/* xcrypt.&
 *
 * A simple involutional encryption program.
 *
 * For theory and expanation,
 * Cf. Computer Language Feb 1987 p. 69 (Alan Filipski)
 *
 */
#include <stdio.h>
#include  <stdfun.h> 
#include <string.h>

#define NDELAY 5
#define BYTE( x ) ( (x) & 0xff )

long seed[3];
int p[256], s[256], q[256];


static char _sccsid[] = { " %M% %I% %H% " };

int main( argc, argv )
int argc;
char *argv[];
{
	void shuffle();
	int rand8();

	char *key;
	int i, c, r0, r1, r2, keylen;

	if( argc != 2 )    {
		fprintf( stderr, "Usage: %s key < infile > outfile.\n                   or\n       ... | %s key | ...\n", argv[0], argv[0] );
		exit( 1 );
	}
	if( ( keylen = strlen( key = *++argv ) ) < 4 )    {
		fprintf( stderr, "Key must be at least 4 characters.\n" );
		exit( 1 );
	}

	for( i = 0; i < 4; i++ )    {
		seed[0] = seed[0] << 8 | key[i];
		seed[1] = seed[1] << 8 | key[(keylen - 1) - i];
		seed[2] = seed[2] << 8 | key[(keylen / 2) + i - 2];
	}

	while( *key )
		*key++ = 0;

	for( i = 0; i < 256; i++ )
		p[i] = i;

	shuffle( p, 256, 0 );
	for( i = 0; i < NDELAY; i++ )    {
		shuffle( p, 256, 0 );
		shuffle( p, 256, 1 );
		shuffle( p, 256, 2 );
	}

	for( i = 0; i < 256; i += 2 )
		s[ s[ p[i] ] = p[i + 1] ] = p[i];

	shuffle( p, 256, 1 );
	for( i = 0; i < 256; i++ )
		q[ p[i] ] = i;

	while( ( c = getchar() ) != EOF )    {
		r0 = rand8( 0 );
		r1 = rand8( 1 );
		r2 = rand8( 2 );
		c = (c ^ r2) + r0;
		c = q[BYTE( s[BYTE( p[BYTE( c )] + r1)] - r1)];
		c = (c - r0) ^ r2;
		putchar( (char)c );
	}

	exit( 0 );
	return( 0 );

} /* End main() */

/* Return a pseudo random byte from one of several streams.  The ith stream
 * is seeded by the global long seed[i].
 */
int rand8( i )
int i;
{
	seed[i] = seed[i] * 1103515245L + 12345;
	return( BYTE( seed[i] >> 19 ) );
}

/* Randomly permute the items in the array[size] using the ith random stream
 * from rand8()
 */
void shuffle( a, size, rstream )
int size, *a, rstream;
{
	int rand8();

	register int i, j, temp;

	for( i = size - 1; i >= 0; i-- )    {
		j = rand8( rstream ) % (i + 1);
		temp = a[i];
		a[i] = a[j];
		a[j] = temp;
	}
}