问题:

I have a problem with CBC mode when I try to encrypt/decrypt some text using php's mcrypt extension. I've created a class to perform this operations, it works fine with other modes but CBC.

The problem is as follow:

I use the clear text Even in cryptography, silence is golden. I do the encryption part, no problem till this point. But each time I try to decrypt, I get something like this: . As you can see, the first 8 characters of the text are wrong. I don't know what may be causing this behavior.

The parts of my class which handle these operations are:

public function encrypt($data)     {         $cypher = $this->_getCypher();         $iv = $this->_getIv($cypher);          return trim(base64_encode(mcrypt_encrypt($cypher, self::KEY, $data, MCRYPT_MODE_CBC, $iv)));     }      public function decrypt($data)     {         $cypher = $this->_getCypher();         $iv = $this->_getIv($cypher);          return trim(mcrypt_decrypt($cypher, self::KEY, base64_decode($data), MCRYPT_MODE_CBC, $iv));     }      protected function _getCypher()     {         return self::$_cyphers[$this->_algorithm];     }      protected function _getIv($cypher)     {         return mcrypt_create_iv(mcrypt_get_iv_size($cypher, MCRYPT_MODE_CBC), MCRYPT_RAND);     }

And the algorithm used for above example is 3DES. As I said before, using other mode, such as ECB, everything works fine.

Any suggestions ?

回答1:

You need to remember the IV that you used for encryption and use that again for decryption.

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