Allow specifying old signing keys with the public key and key ID only

dendrite-sample.monolith.yaml

@@ -18,12 +18,17 @@ global:
   private_key: matrix_key.pem
 
   # The paths and expiry timestamps (as a UNIX timestamp in millisecond precision)
-  # to old signing private keys that were formerly in use on this domain. These
+  # to old signing keys that were formerly in use on this domain name. These
   # keys will not be used for federation request or event signing, but will be
   # provided to any other homeserver that asks when trying to verify old events.
   old_private_keys:
+  #  If the old private key file is available:
   #  - private_key: old_matrix_key.pem
   #    expired_at: 1601024554498
+  #  If only the public key and key ID is known:
+  #  - public_key: mn59Kxfdq9VziYHSBzI7+EDPDcBS2Xl7jeUdiiQcOnM=
+  #    key_id: ed25519:mykeyid
+  #    expired_at: 1601024554498
 
   # How long a remote server can cache our server signing key before requesting it
   # again. Increasing this number will reduce the number of requests made by other

dendrite-sample.polylith.yaml

@@ -18,12 +18,17 @@ global:
   private_key: matrix_key.pem
 
   # The paths and expiry timestamps (as a UNIX timestamp in millisecond precision)
-  # to old signing private keys that were formerly in use on this domain. These
+  # to old signing keys that were formerly in use on this domain name. These
   # keys will not be used for federation request or event signing, but will be
   # provided to any other homeserver that asks when trying to verify old events.
   old_private_keys:
+  #  If the old private key file is available:
   #  - private_key: old_matrix_key.pem
   #    expired_at: 1601024554498
+  #  If only the public key and key ID is known:
+  #  - public_key: mn59Kxfdq9VziYHSBzI7+EDPDcBS2Xl7jeUdiiQcOnM=
+  #    key_id: ed25519:mykeyid
+  #    expired_at: 1601024554498
 
   # How long a remote server can cache our server signing key before requesting it
   # again. Increasing this number will reduce the number of requests made by other

federationapi/routing/keys.go

@@ -160,7 +160,7 @@ func localKeys(cfg *config.FederationAPI, validUntil time.Time) (*gomatrixserver
 	for _, oldVerifyKey := range cfg.Matrix.OldVerifyKeys {
 		keys.OldVerifyKeys[oldVerifyKey.KeyID] = gomatrixserverlib.OldVerifyKey{
 			VerifyKey: gomatrixserverlib.VerifyKey{
-				Key: gomatrixserverlib.Base64Bytes(oldVerifyKey.PrivateKey.Public().(ed25519.PublicKey)),
+				Key: gomatrixserverlib.Base64Bytes(oldVerifyKey.PublicKey),
 			},
 			ExpiredTS: oldVerifyKey.ExpiredAt,
 		}

setup/config/config.go

@@ -231,24 +231,40 @@ func loadConfig(
 		return nil, err
 	}
 
-	for i, oldPrivateKey := range c.Global.OldVerifyKeys {
-		var oldPrivateKeyData []byte
+	for _, key := range c.Global.OldVerifyKeys {
+		switch {
+		case key.KeyID == "":
+			return nil, fmt.Errorf("key ID must be specified for old_verify_keys")
 
-		oldPrivateKeyPath := absPath(basePath, oldPrivateKey.PrivateKeyPath)
-		oldPrivateKeyData, err = readFile(oldPrivateKeyPath)
-		if err != nil {
-			return nil, err
+		case len(key.PublicKey) == ed25519.PublicKeySize:
+			continue
+
+		case len(key.PublicKey) > 0:
+			return nil, fmt.Errorf("the public_key is the wrong length")
+
+		case key.PrivateKeyPath == "":
+			return nil, fmt.Errorf("a private_key path must be specified if public_key isn't")
+
+		default:
+			var oldPrivateKeyData []byte
+			oldPrivateKeyPath := absPath(basePath, key.PrivateKeyPath)
+			oldPrivateKeyData, err = readFile(oldPrivateKeyPath)
+			if err != nil {
+				return nil, fmt.Errorf("failed to read %q: %w", oldPrivateKeyPath, err)
+			}
+
+			// NOTSPEC: Ordinarily we should enforce key ID formatting, but since there are
+			// a number of private keys out there with non-compatible symbols in them due
+			// to lack of validation in Synapse, we won't enforce that for old verify keys.
+			keyID, privateKey, perr := readKeyPEM(oldPrivateKeyPath, oldPrivateKeyData, false)
+			if perr != nil {
+				return nil, fmt.Errorf("failed to parse %q: %w", oldPrivateKeyPath, perr)
+			}
+
+			key.KeyID = keyID
+			key.PrivateKey = privateKey
+			key.PublicKey = privateKey.Public().(ed25519.PublicKey)
 		}
-
-		// NOTSPEC: Ordinarily we should enforce key ID formatting, but since there are
-		// a number of private keys out there with non-compatible symbols in them due
-		// to lack of validation in Synapse, we won't enforce that for old verify keys.
-		keyID, privateKey, perr := readKeyPEM(oldPrivateKeyPath, oldPrivateKeyData, false)
-		if perr != nil {
-			return nil, perr
-		}
-
-		c.Global.OldVerifyKeys[i].KeyID, c.Global.OldVerifyKeys[i].PrivateKey = keyID, privateKey
 	}
 
 	c.MediaAPI.AbsBasePath = Path(absPath(basePath, c.MediaAPI.BasePath))

setup/config/config_global.go

@@ -27,7 +27,7 @@ type Global struct {
 	// Information about old private keys that used to be used to sign requests and
 	// events on this domain. They will not be used but will be advertised to other
 	// servers that ask for them to help verify old events.
-	OldVerifyKeys []OldVerifyKeys `yaml:"old_private_keys"`
+	OldVerifyKeys []*OldVerifyKeys `yaml:"old_private_keys"`
 
 	// How long a remote server can cache our server key for before requesting it again.
 	// Increasing this number will reduce the number of requests made by remote servers
@@ -127,6 +127,9 @@ type OldVerifyKeys struct {
 	// The private key itself.
 	PrivateKey ed25519.PrivateKey `yaml:"-"`
 
+	// The public key, in case only that part is known.
+	PublicKey ed25519.PublicKey `yaml:"public_key"`
+
 	// The key ID of the private key.
 	KeyID gomatrixserverlib.KeyID `yaml:"-"`