What can be used to link UNIX authentication with other mechanisms?

SYSKEY was a Windows NT/2000/XP feature for encrypting the SAM database, not a UNIX authentication linking mechanism.

PAM provides a modular, configurable authentication framework on UNIX and Linux systems that allows administrators to plug in multiple authentication backends - such as LDAP, Kerberos, or one-time password systems - without modifying individual applications. This makes it the definitive mechanism for linking UNIX authentication with other systems, as each module can be stacked and configured independently per service.

S/KEY is a one-time password (OTP) authentication scheme, not a framework for linking or extending UNIX authentication to other mechanisms.

Shadow refers to the /etc/shadow file that stores hashed user passwords on UNIX systems, not a mechanism for integrating authentication with external systems.