You want to decrypt a string.
You've encrypted a string and are ready to decrypt and use it.
$value = Crypt::decrypt($encrypted);
You must 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 the application that encrypts a value must also decrypt the value.
The application must call
Crypt::setKey() prior to decrypting to match the key to the value used for encrypting. See Setting the Encryption Key.