Super simple LFSR cipher in C

cipher.c

#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>

uint64_t sr;

uint8_t shift( )
{
	//Taps at 64, 63, 61, 60
	uint8_t shiftin = 0;
	shiftin ^= ( ( sr & ( 1ULL << 0 ) ) >> 0 );
	shiftin ^= ( ( sr & ( 1ULL << 1 ) ) >> 1 );
	shiftin ^= ( ( sr & ( 1ULL << 60 ) ) >> 60 );
	shiftin ^= ( ( sr & ( 1ULL << 59 ) ) >> 59 );
	sr = ( sr << 1 ) | ( shiftin );
	return sr & 0xff;
}

int main( int argc, char **argv )
{
	char c;
		
	if ( argc < 2 )
	{
		printf( "Please specify cipher integer!\n" );
		return 1;
	}
	
	sr = atoll( argv[1] );
	
	while ( ( c = getchar( ) ) != EOF )
		putchar( c ^ shift( ) );
	
	return 0;
}