From: utzoo!decvax!genrad!john
Newsgroups: net.jokes
Title: Once again, encryption explained.
Article-I.D.: genrad.2345
Posted: Thu Apr 21 14:26:47 1983
Received: Wed Apr 27 07:32:56 1983

/*
 *    Jokes that might be considered offensive are normally transmitted
 * "encrypted" via a rotation 13 algorithm.  Since you must voluntarily
 * "decrypt" the joke to read it, no one can complain about being
 * offended.  If offensive jokes are transmitted in plaintext, then
 * certain sites will refuse to receive or pass on net.jokes.
 * It was suggested that the subject line of encrypted jokes
 * explain the TYPE of offending material, but nobody seems to do that
 * anymore.
 *
 *    The following program is a filter written in C which will encrypt
 * or decrypt jokes.
 */

#include <stdio.h>
main() {
	int c;
	while ((c = getchar()) != EOF) {
		if (c >= 'A' && c <= 'M' || c >= 'a' && c <= 'm')
			c += 13;
		else if (c > 'M' && c <= 'Z' || c > 'm' && c <= 'z')
			c -= 13;
	putchar (c);
	}
}