Remembering old passwords (was 60-second timeout in Unix login)

[Daniel R. Levy]

In article <18083 at topaz.rutgers.edu>, ron at topaz.rutgers.edu (Ron Natalie) writes:
> Actually at BRL, it remembers all past passwords that everyone used and
> won't ever let you reuse them (or use the "passwd" program to set too
> accounts to the same password).

How is this implemented without saving passwords somewhere in the clear?
Also -- if "passwd" unexpectedly refuses to let a user set a proposed password
he has chosen, it would be a tipoff that he has stumbled over somebody else's
current password.
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