cryptsetup is remarkably resilient against having random parts of the file overwritten. I can overwrite the first 16KB of my test device and it doesn't seem to even notice (or, more likely, is silently recovering).

Overwriting 10MB starting at 1MB into the block device? No big deal. No corruption.

There are parts I can corrupt which will cause problems, but I expected it to refuse to unlock when I destroyed the beginning of the file.

And if you have any idea of what I'm talking about right now, we should follow each other. 😆