I am using one ssh account for all my Subversion users. They send me their public keys and I put them in .ssh/authorized_key of the svn account, then they can check out the code from Subversion using ssh tunnel.
So far everything works fine. The problem though is that I want to invalidate keys that have not been used for some time (say one month). Does anyone know a way to make sshd log the public key when a user signs in?