/**
* Decompress Public Key
*
* Accepts a y_byte, 02 or 03 indicating whether the Y coordinate is
* odd or even, and $passpoint, which is simply a hexadecimal X coordinate.
* Using this data, it is possible to deconstruct the original
* uncompressed public key.
*
* @param $key
* @return array|bool
*/
public static function decompress_public_key($key)
{
$math = EccFactory::getAdapter();
$y_byte = substr($key, 0, 2);
$x_coordinate = substr($key, 2);
$x = self::hex_decode($x_coordinate);
$theory = EccFactory::getNumberTheory($math);
$generator = EccFactory::getSecgCurves($math)->generator256k1();
$curve = $generator->getCurve();
try {
$x3 = $math->powmod($x, 3, $curve->getPrime());
$y2 = $math->add($x3, $curve->getB());
$y0 = $theory->squareRootModP($y2, $curve->getPrime());
if ($y0 == null) {
throw new \InvalidArgumentException("Invalid public key");
}
$y1 = $math->sub($curve->getPrime(), $y0);
$y = $y_byte == '02' ? $math->mod($y0, 2) == '0' ? $y0 : $y1 : ($math->mod($y0, 2) !== '0' ? $y0 : $y1);
$y_coordinate = str_pad($math->decHex($y), 64, '0', STR_PAD_LEFT);
$point = $curve->getPoint($x, $y);
} catch (\Exception $e) {
throw new \InvalidArgumentException("Invalid public key");
}
return array('x' => $x_coordinate, 'y' => $y_coordinate, 'point' => $point, 'public_key' => '04' . $x_coordinate . $y_coordinate);
}
