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Comment by tptacek | Hacker News Reader

The funny thing about that comment is that I've worked in several environmens where DHE was not only not preferred, but explicitly banned. Not only that, but DHE is even more complicated than SSL as most people understand it --- if you're in favor of it, you're making an even stronger argument for TLS (DH is remarkably easy to screw up).

Either way, the question of whether you're using ephemeral keys or not is not relevant to this discussion. Nobody is going to choose backup software based on whether the encrypted transport has forward secrecy.

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The main thing I am trying to say is that, if you understand crypto enough to configure your TLS optimally, you can do the crypto without TLS. And, if you don't understand crypto and TLS this well, you shouldn't be trying to store the public's private data in a way that you claim to be secure. (This isn't about ZumoDrive; They may know quite a lot about these issues, and their choices might very well be optimal for the use cases they are trying to address.)

It would be very interesting to read why people would ban DHE. I know DHE key exchange is tricky in TLS because TLS doesn't allow proper negotiation of the parameters like it does with ECDHE and there are no commonly agreed-upon (named) DHE parameters like there are for ECDHE and IPSEC. Luckily, things are pretty straightforward if you use the Suite B profile of TLS (RFC5430), AFAICT.

There's a big difference between whether people would choose backup software based on the algorithms being used, and what algorithms should be implemented. Many people would choose backup software without any encryption if it was easy to use and cheap.

This is so absolutely, completely untrue I hardly know where to start.

Knowing the difference between a an ephemeral keying scheme and a basic RSA key exchange does not make you prepared to implement either.

The Tor developers thought they knew enough to implement DH. Roger Dingledine has publications going back before 2000 and did his doctorate under Rivest. But they forgot to check DH parameters for 0-mod-p (a mistake lots of people manage to make) and so fielded an anonymity network that provided very little anonymity for several years.

Cryptography isn't rocket science. It's more like pyrotechnics. There aren't that many things to remember (though there are more of them than most people think; look how the browsers screwed up RSA a few years ago). It's just that the things you need to remember aren't obvious, and when you get them wrong, you blow your f'ing hands off.

If you choose a bad implementation of your crypto primitives (ones that don't make these kinds of checks) then you will get bad results. That's not surprising. The same thing happens when you choose a bad TLS implementation. Your argument is that you should use a well-known TLS stack because the bugs have been shaken out over the years. I agree with that. But, the same thing applies to the lower-level crypto protocol implementations. The common TLS implementations are implemented with the common crypto primitive implementations. For example, IIRC the checks of D-H parameters in OpenSSL are in the D-H code, not in the TLS code. So, you don't need to use the TLS part of OpenSSL to get the benefits of its carefully-written D-H implementation.

I'm not saying that anybody should go out and implement all the primitives (D-H, ECC, AES, GCM) themselves.

What's the recommendation you would make to a generalist developer on a library they can safely use to implement their own secure transport with ephemeral keying?

You sound very clueful but I don't think you've thought this through. Off the top of my head I can think of zero popular cryptosystems that haven't been burned by terrible implementation flaws. I just told you that Tor, which was implemented by someone who got a PhD under Rivest, managed to screw up DH. Your response is, "if you picked better libraries, you'd do better than Roger Dingledine". No, you wouldn't.

The specific bug you mentioned really was the result of writing/choosing a bad D-H library. But, I agree with your your point that, even if you have a security expert on board, you are likely to make mistakes.

If Tor would have used an off-then-shelf D-H implementation that already performed that check, it wouldn't have had the vulnerability. It is definitely easier to pick a TLS implementation that is unlikely to have such flaws (now)--just pick what lots of others have been using for a long time. You could do the same with crypto primitive implementations, but you run into problems because the interfaces for different libraries are all different, and in particular the preconditions and postconditions of various operations might be different. So, user of a crypto library has to fully understand all the details in order to verify what is his responsibility and what is the library's responsibility. It would be great if there was a specification that specified what every D-H implementation must do, and what the user of a D-H implementation must do to be safe.

Actually, a similar problem exists with TLS. Right now there is a draft TLS spec. that attempts to standardize how TLS implementations check a server's certificate against the server's domain name. Since there's no standard way of doing it, its harder to verify that a given TLS implementation is actually doing it the right way. And, actually, this is something that OpenSSL and other TLS implementations punt on and force the application to deal with manually. As I'm sure you've seen, lots of applications actually don't even check at all!

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