XOR

<?php
if (PHP_SAPI !== 'cli') exit();
if ($argc !== 3 || in_array($argv[1], ['--help', '-h'])) {
  echo "Usage: {$argv[0]} INFILE OUTFILE", PHP_EOL;
  exit();
}
$in = fopen($argv[1], 'r');
$out = fopen($argv[2], 'w');
$m = 0x80;
for ( ; false !== ($c = fgetc($in)); $m++) {
  if ($m == 0xff) $m = 0x80;
  fwrite($out, chr(ord($c) ^ $m));
}