkFetch(req: IncomingMessage): Promise<number> {
 		}
 
 		// HTTP-Signatureの検証
-		const httpSignatureValidated = httpSignature.verifySignature(
+		let httpSignatureValidated = httpSignature.verifySignature(
 			signature,
 			authUser.key.keyPem,
 		);
 
+		// If signature validation failed, try refetching the actor
+		if (!httpSignatureValidated) {
+			authUser.key = await dbResolver.refetchPublicKeyForApId(authUser.user);
+
+			if (authUser.key == null) {
+				return 403;
+			}
+
+			httpSignatureValidated = httpSignature.verifySignature(
+				signature,
+				authUser.key.keyPem,
+			);
+		}
+
 		if (!httpSignatureValidated) {
 			return 403;
 		}

packages/backend/src/remote/activitypub/db-resolver.ts

@@ -17,7 +17,7 @@ import { Cache } from "@/misc/cache.js";
 import { uriPersonCache, userByIdCache } from "@/services/user-cache.js";
 import type { IObject } from "./type.js";
 import { getApId } from "./type.js";
-import { resolvePerson } from "./models/person.js";
+import { resolvePerson, updatePerson } from "./models/person.js";
 import {redisClient, subscriber} from "@/db/redis.js";
 
 const publicKeyCache = new Cache<UserPublickey | null>("publicKey", 60 * 30);
@@ -152,7 +152,7 @@ export default class DbResolver {
 	 */
 	public async getAuthUserFromKeyId(keyId: string): Promise<{
 		user: CacheableRemoteUser;
-		key: UserPublickey;
+		key: UserPublickey | null;
 	} | null> {
 		const key = await publicKeyCache.fetch(
 			keyId,
@@ -204,6 +204,15 @@ export default class DbResolver {
 			key,
 		};
 	}
+
+	public async refetchPublicKeyForApId(user: CacheableRemoteUser): Promise<UserPublickey | null> {
+		await updatePerson(user.uri!, undefined, undefined, user);
+		let key = await UserPublickeys.findOneBy({ userId: user.id });
+		if (key != null) {
+			await publicKeyByUserIdCache.set(user.id, key);
+		}
+		return key;
+	}
 }
 
 subscriber.on("message", async (_, data) => {