/* 
 * This is a simple program to scramble a file to prevent casual users
 * from grabbing test versions
 */
#include <stdio.h>

/*
 * rcode key < infile > outfile
 */
int main( argc, argv )
int argc;
char **argv;
{
    unsigned char mapping[256], mask, block[4096];
    char          *key;
    int i, k, nread;

    /* Get the key from input */
    if (argc != 2) {
	fprintf( stderr, "Key required\n" );
	exit( 1 );
    }
    
    /* Fill the mapping from the key.  Note that the key only has
       a subrange of set bits */
    key = argv[1];
    for (i=0; i<256; i++) {
	if (!*key) key = argv[1];
	mask = *key++ - ' ';
	if (i & mask) mask |= 0xa0;
	mapping[i] = mask;
    }

    /* Read stdin, xor with rotating mapping, and write to stdout */
    k = 0;
    while ((nread = fread( block, 1, 4096, stdin )) > 0) {
	
	for (i=0; i<nread; i++) {
	    block[i] = block[i] ^ mapping[k++];
	    if (k == 256) k = 0;
	}
	fwrite( block, 1, nread, stdout );
    }

    return 0;
}

.