From 080ad4c0a0e8b81d243f82ce206bf656cdf0a6fe Mon Sep 17 00:00:00 2001
From: Toni Spets <toni.spets@beeper.com>
Date: Tue, 14 Oct 2025 13:32:02 +0300
Subject: [PATCH] crypto: Allow decrypting message content without event id or
 ts

Replay attack prevention shouldn't store empty event id or ts to
database if we're decrypting without them. This may happen if we are
looking into a future delayed event for example as it doesn't yet have
those.

We still prevent doing that if we already know them meaning we have
gotten the actual event through sync as that's also when a delayed event
would move from scheduled to finalised and then it also contains those
fields.
---
 crypto/sql_store.go  | 14 ++++++++++++++
 crypto/store.go      |  3 +++
 crypto/store_test.go | 12 +++++++++++-
 3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/crypto/sql_store.go b/crypto/sql_store.go
index 13940d79..ca75b3f6 100644
--- a/crypto/sql_store.go
+++ b/crypto/sql_store.go
@@ -664,6 +664,20 @@ func (store *SQLCryptoStore) IsOutboundGroupSessionShared(ctx context.Context, u
 // ValidateMessageIndex returns whether the given event information match the ones stored in the database
 // for the given sender key, session ID and index. If the index hasn't been stored, this will store it.
 func (store *SQLCryptoStore) ValidateMessageIndex(ctx context.Context, senderKey id.SenderKey, sessionID id.SessionID, eventID id.EventID, index uint, timestamp int64) (bool, error) {
+	if eventID == "" && timestamp == 0 {
+		var notOK bool
+		const validateEmptyQuery = `
+		SELECT EXISTS(SELECT 1 FROM crypto_message_index WHERE sender_key=$1 AND session_id=$2 AND "index"=$3)
+		`
+		err := store.DB.QueryRow(ctx, validateEmptyQuery, senderKey, sessionID, index).Scan(&notOK)
+		if notOK {
+			zerolog.Ctx(ctx).Debug().
+				Uint("message_index", index).
+				Msg("Rejecting event without event ID and timestamp due to already knowing them")
+		}
+		return !notOK, err
+	}
+
 	const validateQuery = `
 	INSERT INTO crypto_message_index (sender_key, session_id, "index", event_id, timestamp)
 	VALUES ($1, $2, $3, $4, $5)
diff --git a/crypto/store.go b/crypto/store.go
index 8b7c0a96..7620cf35 100644
--- a/crypto/store.go
+++ b/crypto/store.go
@@ -525,6 +525,9 @@ func (gs *MemoryStore) ValidateMessageIndex(_ context.Context, senderKey id.Send
 	}
 	val, ok := gs.MessageIndices[key]
 	if !ok {
+		if eventID == "" && timestamp == 0 {
+			return true, nil
+		}
 		gs.MessageIndices[key] = messageIndexValue{
 			EventID:   eventID,
 			Timestamp: timestamp,
diff --git a/crypto/store_test.go b/crypto/store_test.go
index 8aeae7af..7a47243e 100644
--- a/crypto/store_test.go
+++ b/crypto/store_test.go
@@ -75,8 +75,13 @@ func TestValidateMessageIndex(t *testing.T) {
 		t.Run(storeName, func(t *testing.T) {
 			acc := NewOlmAccount()
 
+			// Validating without event ID and timestamp before we have them should work
+			ok, err := store.ValidateMessageIndex(context.TODO(), acc.IdentityKey(), "sess1", "", 0, 0)
+			require.NoError(t, err, "Error validating message index")
+			assert.True(t, ok, "First message validation should be valid")
+
 			// First message should validate successfully
-			ok, err := store.ValidateMessageIndex(context.TODO(), acc.IdentityKey(), "sess1", "event1", 0, 1000)
+			ok, err = store.ValidateMessageIndex(context.TODO(), acc.IdentityKey(), "sess1", "event1", 0, 1000)
 			require.NoError(t, err, "Error validating message index")
 			assert.True(t, ok, "First message validation should be valid")
 
@@ -94,6 +99,11 @@ func TestValidateMessageIndex(t *testing.T) {
 			ok, err = store.ValidateMessageIndex(context.TODO(), acc.IdentityKey(), "sess1", "event1", 0, 1000)
 			require.NoError(t, err, "Error validating message index")
 			assert.True(t, ok, "First message validation should be valid")
+
+			// Validating without event ID and timestamp must fail if we already know them
+			ok, err = store.ValidateMessageIndex(context.TODO(), acc.IdentityKey(), "sess1", "", 0, 0)
+			require.NoError(t, err, "Error validating message index")
+			assert.False(t, ok, "First message validation should be invalid")
 		})
 	}
 }