The tcb package consists of three components: pam_tcb, libnss_tcb, and
libtcb. pam_tcb is a PAM module which supersedes pam_unix and pam_pwdb.
It also implements the tcb password shadowing scheme (see tcb(5) for
details). The tcb scheme allows many core system utilities (passwd(1)
being the primary example) to operate with little privilege. libnss_tcb
is the accompanying NSS module. libtcb contains code shared by the
PAM and NSS modules and is also used by programs from the shadow-utils
package.