Thought experiment on protocols and noise

I hesitate to call this an interview question because I don’t think on-the-spot puzzle solving equates to a good engineering hire. On the other hand, I try to explore some simple thought experiments with candidates that have a security background.

One of these involves a protocol that has messages authenticated by an HMAC. There’s a message (with appropriate internal format) and a SHA-256 HMAC that covers it. As the implementer, you receive a message that doesn’t verify. In other words, your calculated MAC isn’t the same as the one in the message. What do you do?

“Throw an error” is a common response. But is there something more clever you could do? What if you could tell whether the message had been tampered with or if this was an innocent network error due to noise? How could you do that?

Some come up with the idea of calculating the Hamming distance or other comparison between the computed and message HMACs. If they are close, it’s unlikely that the message had been corrupted, due to the avalanche property of secure hash functions. If not, it may be a bit flip in the message, possibly due to an attack.

Ok, you can distinguish whether the MAC had a small number of errors or the message itself. Is this helpful, and is it secure? Consider:

  • If you return an error, which one do you return? At what point in the processing?
  • Does knowing which type of error occurred help an attacker? Which kind of attacker?
  • If you chose to allow up to 8 flipped bits in the MAC while still accepting the message, is that secure? If so, at what number of bits would it be insecure? Does the position of the bits matter?

There comes a moment when every engineer comes up with some “clever” idea like the above. If she’s had experience attacking crypto, the next thought is one of intense unease. The unschooled engineer has no such qualms, and thus provides full employment for Root Labs.

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