Protect against replay attacks

1 files changed, 18 insertions, 1 deletions

diff --git a/src/Olm.cpp b/src/Olm.cpp

index 293b12de..e4ab0aa1 100644
--- a/src/Olm.cpp
+++ b/src/Olm.cpp
@@ -1028,7 +1028,8 @@ send_megolm_key_to_device(const std::string &user_id,
 
 DecryptionResult
 decryptEvent(const MegolmSessionIndex &index,
-             const mtx::events::EncryptedEvent<mtx::events::msg::Encrypted> &event)
+             const mtx::events::EncryptedEvent<mtx::events::msg::Encrypted> &event,
+             bool dont_write_db)
 {
         try {
                 if (!cache::client()->inboundMegolmSessionExists(index)) {
@@ -1043,10 +1044,26 @@ decryptEvent(const MegolmSessionIndex &index,
         std::string msg_str;
         try {
                 auto session = cache::client()->getInboundMegolmSession(index);
+                auto sessionData =
+                  cache::client()->getMegolmSessionData(index).value_or(GroupSessionData{});
 
                 auto res =
                   olm::client()->decrypt_group_message(session.get(), event.content.ciphertext);
                 msg_str = std::string((char *)res.data.data(), res.data.size());
+
+                if (!event.event_id.empty() && event.event_id[0] == '$') {
+                        auto oldIdx = sessionData.indices.find(res.message_index);
+                        if (oldIdx != sessionData.indices.end()) {
+                                if (oldIdx->second != event.event_id)
+                                        return {DecryptionErrorCode::ReplayAttack,
+                                                std::nullopt,
+                                                std::nullopt};
+                        } else if (!dont_write_db) {
+                                sessionData.indices[res.message_index] = event.event_id;
+                                cache::client()->saveInboundMegolmSession(
+                                  index, std::move(session), sessionData);
+                        }
+                }
         } catch (const lmdb::error &e) {
                 return {DecryptionErrorCode::DbError, e.what(), std::nullopt};
         } catch (const mtx::crypto::olm_exception &e) {