Hacker News Reader

Comment by ott2 | Hacker News Reader

So, Mr Smith wants to add a bunch of elliptic curve options (using the NIST curves, weakest P-256 first), while removing the widely used TLS_RSA_WITH_AES_256_CBC_SHA256 due to "concerns" about "performance" and an unsubstantiated argument that ephemeral key exchange is somehow always better. Hmm.

View on HN

Mr. Smith joins pretty much the entire mainstream of cryptography in urging people to switch away from RSA and towards ECC. The NIST P-256 curve is the most common ECC curve used. It was generated by picking a prime that is fast to compute with and hashing a string with SHA-1.

As djb pointed out in "Security dangers of the NIST curves", SHA-1 does not prove much. If NSA knows a weak class of curves, they try as many strings as they want until SHA-1 of the string hits a weak curve.

It doesn't prove much; you can always just generate a new P-256-alike with the instructions NIST provided for you in that document.

Who can generate? Aren't the values fixed by the standards, mustn't both client and servers use the same as long as they support the given standard?

The standard provides a standard set of curve parameters and a NIST-sanctioned way of generating new curve parameters using the exact same method.

How can be any other parameters than the standard ones used in the current browsers and servers? I think they can't, am I right?

And how can browsers start to use any other parameters before they standardise them? I think they can't?

This is true but not particularly meaningful to me, because you can't really do anything new with crypto at all without some kind of software update. For instance, the primes and generator for conventional number theoretic DH are also pre-generated and baked into a standard.

So maybe we should generate our own curves. I propose something as follows:

1. Locate a public string. A tweet or a quote should suffice.

2. SHA-512 the string to obtain a seed.

3. Use that seed to generate b, and calculate N = #E(Fp) = n * h, and choose a base point P. Of course we need to ensure that these parameters are safe against known attacks.

4. Mandate that the new set of parameters MUST be supported wherever NIST prime curves are supported.

The last step is probably the most difficult. You don't need that if you don't need to interoperate with other implementations though.

The only thing you want not to happen is for software to start generating and negotiating its own curves, because that then requires all interoperable implementations to parse and validate random curves from attackers.

No, I didn't say that everyone generates their own curves. I meant the security community should generate our own curves. Somebody should email Thomas Pornin.

Sorry, I didn't mean to imply you were saying that.

There are already several alternative curve sets, satisfying various degrees of paranoia:

- http://certivox.org/display/EXT/CertiVox+Standard+Curves

- http://tools.ietf.org/html/rfc5639

- curve25519 and the other djb et al curves.

TLS_RSA_WITH_AES_256_CBC_SHA256 is not forward secret (if you have the certificate private key you can passively decrypt all past/future sessions) so removing it is a great idea. I'm only responding to what you said and not making a judgement about the rest of the guy's suggestions.

Beware of random cryptographers bearing suggestions? #HUMINT

Some of the OpenSSH and IETF mailing list archives about SSH also make interesting reading. Exactly why does RFC 4253 mandate Group 1 and Group 14 as the only required key exchange mechanisms? (They were defined by RFC 2412, written by someone who was "assigned to the DARPA Information Technology Office".) However, perhaps it would be better to fork a separate thread for such speculations.

I really hope TLS crypto standards won't be set (or changes rejected) based on who proposed a particular change.

Changes should stand on their own merits. I don't see why we should trust anyone, random or not. It's not about trusting people, it's about trusting algorithms. So it doesn't really matter who in particular proposes changes.

In a ideal world, but if you get a lot of "contributions" some might slip through. Remember, no one can match NSA's budget, manpower and maybe brainpower.

Reply on news.ycombinator.com