Fix decryption error
If you’re seeing Unable to decrypt message or similar errors in a room, here’s a list of possible steps you can take to try to remedy the situation. Note that the instructions are relative to Element, but in principle similar steps can be taken for other clients.
A detailed explanation is available on neko.dev.
We assume the sender’s and recipient’s devices are cross-signed.
If at least one of your devices can read the messages
- Open the device that can read the messages (“device A” thereafter) and navigate to the room that those messages are in.
- Open the device that cannot read the messages (“device B” thereafter) and navigate to the same room.
- The message keys should start coming in a few moments.
If the above doesn’t work
- Navigate to Settings => Security & Privacy on both devices.
- On device A, click “Export E2E room keys”. Follow the instructions.
- Move the resulting key file to device B.
- On device B, click “Import E2E room keys”. Follow the instructions.
If none of your devices can read the messages
- If key backup is enabled for your account, go to Room Settings => Security & Privacy => Restore from Backup.
- While the sender is online, ask them to navigate to the room, then click “resend key requests” on your client (if exists).
Note that if you logged out of every device when you received the message, the message cannot be recovered. (Dehydrated devices will eventually fix this issue, but it will not recover anything that is already lost.)
Prevention
If all above methods are exhausted, then the messages are very unlikely to be recovered. There are, however, a few things that you can do to prevent such annoyances from happening again:
- Unless you understand what it does, have the sender disable “Never send encrypted messages to unverified sessions in this room from this session” (in Room Settings => Security & Privacy), as well as “Never send encrypted messages to unverified sessions from this session” (in User Settings => Security & Privacy).
- On PC Element, have the sender open your profile and show the recipient’s active sessions under “Security”. Check if the recipient’s current session is there. If not, please verify whether the homeservers and clients are working correctly. Note that some experimental features on PC Element (especially Rust Crypto) may interfere with encryption.
- Have the sender enter
/discardsession (on Element; /rotate-megolm-session on Nheko) in the room (unlikely to fix existing issue but could prevent future issues).