There's certainly a case here. If Barr goes forward with his nutty anti-encryption push, I hope the opposition uses this argument. Google lists definition #1 for "arms" as "weapons", and definition #2 of "weapons" as "a means of gaining an advantage or defending oneself in a conflict or contest."

I doubt any would argue that un-breakable (at least, theoretically) comms would provide no tactical advantage. If anyone would, I'd refer him to the second world war as a prominent example. This argument could help built a bi-partisan pro-cryptography group by pulling in support from the 2A crowd. I support cryptography remaining fully legal (even if classified as a munition or armament) for the same reason I support the 2A, practical security considerations aside.

Well they made that argument about cryptography being a munition and I don't see anyone advancing a 2nd Amendment challenge against those laws.