===========
 CryptServ
===========

Deadline: October 24th, 2014.

.. sectnum::

Instructions
============

You must implement a program ``cryptserv`` working as follows.

``cryptserv`` must takes two command-line arguments; a numeric
encryption/decryption key, and a filesystem path to use as Unix domain
socket (``PF_UNIX/AF_UNIX``).  It must then listen for connections on
that socket. When a client connects, ``cryptserv`` must
encrypt/decrypt the received data stream from that client (cf
Cryptography_ below) and send the results back to the client. The
program must support exchanging data with multiple clients
simultaneously.

For extra credits you may implement the following:

- Detaching as a daemon when the command-line option ``-d`` is specified.
- Non-blocking I/O: communication
  with other clients can go on even if a client is blocked.
- Support for listening to multiple sockets/addresses.

Constraints:

- You must only depend on standard C functions (either from ISO C 1999/2011 or
  POSIX).
- You must not use ``system`` or any other mechanism that invokes an
  external program.

Cryptography
============

The encryption (or decryption) of the data stream from one client  must
use the following algorithm:

- first the C library's random generator state must be initialized
  (``initstate``) using the key as initial seed and
  a seed array of 256 bytes.
- each successive byte in the input stream must be XORed with the lowest
  8 bits of successive calls to ``random``:

  .. code:: c

      char buf[256];
      initstate(key, buf, 256);
      setstate(buf);
      out[0] = inp[0] ^ (random() & 0xff);
      out[1] = inp[1] ^ (random() & 0xff);
      // ...

Be sure that each client connection uses its own random state!

Example use
===========

The following examples uses Netcat (``nc``), a quasi-standard “swiss army” network tool:

::

   # in one session
   $ ./cryptserv 0xdeadbeef /tmp/mysock

   # in another session
   $ echo hello >msg.txt
   $ nc -U /tmp/mysock <msg.txt >msg.enc
   $ nc -U /tmp/mysock <msg.enc >msg2.txt
   $ cmp msg.txt msg.enc || echo OK
   $ cmp msg.txt msg2.txt && echo OK

   # in yet another session
   $ nc -U /tmp/mysock </dev/urandom >/dev/null &
   $ echo hello | nc -U /tmp/mysock && echo OK

Grading
=======

- 4 points if ``cryptserv`` works with 1 client at a time, in constant memory space.
- +2 points if ``cryptserv`` works with any number of clients simultaneously, in
  memory space linear with the number of clients connected.
- +1 point if all of the above, and the program detaches as a daemon properly if ``-d`` is given.
- +1 points if non-blocking I/O is properly implemented.
- +1 points if the program can listen to multiple sockets/addresses
  (explain your command-line syntax in an accompanying ``README`` file.)
- +1 point if an enclosed ``README`` file contains an explanation that
  properly identifies the encryption algorithm used in this assignment
  and suggests simple improvements to make it more secure.

----

Copyright and licensing
=======================

Copyright © 2014, Raphael ‘kena’ Poss.  Permission is granted to
distribute, reuse and modify this document and other documents for the
Systems Programming course by the same author according to the terms
of the Creative Commons Attribution-ShareAlike 4.0 International
License.  To view a copy of this license, visit
`http://creativecommons.org/licenses/by-sa/4.0/
<http://creativecommons.org/licenses/by-sa/4.0/>`_.