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.
Basic checks
- 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 your active sessions under “Security”. Check if your current session is there. If not, please verify whether your homeserver is working correctly.
- If you logged out of every device when you received the message, the message cannot be recovered, for now. (Dehydrated devices will eventually fix this issue)
If none of the above applies to you, continue reading.
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.
For all other situations
Try the following in no specific order:
- 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 device.
- Enter
/discardsession in the room (unlikely to fix existing issue but could prevent future issues).
If you have exhausted all the steps and they don’t produce improvements on future messages either, it may very well be an issue regarding your homeserver, or a protocol bug.