1   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.

2   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:

  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!

3   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

4   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.

5   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/.