Check username/password in Linux without root

Question

If I have a username and password pair how can I verify that they are actually correct in a Linux system? I know I can use passwd to do so but I want to do it programatically using C.

I should not require root privileges (so reading the shadow file is not an option).

Thank you.

Answer 1

If you are using a PAM, you might be able to make use of checkpassword-pam.

The manual has an example command (with debugging) which should give you a good place to start.

echo -e "username\0password\0timestamp\0" \
    | checkpassword-pam -s SERVICE \
    --debug --stdout -- /usr/bin/id 3<&0