In the operation of a normal Linux system there are many secrets stored on behalf of a user. Wifi passwords, passwords from web sites, etc. Ideally you want them to be quickly and conveniently accessible to the rightful user but also be as difficult as possible for hostile parties to access.
The solution in GNOME and KDE is to have a wallet that is encrypted to store such passwords, the idea is that if a hostile party gets access to a PC that doesn’t use full disk encryption then the secrets will be protected. This is an OK feature. In early versions it required entering a password every time you logged in. The current default mode of operation is to have the login password used to decrypt the wallet which is very convenient.
The problem is the case where the user login password has a scope larger than the local PC, EG a domain login password for Active Directory, Kerberos, or similar systems. In such a case if an attacker gets the encrypted wallet that could facilitate a brute force attack on the password used for domain logins.
I think that a better option for this would be to store wallets in a directory that the user can’t access directly, EG a mode 1770 directory with group “wallet”. Then when logging in a PAM process running as root could open the wallet and pass a file handle to a process running in the context of the user. For access apart from login there could be SETGID programs to manage it which could require authenticating the user’s password before any operation that exports the data so that a vulnerability in a web browser or other Internet facing program can’t just grab the file contents.
Storing the data in a file that needs a SETGID or root owned process to access it doesn’t preclude the possibility of encrypting that file. The same encryption options would be available including encrypting with the login password and unlocking at login time via PAM. The difference is that a brute force attack to discover the login password would first require breaking the security of one of those SETGID programs to get access to the raw data – direct attacks by running the wallet open command repeatedly could be managed by the usual rate limiting mechanisms and logging in the system logs.
The same methods could be used for protecting the secret keys for GPG and SSH which by default are readable by all processes running in the user context and encrypted with a passphrase.
The next issue to consider is where to store such an restricted directory for wallets. Under the user home directory would give the advantage of having the same secrets operate over a network filesystem and not need anything special in backup configuration. Under /var/lib would give the advantage of better isolation from all the less secret (in a cryptographic sense) data in the user home directories.
What do you think?