Essentially, what I described above (picking the ring in the inventory and using it on the message) has been done manually. By doing that, no message of an "incompatible" action appeared but nothing happened either. Clicking on the message with the ring, simply resulted in the disappearance of the ring (as being used, indeed) but nothing happening following that (no message appearing decoded, not even after re-reading the message after the use of the ring on it).