Forward Secrecy for Google HTTPS | Hacker News Reader

Forward Secrecy for Google HTTPS

imperialviolet.org

97 pointsdpeck19 comments

What worries me here, is a cargo-cult following of this advice.

Ephemeral Diffie-Helman adds an order of magnitude to the session initiation overhead (nginx got a bad rap a little while back, becuase it has kEDH enabled by default). I don't want to see people slide back into the "SSL is too expensive" mindset, because they don't understand the SSL options they are using.

I think this is a reasonable concern about DHE, but this blog post was specifically intended to rebut the concern: they optimized DHE and use EC (faster, smaller keys), and managed to get it deployed across all of Google. The subtext is that one reason DHE isn't widely deployed is poor implementation, and they've taken a step towards correcting it.

Thanks - I missed the DHE optimizations, and it's really nice to see that these improvements were immediately fed back into OpenSSL.

The post made it sound like just the bugfixes are in 1.0.0e, but the performance improvements may have not been accepted yet. I wonder if that is the case?

This is good to see, and I really appreciate the work google has put into making tls/ssl more pervasive.

I do wonder about the performance differences between their faster DHE implementation and what they were using before (RSA-RC4-SHA). I wish they had provided a bit of data on that.

It kind of has to be less performant, just because ECDHE uses more bignum crypto primitives.

Yeah, that makes sense.

I was mostly curious about how much faster their new implementation was over the old, and how much slower it was than RSA-RC4-SHA.

I think that with the session resumptions they got by using session tickets, this could easily end up being a net gain for them.

DH over elliptic curves is definitely fast enough for general purpose use, but on low-power devices it could be problematic. I think a better approach is the one I used in spiped: Each connection can pick whether to do a DH computation and get forward secrecy or to instead use a DH exponent of zero and still get regular secrecy.

Alas, SSL doesn't support such a mechanism.

Why can't the client just not list the DH cipher suites in the Client Hello?

People might turn on EDH and then decide it's too expensive, or if they come under attack, they can easily configure it off again.

Unlike, say, the new mandatory 2048 bit RSA key length requirements.

Seems like a lot of folks missed the section of the post explaining their implementation and use of _ECDHE_ not _EDH_. The performance impact of ECDHE is surprisingly small.

It doesn't help that Apache has a setting "kEDH - Ephemeral (temp.key) Diffie-Hellman key exchange (no cert)" and "EDH - all ciphers using Ephemeral Diffie-Hellman key exchange" that (I think) maps to what IETF TLS calls "DHE - Ephemeral DH".

I get these confused all the time, and now they add in an "EC" prefix!

More great work from Google legitimizing and advocating for strong HTTPS defaults. I'm looking forward to this being broadly adopted.

Securely Yours,

Lil' B

I'm sure these guys have thought about the longevity of data at rest when selecting the security strengths on the EC curves and RSA keys. These chosen parameters are quite high and they link to an article which mentions a period of up to 2040 before having to be re-encrypted or expected potentially be broken.

Make your own conclusions about that.

I'm not certainly a fan of session resumption and renegotiation's from a potential security aspect, anyone care to persuade me apart from a performance standpoint?

Look at it this way: session resumption enables you not start the handshake all over again from scratch for every TCP connection your browser makes. This mean you can actually do more secure things like preferring DHE on the initial handshake because it happens much less often.

The session ticket support enables Google to use session resumption across their massively load-balanced SSL terminators.

I don't think Google is using renegotiation in this architecture, but I think we got renegotiation nailed down tight with RFC 5746.

I am not a cypherpunk, but can/should we differentiate between 1. encouraging those running OpenSSL/TLS servers to use a. sane defaults that allow forward secrecy and b. ephemeral TLS session ticket keys, 2. using Google's in-house EC cipher implementations and 3. doing 1. and 2.?

We could do 1. only and still reap some benefit, correct?

There are environments where ephemeral DH keys violate policy (those sessions can't be passively monitored; some sessions must be), which could militate against PFS suites becoming the default.

DHE is good, but I'm not sure forward secrecy is as pressing a security issue as simple adoption of TLS is. I admire Google's continued efforts to set the standard for its secure deployment, though.

Look at it from Google's point of view though:

Now they won't have "lawfully intercepted" packet captures showing up in their mailbox with demands that they decrypt them using their private key.

Correct!

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