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(libc.info)Cryptographic Functions

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DES Encryption and Password Handling
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   On many systems, it is unnecessary to have any kind of user
authentication; for instance, a workstation which is not connected to a
network probably does not need any user authentication, because to use
the machine an intruder must have physical access.

   Sometimes, however, it is necessary to be sure that a user is
authorized to use some service a machine provides--for instance, to log
in as a particular user id (Note: Users and Groups).  One traditional
way of doing this is for each user to choose a secret "password"; then,
the system can ask someone claiming to be a user what the user's
password is, and if the person gives the correct password then the
system can grant the appropriate privileges.

   If all the passwords are just stored in a file somewhere, then this
file has to be very carefully protected.  To avoid this, passwords are
run through a "one-way function", a function which makes it difficult to
work out what its input was by looking at its output, before storing in
the file.

   The GNU C library already provides a one-way function based on MD5
and for compatibility with Unix systems the standard one-way function
based on the Data Encryption Standard.

   It also provides support for Secure RPC, and some library functions
that can be used to perform normal DES encryption.

Legal Problems

This software can get you locked up, or worse.

getpass

Prompting the user for a password.

crypt

A one-way function for UNIX passwords.

DES Encryption

Routines for DES encryption.