"User authentication has always been a problem. Build it into a program, and it's hard to change. Leave it out, and you have no security at all. Now there's an alternative: PAM, or Pluggable Authentication Modules.
PAM provides an interface that programs can use to connect to whatever authentication methods are desired. Authentication can be as trivial as the user typing "hello world", as complex as biometrics, or as prosaic as passwords.
PAM isn't the only way to do this. GSS-API, defined in RFCs 1508 and 1509, is a similar authentication interface protocol. Kerberos, SSh, and Heimdal are different, and focus on securing the authentication process itself rather than linking applications to authentication modules."
