Secure SSL, “Tales of Transport Layer Security at Twitter” from 2013 B-Sides San Francisco

SSL++; Tales of Transport Layer Security at Twitter

I am happy to have attended this talk, at 2013 B-Sides San Francisco, by @jimio, a Twitter employee, on SSL security and how to create a secure SSL site. The title was  “SSL++ : Tales of Transport Layer Security at Twitter” and it was definitely a good way to wake up and start the day. Twitter was able to switch to exclusive-SSL and netted out to a faster site with SSL. In this talk, he discussed why and how.

Twitter SSL


First point:  I am indebted to the speaker for prompting me to do a bit of reading about the CRIME and BEAST SSL/TLS attacks. I am primarily a software architect but of course at each job on my resumé I have picked up very interesting domain knowledge and crypto is full of things like CRIME and BEAST that do not occur to you as you use or design a crypto algorithm.  To summarize for the benefit of those who need it (and presage a little some of the similar inject-then-diagnose approaches to acquiring crypto keys I will be writing about w.r.t. other talks I attended), the CRIME attack works by injecting content into TLS compressed headers (or indeed it is useful for any encrypted compressed information) and then observing the resulting size of the compressed information relying on the fact that the compression algorithm economizes on repeats.  That is, if your injected content causes the size to increase then it is probably not in the original content.  If the size does not increase (or very little), it probably is in the content.  So one can guess and hone in on the compressed content without having to know the crypto key.  BEAST works by injecting content that is 15 bytes, then 14, then 13, … down to zero so that at each iteration the last byte of the content is the only unknown byte and one only has to brute force 256 combinations rather than 2^128.  This reminds me of Schuyler Towne’s talk about how to get into those Base-10 suitcase locks.  Typically a session cookie is being pursued with this attack.

Transport Layer Security at Twitter

Okay, there’s the preamble. The balance of this talk was about not so much about exotic SSL vulnerabilities like those discussed above, but simply vulnerabilities stemming from not thoroughly using SSL.  Sometimes this can mean the login page is in SSL (lovely, protects password) but the cookie is in cleartext (bollocks).  So it needs to be SSL everywhere.  Twitter instituted such a change at one point and gave customers the ability to opt out and about 1% did.  However, even when you think you are fully SSL, there are still CSRFish things people can do like <img src=””> which can prompt GETs over HTTP thereby revealing the user’s cookie even if the response is innocuous.  The speaker discussed man in the middle attacks though not of what you the reader are likely to have been hearing about lately but the simpler variety of intercept the SSL and broker it as HTTP to the server and thereby read all the content unencrypted.  Again, the countermeasure here is absolutely airtight SSL on the site.  And then there are things like #!/dir or anything similar where everything past the # does not get sent to the server and is instead processed with client side script.  That one actually transcends the thesis of this talk.  Certainly it is an SSL issue but it is a whole-bunch-of-other-things issue as well.  Prior to working in information security, I worked at a company where we were doing loads of this kind of stuff in a web application and also calculating cookies in client-side jsp (!)… 13 years ago… more naive times.  The management hired a security firm to audit and that is how we found out about this stuff.  We weren’t developing an E-commerce site, it was more of an internal-use site but of course one wants to be secure even in that environment.

Every request should be SSL

The overall goal is to get all requests internal and external to your site to be SSL.  Obviously you can control the former but not fully the latter.  So you can do the best you can on the latter.  For example, canonical linkrel always with an https.  Google’s crawlers respect this but Bing and Yahoo don’t.  There is some partisanship apparently that it is unseemly to use linkrel in this fashion (it is not canonical to use canonical this way :-)?) but as you can imagine, the speaker rejects such arbitrary religious arguments as do I.  Then there is the issue of people not typing fully qualified links with protocol into their browsers (it’s been a while since 1992 after all).  Of course you expect any browser to GET but interestingly Twitter apparently convinced Chrome developers to put an “if (it is twitter) {assume HTTPS}” line in their code.  More measures to encourage clients to request nothing but SSL include the <strict-transport-security> tag and CSP.

Pros & Cons of Cert-Pinning

At this point he spoke about cert-pinning which I wrote up extensively with regard to another talk so suffice to say, it is a good idea wherever feasible.  Mobile apps were the focus of that other talk and the disadvantage to cert pinning was redeployment of all in-the-field apps to use the new baked-in cert when the cert needs to be changed.  These would be things like standalone games that communicate with a server.  So if you are building a web application that is exclusively used as such and is therefore inherently self-deploying, that concern is lessened though I suppose it requires savvy users/browsers to maintain client-side trusted  certs and not capriciously ok new ones.

Performance issues with encrypted SSL, Not Really

The speaker concluded by addressing performance considerations of going exclusively encrypted.  In short, he said optimize other areas of your website to buy back the performance lost by going SSL, which is not that significant to begin with.  The advantages far outweigh the liabilities of performance.  Further, his company (Twitter) is a case in point.  They cleaned up their code as part of the switch to exclusive-SSL and netted out to a faster site with SSL.

I’m finding that a common denominator in a lot of these talks is “the more things change the more they stay the same” and possibly “there is one (web developer) born every minute.”  The exotic sexy (in the nerd sense) vulnerabilities command our attention as we want to stay ahead of the bleeding edge but the old vulnerabilities (particularly as they combine with new ones) keep resurfacing and constant vigilance implies remembering them as much as it does staying abreast of new developments. Our CEO, Dan Kuykendall likes to refer to it as Where’s Waldo (link to blog post) or Leisure Suit Larry. They same old things just keep popping up in new places.

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