fix: 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](https://github.com/mastodon/mastodon/blob/fc9ab61448af94513524f14f5fd287d2a26ceeab/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/core/activitypub/ApDbResolverService.ts

@@ -164,6 +164,19 @@ export class ApDbResolverService implements OnApplicationShutdown {
 		};
 	}
 
+	/**
+	 * Miskey User -> Refetched Key
+	 */
+	@bindThis
+	public async refetchPublicKeyForApId(user: MiRemoteUser): Promise<MiUserPublickey | null> {
+		await this.apPersonService.updatePerson(user.uri!);
+		const key = this.userPublickeysRepository.findOneBy({ userId: user.id });
+		if (key != null) {
+			await this.publicKeyByUserIdCache.set(user.id, key);
+		}
+		return key;
+	}
+
 	@bindThis
 	public dispose(): void {
 		this.publicKeyCache.dispose();

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

@@ -104,7 +104,18 @@ export class InboxProcessorService {
 		}
 
 		// HTTP-Signatureの検証
-		const httpSignatureValidated = httpSignature.verifySignature(signature, authUser.key.keyPem);
+		let httpSignatureValidated = httpSignature.verifySignature(signature, authUser.key.keyPem);
+
+		// If signature validation failed, try refetching the actor
+		if (!httpSignatureValidated) {
+			authUser.key = await this.apDbResolverService.refetchPublicKeyForApId(authUser.user);
+
+			if (authUser.key == null) {
+				throw new Bull.UnrecoverableError('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) {