It's a good point; trying to represent it on a detailed protocol diagram would be tricky, I think a spatial indication (e.g. a line extending from the signature to a box containing the kx messages) would be more appropriate there. Perhaps having it 'fade out' as it went back in time would be a good way to indicate that it was retroactive.

But, for my immediate purposes, I'm representing things at least one abstraction level up from that, if not two - my diagram doesn't even state exactly what protocol is in use (the surrounding text does and in fact it's also a fixed part of the context, because this is really a small improvement to a long-established system).

In the confidentiality case it suffices to draw red lines inside a box and black lines between boxes; I'm just looking for an analogous way of indicating that there are some worthwhile integrity protection properties too, ideally something that I can do easily in Gliffy l-)