I have found some minor issues with automatic ticket renewal while
If random ccache file names are used the name of the ccache file should
be kept if the user is not logged in, but the TGT is still renewable.
If a user logs in and out repeatedly and random names are used a new
hash entry is created for every new ticket. The old entries just eat
away some memory because the related ccache file is already deleted.
Using the user name as the hash key solves this, because currently sssd
(and MIT Kerberos) support one ccache per user.