Application of standard techniques to new data: is it publishable?
You might consider sitting down with the people arguing for publication to try and understand why they consider it to be (potentially) publishable.  It's possible, for example, that this new protocol addresses some issues not addressed by existing protocols; or addresses them more efficiently, or robustly, or is otherwise better in some way.  In other words, it's possible that the protocol by itself (without your security analysis) is not publishable, and your security analysis by itself (without the novelties of the new protocol) is not publishable, but the two taken together may possibly be publishable.