From dd7d54b43ca55ba54d94140d612c450c4a457824 Mon Sep 17 00:00:00 2001
From: Neil Alexander <neilalexander@users.noreply.github.com>
Date: Fri, 25 Sep 2020 10:01:45 +0100
Subject: [PATCH] Allow configuring old verify keys

---
 federationapi/routing/keys.go    | 12 ++++++++++--
 internal/config/config.go        | 14 ++++++++++++++
 internal/config/config_global.go | 20 ++++++++++++++++++++
 3 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/federationapi/routing/keys.go b/federationapi/routing/keys.go
index 785be0903..17762b03e 100644
--- a/federationapi/routing/keys.go
+++ b/federationapi/routing/keys.go
@@ -136,6 +136,8 @@ func localKeys(cfg *config.FederationAPI, validUntil time.Time) (*gomatrixserver
 	var keys gomatrixserverlib.ServerKeys
 
 	keys.ServerName = cfg.Matrix.ServerName
+	keys.TLSFingerprints = cfg.TLSFingerPrints
+	keys.ValidUntilTS = gomatrixserverlib.AsTimestamp(validUntil)
 
 	publicKey := cfg.Matrix.PrivateKey.Public().(ed25519.PublicKey)
 
@@ -145,9 +147,15 @@ func localKeys(cfg *config.FederationAPI, validUntil time.Time) (*gomatrixserver
 		},
 	}
 
-	keys.TLSFingerprints = cfg.TLSFingerPrints
 	keys.OldVerifyKeys = map[gomatrixserverlib.KeyID]gomatrixserverlib.OldVerifyKey{}
-	keys.ValidUntilTS = gomatrixserverlib.AsTimestamp(validUntil)
+	for _, oldVerifyKey := range cfg.Matrix.OldVerifyKeys {
+		keys.OldVerifyKeys[oldVerifyKey.KeyID] = gomatrixserverlib.OldVerifyKey{
+			VerifyKey: gomatrixserverlib.VerifyKey{
+				Key: gomatrixserverlib.Base64Bytes(oldVerifyKey.PrivateKey),
+			},
+			ExpiredTS: oldVerifyKey.ExpiredAt,
+		}
+	}
 
 	toSign, err := json.Marshal(keys.ServerKeyFields)
 	if err != nil {
diff --git a/internal/config/config.go b/internal/config/config.go
index d75500db5..81935f3fc 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -232,6 +232,20 @@ func loadConfig(
 		return nil, err
 	}
 
+	for _, oldPrivateKey := range c.Global.OldVerifyKeys {
+		var oldPrivateKeyData []byte
+
+		oldPrivateKeyPath := absPath(basePath, oldPrivateKey.PrivateKeyPath)
+		oldPrivateKeyData, err = readFile(oldPrivateKeyPath)
+		if err != nil {
+			return nil, err
+		}
+
+		if oldPrivateKey.KeyID, oldPrivateKey.PrivateKey, err = readKeyPEM(oldPrivateKeyPath, oldPrivateKeyData); err != nil {
+			return nil, err
+		}
+	}
+
 	for _, certPath := range c.FederationAPI.FederationCertificatePaths {
 		absCertPath := absPath(basePath, certPath)
 		var pemData []byte
diff --git a/internal/config/config_global.go b/internal/config/config_global.go
index 03f522be4..d210a3aca 100644
--- a/internal/config/config_global.go
+++ b/internal/config/config_global.go
@@ -22,6 +22,11 @@ type Global struct {
 	// prefix "ed25519:".
 	KeyID gomatrixserverlib.KeyID `yaml:"-"`
 
+	// 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"`
+
 	// 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
 	// for our key, but increases the period a compromised key will be considered valid
@@ -60,6 +65,21 @@ func (c *Global) Verify(configErrs *ConfigErrors, isMonolith bool) {
 	c.Metrics.Verify(configErrs, isMonolith)
 }
 
+type OldVerifyKeys struct {
+	// Path to the private key.
+	PrivateKeyPath Path `yaml:"private_key"`
+
+	// The private key itself.
+	PrivateKey ed25519.PrivateKey `yaml:"-"`
+
+	// The key ID of the private key.
+	KeyID gomatrixserverlib.KeyID `yaml:"-"`
+
+	// When the private key was designed as "expired", as a UNIX timestamp
+	// in millisecond precision.
+	ExpiredAt gomatrixserverlib.Timestamp `yaml:"expired_at"`
+}
+
 // The configuration to use for Prometheus metrics
 type Metrics struct {
 	// Whether or not the metrics are enabled