求一个PHP函数的Python版本

function _authcode($string, $operation = 'DECODE', $key = '', $expiry = 0) {

	$ckey_length = 4;



	$key = md5($key ? $key : UC_KEY);

	$keya = md5(substr($key, 0, 16));

	$keyb = md5(substr($key, 16, 16));

	$keyc = $ckey_length ? ($operation == 'DECODE' ? substr($string, 0, $ckey_length): substr(md5(microtime()), -$ckey_length)) : '';



	$cryptkey = $keya.md5($keya.$keyc);

	$key_length = strlen($cryptkey);



	$string = $operation == 'DECODE' ? base64_decode(substr($string, $ckey_length)) : sprintf('%010d', $expiry ? $expiry + time() : 0).substr(md5($string.$keyb), 0, 16).$string;

	$string_length = strlen($string);



	$result = '';

	$box = range(0, 255);



	$rndkey = array();

	for($i = 0; $i <= 255; $i++) {

		$rndkey[$i] = ord($cryptkey[$i % $key_length]);

	}



	for($j = $i = 0; $i < 256; $i++) {

		$j = ($j + $box[$i] + $rndkey[$i]) % 256;

		$tmp = $box[$i];

		$box[$i] = $box[$j];

		$box[$j] = $tmp;

	}



	for($a = $j = $i = 0; $i < $string_length; $i++) {

		$a = ($a + 1) % 256;

		$j = ($j + $box[$a]) % 256;

		$tmp = $box[$a];

		$box[$a] = $box[$j];

		$box[$j] = $tmp;

		$result .= chr(ord($string[$i]) ^ ($box[($box[$a] + $box[$j]) % 256]));

	}



	if($operation == 'DECODE') {

		if((substr($result, 0, 10) == 0 || substr($result, 0, 10) - time() > 0) && substr($result, 10, 16) == substr(md5(substr($result, 26).$keyb), 0, 16)) {

			return substr($result, 26);

		} else {

				return '';

			}

	} else {

		return $keyc.str_replace('=', '', base64_encode($result));

	}



}