Re: [openpgp] The combinatorial complexity of OpenPGPv4

[David Gil <[e-mail]>]

On Friday, March 13, 2015 6:20 PM, Falcon Darkstar Momot <[e-mail]> wrote:

> I feel like perhaps this type of exhaustive testing is neither necessary
> nor expected, and that a few end-to-end tests designed to exercise edge
> cases could be combined with more exhaustive unit tests to achieve
> reasonable results. 


The difficulty, as always, is proving that an actual implementation is modular. In the case of OpenPGP, it really isn't: A lot of data has to get carried between each stage to ensure conformance with the high-level semantics.


In contrast, suppose that I wanted to exhaustively test the different ways of doing authenticated encryption with TweetNaCl. That's exactly 1 test, which I can then test for 2^32 different parameter values or so quite easily.

That type of exhaustive testing is routinely (though not routinely enough) done, and it is expected for critical software.

> Protocol modularity is not evil.

Modularity is neutral. "Agility", as folks like to call it, is evil.

The name-sake article of Google and Yahoo's End-to-End project has some good arguments for avoiding redundancy in systems design: 


http://web.mit.edu/saltzer/www/publications/endtoend/endtoend.pdf

(I'd note that I think the article argues for its position a bit too strongly. It's clear that transport-layer and message-level encryption are both necessary, for example, because they have fundamentally different semantics.)