2024-10-31
Pam_unix and your system's supported password algorithms
The Linux login passwords that wind up in /etc/shadow can be
encrypted (well, hashed) with a variety of algorithms, which you
can find listed (and sort of documented) in places like Debian's
crypt(5) manual page.
Generally the choice of which algorithm is used to hash (new)
passwords (for example, when people change them) is determined by
an option to the pam_unix PAM
module.
You might innocently think, as I did, that all of the algorithms your system supports will all be supported by pam_unix, or more exactly will all be available for new passwords (ie, what you or your distribution control with an option to pam_unix). It turns out that this is not the case some of the time (or if it is actually the case, the pam_unix manual page can be inaccurate). This is surprising because pam_unix is the thing that handles hashed passwords (both validating them and changing them), and you'd think its handling of them would be symmetric.
As I found out today, this isn't necessarily so. As documented in the Ubuntu 20.04 crypt(5) manual page, 20.04 supports yescrypt in crypt(3) (sadly Ubuntu's manual page URL doesn't seem to work). This means that the Ubuntu 20.04 pam_unix can (or should) be able to accept yescrypt hashed passwords. However, the Ubuntu 20.04 pam_unix(8) manual page doesn't list yescrypt as one of the available options for hashing new passwords. If you look only at the 20.04 pam_unix manual page, you might (incorrectly) assume that a 20.04 system can't deal with yescrypt based passwords at all.
At one level, this makes sense once you know that pam_unix and crypt(3) come from different packages and handle different parts of the work of checking existing Unix password and hashing new ones. Roughly speaking, pam_unix can delegate checking passwords to crypt(3) without having to care how they're hashed, but to hash a new password with a specific algorithm it has to know about the algorithm, have a specific PAM option added for it, and call some functions in the right way. It's quite possible for crypt(3) to get ahead of pam_unix for a new password hashing algorithm, like yescrypt.
(Since they're separate packages, pam_unix may not want to implement this for a new algorithm until a crypt(3) that supports it is at least released, and then pam_unix itself will need a new release. And I don't know if linux-pam can detect whether or not yescrypt is supported by crypt(3) at build time (or at runtime).)
PS: If you have an environment with a shared set of accounts and passwords (whether via LDAP or your own custom mechanism) and a mixture of Ubuntu versions (maybe also with other Linux distribution versions), you may want to be careful about using new password hashing schemes, even once it's supported by pam_unix on your main systems. The older some of your Linuxes are, the more you'll want to check their crypt(3) and crypt(5) manual pages carefully.