From 974f7dc5446f25090b5cf35f53579a5bdd437d58 Mon Sep 17 00:00:00 2001
From: Tulir Asokan <tulir@maunium.net>
Date: Thu, 19 Feb 2026 14:10:20 +0200
Subject: [PATCH] crypto/decryptmegolm: allow device key mismatches, but mark
 as untrusted

---
 crypto/decryptmegolm.go | 8 +++++++-
 id/trust.go             | 7 ++++++-
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/crypto/decryptmegolm.go b/crypto/decryptmegolm.go
index 77a64b1e..9753eabd 100644
--- a/crypto/decryptmegolm.go
+++ b/crypto/decryptmegolm.go
@@ -124,7 +124,13 @@ func (mach *OlmMachine) DecryptMegolmEvent(ctx context.Context, evt *event.Event
 					Msg("Couldn't resolve trust level of session: sent by unknown device")
 				trustLevel = id.TrustStateUnknownDevice
 			} else if device.SigningKey != sess.SigningKey || device.IdentityKey != sess.SenderKey {
-				return nil, ErrDeviceKeyMismatch
+				log.Debug().
+					Stringer("session_sender_key", sess.SenderKey).
+					Stringer("device_sender_key", device.IdentityKey).
+					Stringer("session_signing_key", sess.SigningKey).
+					Stringer("device_signing_key", device.SigningKey).
+					Msg("Device keys don't match keys in session, marking as untrusted")
+				trustLevel = id.TrustStateDeviceKeyMismatch
 			} else {
 				trustLevel, err = mach.ResolveTrustContext(ctx, device)
 				if err != nil {
diff --git a/id/trust.go b/id/trust.go
index 04f6e36b..6255093e 100644
--- a/id/trust.go
+++ b/id/trust.go
@@ -16,6 +16,7 @@ type TrustState int
 
 const (
 	TrustStateBlacklisted          TrustState = -100
+	TrustStateDeviceKeyMismatch    TrustState = -5
 	TrustStateUnset                TrustState = 0
 	TrustStateUnknownDevice        TrustState = 10
 	TrustStateForwarded            TrustState = 20
@@ -23,7 +24,7 @@ const (
 	TrustStateCrossSignedTOFU      TrustState = 100
 	TrustStateCrossSignedVerified  TrustState = 200
 	TrustStateVerified             TrustState = 300
-	TrustStateInvalid              TrustState = (1 << 31) - 1
+	TrustStateInvalid              TrustState = -2147483647
 )
 
 func (ts *TrustState) UnmarshalText(data []byte) error {
@@ -44,6 +45,8 @@ func ParseTrustState(val string) TrustState {
 	switch strings.ToLower(val) {
 	case "blacklisted":
 		return TrustStateBlacklisted
+	case "device-key-mismatch":
+		return TrustStateDeviceKeyMismatch
 	case "unverified":
 		return TrustStateUnset
 	case "cross-signed-untrusted":
@@ -67,6 +70,8 @@ func (ts TrustState) String() string {
 	switch ts {
 	case TrustStateBlacklisted:
 		return "blacklisted"
+	case TrustStateDeviceKeyMismatch:
+		return "device-key-mismatch"
 	case TrustStateUnset:
 		return "unverified"
 	case TrustStateCrossSignedUntrusted: