RFC 9421 signatures are verified, not refused

WordPress's ActivityPub plugin (and Ghost and Fedify) sign with RFC 9421
first and fall back to draft-cavage only after a refusal, so each first
delivery cost two requests and a 401 in our statistics. Now a request
carrying Signature-Input is verified as an HTTP message signature: its
covered components (the method and our own public target, the body's
Content-Digest), its created and expires, with the actor's RSA key under
PKCS#1 v1.5 or PSS. Deliveries and signed fetches both take it; the
ledger names the scheme (rfc9421:rsa-v1_5-sha256). What PrivaPub sends
stays draft-cavage, which every server reads. Ed25519 waits for FEP-521a
keys.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub.Tests/Support/RemoteActor.cs

@@ -68,6 +68,22 @@ namespace PrivaPub.Tests.Support
 			return request;
 		}
 
+		// the same delivery signed as RFC 9421 does it (WordPress, Ghost, Fedify), for `signedFor` when it is not where the
+		// request goes
+		public HttpRequestMessage MessageSignedPost(string path, JsonNode activity, string signedFor = default, string origin = "https://privapub.test")
+		{
+			var body = Encoding.UTF8.GetBytes(activity.ToJsonString());
+			var request = new HttpRequestMessage(HttpMethod.Post, new Uri(origin + (signedFor ?? path))) { Content = new ByteArrayContent(body) };
+			request.Content.Headers.TryAddWithoutValidation("Content-Type", "application/activity+json");
+			MessageSignatures.Sign(request, KeyId, _key.ExportPkcs8PrivateKeyPem(), body);
+			request.RequestUri = new Uri(origin + path);
+			return request;
+		}
+
+		public string PrivateKeyPem => _key.ExportPkcs8PrivateKeyPem();
+
+		public string PublicKeyPem => _key.ExportSubjectPublicKeyInfoPem();
+
 		string Signature(string signingString, string headers)
 		{
 			var signature = Convert.ToBase64String(_key.SignData(Encoding.UTF8.GetBytes(signingString), HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1));