passwd \- change login password
This command changes (or installs) a password
(your own name by default).
The program prompts for the old password and then for the new one.
The caller must supply both.
The new password must be typed twice, to forestall mistakes.
New passwords must be at least four characters long if they use
a sufficiently rich alphabet and at least six characters long
These rules are relaxed if you are insistent enough.
Only the owner of the name or the super-user may change a password;
the owner must prove he knows the old password.
login(1), passwd(5), crypt(3)
Robert Morris and Ken Thompson,
.I UNIX password security
The password file information should be kept in a different data structure
would probably be suitable.