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<h2 class="chapter">32 DES Encryption and Password Handling</h2>

<!-- %MENU% DES encryption and password handling -->
<p>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.

   <p>Sometimes, however, it is necessary to be sure that a user is authorized
to use some service a machine provides&mdash;for instance, to log in as a
particular user id (see <a href="Users-and-Groups.html#Users-and-Groups">Users and Groups</a>).  One traditional way of
doing this is for each user to choose a secret <dfn>password</dfn>; 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.

   <p>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 <dfn>one-way function</dfn>, a function which makes it difficult to
work out what its input was by looking at its output, before storing in
the file.

   <p>The GNU C library provides a one-way function that is compatible with
the behavior of the <code>crypt</code> function introduced in FreeBSD 2.0. 
It supports two one-way algorithms: one based on the MD5
message-digest algorithm that is compatible with modern BSD systems,
and the other based on the Data Encryption Standard (DES) that is
compatible with Unix systems.

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

<ul class="menu">
<li><a accesskey="1" href="Legal-Problems.html#Legal-Problems">Legal Problems</a>:               This software can get you locked up, or worse. 
<li><a accesskey="2" href="getpass.html#getpass">getpass</a>:                      Prompting the user for a password. 
<li><a accesskey="3" href="crypt.html#crypt">crypt</a>:                        A one-way function for passwords. 
<li><a accesskey="4" href="DES-Encryption.html#DES-Encryption">DES Encryption</a>:               Routines for DES encryption. 
</ul>

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