Last week, a backdoor was discovered in xz-utils. The backdoor processes commands sent using RSA public keys as a covert channel. In order to prevent anyone else from using the backdoor, the threat actor implemented a cryptographic signature check on the payload.

I have seen a number of people claim that this would necessarily result in an obviously invalid RSA public key, or at least one with no corresponding private key.

This is incorrect, and someone nerd sniped me into proving it.

If you’re like most people, there’s a good chance that it’s been years since you’ve sent an email that wasn’t cryptographically signed. You don’t use PGP, you say? Well, even if you are not signing your email, your provider is almost certainly doing it for you. Plausible deniability has been tossed aside in the name of stopping spam, but it doesn’t have to be.

Sometimes hacking requires doing things that, while possible to do with some algorithm, simply aren’t supported by any existing implementation. Usually for good reason. A good example of this that I’ve run into in the past is needing to initialize a hash algorithm with a specific state. There’s really not any reason to do this unless you’re trying to execute a length extension attack, and with the exception of HashPump (which was written specifically for that use case) I’m not aware of any library that supports it. I recently ran into this with problem with RSA.