Document restricted key setup for tunnel-only access in README, including creation, configuration, and troubleshooting steps

README.md

@@ -70,6 +70,94 @@ Packaged example: /usr/share/backtunnel/profiles.ini
 
 ---
 
+## 🔒 Temporary, tunnel-only access (restricted key)
+
+By default, if you install a normal SSH key on the sharing machine, that key could also be used for direct SSH (if the server is reachable). To keep access **strictly temporary** and **usable only through the reverse tunnel**, use a **restricted key** in `authorized_keys`.
+
+This approach:
+- **Allows SFTP/sshfs only** (no shell).
+- **Only works via the reverse tunnel** (server sees the client as `127.0.0.1`).
+- **Stops working** automatically when the reverse tunnel (started by `backtunnel-share`) ends.
+
+### 1) Create a dedicated key on the client (access side)
+
+```bash
+ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519_backtunnel -C backtunnel
+```
+
+### 2) Copy the public key to the server via the tunnel once
+
+You’ll be prompted for the server password this one time, while the tunnel is up.
+
+```bash
+ssh-copy-id -p 2222 -i ~/.ssh/id_ed25519_backtunnel.pub user@localhost
+```
+
+### 3) Restrict that key in authorized_keys on the server
+
+Edit the newly added line for this key in ~/.ssh/authorized_keys on the server and prefix it with:
+
+```ini
+from="127.0.0.1",command="internal-sftp",restrict
+```
+
+The final line should look like:
+
+```ini
+from="127.0.0.1",command="internal-sftp",restrict ssh-ed25519 AAAAC3... backtunnel
+```
+
+- from="127.0.0.1" limits use to connections that arrive via the reverse tunnel.
+- command="internal-sftp" forces SFTP only (sshfs uses SFTP).
+- restrict implies no-pty,no-agent-forwarding,no-port-forwarding,no-X11-forwarding.
+
+💡 One-liner (no editor): prepend restrictions while appending your key
+```bash
+( printf 'from="127.0.0.1",command="internal-sftp",restrict '; cat ~/.ssh/id_ed25519_backtunnel.pub ) \
+| ssh -p 2222 user@localhost 'umask 077; mkdir -p ~/.ssh; cat >> ~/.ssh/authorized_keys'
+```
+
+### 4) Use as normal with BackTunnel
+
+Start the share on the server:
+
+```bash
+backtunnel-share /path/to/folder with user@REMOTE for 2h
+```
+
+Mount on the client (no password prompts now):
+
+```bash
+mkdir -p ~/remote-rssh
+backtunnel-access /path/to/folder from user@REMOTE -p 2222 -m ~/remote-rssh
+```
+
+### 5) Cleanup (optional)
+
+After you’re done, remove the restricted key line from ~/.ssh/authorized_keys on the server (or keep it for next time—it's safe: it only works via the tunnel, and only for SFTP).
+
+> If you keep it, the key does not grant shell access and cannot be used over the network directly thanks to from="127.0.0.1".
+
+---
+
+### Troubleshooting
+
+- If sftp -P 2222 user@localhost or sshfs still asks for a password:
+
+    - The restricted key line may be malformed (missing comma or options).
+    - File permissions: ~/.ssh should be 700, authorized_keys should be 600 on the server.
+    - Too many keys tried: you can force the key with:
+
+```bash
+ssh -i ~/.ssh/id_ed25519_backtunnel -p 2222 user@localhost true
+```
+
+- If your mount point is ~-based, don’t quote it (~/remote-rssh is OK; '~/remote-rssh' won’t expand).
+
+If you’d also like a brief “restricted key” note in the man page later, say the word and I’ll hand you a ready-to-paste `.1` section too.
+
+---
+
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