Principle analysis of PHP encryption function in discuz program

// Parameter explanation
// $string: plain text or cipher text
// $operation: DECODE means decryption, others means encryption
// $key: key
// $expiry: ciphertext validity period
function authcode($string, $operation = 'DECODE', $key = '', $expiry = 0) {
// Dynamic key length, the same plaintext will generate different passwords The article relies on dynamic keys
$ckey_length = 4;
// Key
$key = md5($key ? $key : $GLOBALS['discuz_auth_key']);
// Key a will participate in encryption and decryption
$keya = md5(substr($key, 0, 16));
// Key b will be used for data integrity verification
$keyb = md5(substr($key, 16, 16));
// Key c is used to change the generated ciphertext
$keyc = $ckey_length ? ($operation == 'DECODE' ? substr($string, 0, $ckey_length):
　substr(md5(microtime()), -$ckey_length)) : '';
// The key involved in the operation
$cryptkey = $keya.md5($keya.$keyc);
$key_length = strlen($cryptkey);
// Plain text, the first 10 bits are used to save the timestamp and verify the data validity during decryption. Bits 10 to 26 are used to save $keyb (key b). This key will be used during decryption. Verify data integrity
// If decoding, it will start from the $ckey_length bit, because the dynamic key is stored in the $ckey_length bit before the ciphertext to ensure correct decryption
$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);
$rndkey = array();
// Generate key book
for($i = 0 ; $i <= 255; $i++) {
$rndkey[$i] = ord($cryptkey[$i % $key_length]);
}
//Use a fixed algorithm to scramble the key book and increase Randomness seems very complicated, but in fact it does not increase the strength of the ciphertext
for($j = $i = 0; $i < 256; $i++) {
$j = ($j + $box[ $i] + $rndkey[$i]) % 256;
$tmp = $box[$i];
$box[$i] = $box[$j];
$box[$j] = $tmp ;
}
// Core encryption and decryption part
for($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;
// Get the key from the key book, perform XOR, and then convert it into characters
$result .= chr(ord($string[$i]) ^ ($box[($box[$a] + $box[$j]) % 256]));
}
if($operation == 'DECODE') {
// substr($result, 0, 10) == 0 Verify data validity
// substr($result, 0, 10) - time() > 0 Verify data validity
// substr($result, 10, 16) == substr(md5(substr($result, 26).$keyb), 0, 16) Verify data integrity
// Verify data validity, please see the format of unencrypted plaintext
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 {
// Save the dynamic key in the ciphertext. This is why the same plaintext can be decrypted after producing different ciphertexts
// Because the encrypted ciphertext may contain some special characters and may be lost during the copying process, it is encoded with base64
return $keyc.str_replace('=', '', base64_encode($result));
}
} //edit bbs.it-home.org
?>> .
Reminder: The ownership of this function belongs to Kangsheng Chuangxiang and cannot be used freely, haha.