Invalidating unused ssh keys

Solution 1:

Everything is documented in sshd(8), under "AUTHORIZED_KEYS FILE FORMAT" section.

In .ssh/authorized_keys2 add something like environment="SSHKEY=1" at beginning of each line, so it should looks like:

environment="SSHKEY=1" ssh-dss AAAAB3N ...
environment="SSHKEY=2" ssh-rsa AAAAB3N ...

Enable PermitUserEnvironment option in /etc/ssh/sshd_config and restart sshd. Now you can add something like echo $SSHKEY >>.sshlog to ~/.bashrc for logging used ssh keys.

But I think much ease way is backup authorized_keys2 file, remove all keys from it, and just wait until people call/email/im you asking why svn doesn't work. Then you can either restore their key or ask them to resend their keys to you if you unsure which key belong to who. As side effect you'll know who is really working. :)