You want to encrypt a string.
You have private data and want to encrypt it for later decryption.
$encrypted = Crypt::encrypt($value);
You must later decrypt the value with the same key used to encrypt it.
Laravel's encryption routines use
Config::get('app.key') for encryption. This happens internally. Since this value is different for every Laravel application then a value encrypted by the application should only be decrypt with the same application.
The application can call
Crypt::setKey() prior to encrypting. The same key must later be used to decrypt the value. See Setting the Encryption Key.