How to debug/investigate corrupted encrypted data?

116
October 18, 2018, at 4:40 PM

I've got a code base which stores some sensitive data in the database. Before I store the data in the DB I encrypt the data using this Crypto library (docs here).

To decrypt it I use the following

use \Defuse\Crypto\Crypto;
use \Defuse\Crypto\Exception as Ex;
// The following is inside a class, but for clarity I only copy pasted this part    
try {
    return Crypto::decrypt($aStr, Crypt::$cryptoKey);
} catch (Ex\InvalidCiphertextException $ex) { // VERY IMPORTANT
    // Either:
    //   1. The ciphertext was modified by the attacker,
    //   2. The key is wrong, or
    //   3. $ciphertext is not a valid ciphertext or was corrupted.
    // Assume the worst.
    die('The ciphertext has been tampered with! Message:'.$ex->getMessage());
}
// I've got some more catch blocks here but they're not relevant for this question

This code works great, but today I stumbled upon one database record which makes the whole thing die on this InvalidCiphertextException catch. I've manually tried it with some sample code, but I always get the InvalidCiphertextException.

I presume the data is corrupt, but I'm not sure whether that is the case.

The record is in the middle of a table with 15000 records which are all fine and there have been no sudden changes to that part of the code for ages.

Is there any more I can do to investigate (or even decrypt) this? Or is there any more info I can find about this record or a way to debug this further?

All tips are welcome!

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