Authcode function code of a reversible encryption function in UCenter

When you browse the UCenter source code, you will find this function. if necessary, record it. The code is as follows:

Function authcode ($ string, $ operation = 'Decode', $ key = '', $ expiry = 0 ){

$ Ckey_length = 4; // The random key length ranges from 0 to 32;
// Adding a random key can make the ciphertext irregular. even if the original text and the key are identical, the encryption results will be different each time, increasing the difficulty of cracking.
// The larger the value, the larger the ciphertext change law. The ciphertext change is equal to the power of $ ckey_length of 16.
// When this value is 0, no random key is generated

$ 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 (1, 0,255 );

$ Rndkey = array ();
For ($ I = 0; $ I <= 255; $ I ++ ){
$ Rndkey [$ I] = ord ($ cryptkey [$ I % $ key_length]);
}

For ($ j = $ I = 0; I 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 ));
}
}
/*************************************** ************************************/
$ String = authcode ("Hello", "ENCODE", "HTML_TCCJ_AUTH ");
Echo $ string ,'
';
Echo authcode ($ string, "DECODE", "HTML_TCCJ_AUTH "),'
';