PHP Cryptographic Decryption String Example

Encryption and decryption is a very common requirement in a system, and PHP does the same for Web sites that are often used by PHP encryption to decrypt string functions.

Included some of the more classic PHP encryption and decryption function code, share to everyone. Encryption and decryption principle is generally through a certain encryption and decryption algorithm, the key is added to the algorithm, and finally get the encryption and decryption results.

function Authcode ($string, $operation = ' DECODE ', $key = ', $expiry = 0) {//dynamic key length, the same plaintext will generate different ciphertext is dependent on the dynamic key $ckey _lengt   h = 4;    Key $key = MD5 ($key? $key: $GLOBALS [' Discuz_auth_key ']);   Key A will participate in the 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)): ";   Key to participate in the operation $cryptkey = $keya. MD5 ($keya. $KEYC);   $key _length = strlen ($cryptkey); PlainText, the first 10 bits are used to hold the timestamp, verify data validity when decrypting, 10 to 26 bits to save $keyb (key B),//decryption will verify data integrity through this key///If it is decoded, it will start from the $ckey_length bit, because the ciphertext before $ckey_ The length bit holds the dynamic key to ensure that the decryption is correct $string = $operation = = ' DECODE '? Base64_decode (substr ($string, $ckey _length)): sprintf ('%010d ', $expiry? $expiry + Time (): 0). substr (MD5 ($string. $keyb   ), 0, (+). $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% $key _length]); }//using a fixed algorithm to disrupt the key book, adding randomness, as if complex, actually does not increase the intensity of the ciphertext for ($j = $i = 0; $i < $i) {$j = ($j + $box [$i] + $rndkey [$i    ])% 256;    $tmp = $box [$i];    $box [$i] = $box [$j];   $box [$j] = $tmp;    }//Core plus decryption section for ($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;   The key is derived from the key book and then converted into a character $result. = Chr (ord ($string [$i]) ^ ($box [($box [$a] + $box [$j])% 256])); if ($operation = = ' DECODE ') {//Verify data validation, see the format of unencrypted plaintext if (substr ($result, 0, ten) = = 0 | | substr ($result, 0, ten)-t IME () > 0) && substr ($result, ten, +) = = substr (MD5 ($result, $keyb), 0, +)) {return substr ($r    Esult, 26);    } else {return '; }} else {//To keep the dynamic key in ciphertext, which is why the same plaintext, the production of different ciphertext can be decrypted after the reason//because the encrypted ciphertext may be some special characters, the copy process may be lost, so the Base64 code return $KEYC. str   _replace (' = ', ' ', Base64_encode ($result));  }}