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colmmacc's profile
Colm MacCárthaigh
Colm MacCárthaigh
Colm MacCárthaigh
@colmmacc

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Colm MacCárthaigh

@colmmacc

AWS, Apache, Crypto, Irish Music, Haiku, Photography

Seattle
notesfromthesound.com
Joined April 2008

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    1. Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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      With that approach, when we negotiate what options/modes/extensions you're going to use, we do it just once and handcuff ourselves to the corresponding linearized set of state transitions. That is the only order we will then follow. An out of order message shuts everything down.

      1 reply 0 retweets 23 likes
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    2. Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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      That makes our state machine itself tiny, it's here: https://github.com/awslabs/s2n/blob/master/tls/s2n_handshake_io.c#L253 … all it does is increment a value! Btw one of our coding tenets is never to mix control flow and message parsing in the same function. Big win here!

      1 reply 1 retweet 39 likes
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    3. Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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      O.k. so that's the basic approach, but how do we then test it? how do we make sure that all of our valid states are correct? With some help from @galois , we formally verify it! Here's how ...

      1 reply 0 retweets 17 likes
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    4. Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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      So we have a declarative definition of the valid TLS states that come from the RFC. It's written in Cryptol, and is here: https://github.com/awslabs/s2n/blob/master/tests/saw/spec/rfc-handshake.cry#L134 … now that's done by hand and we have to trust it, it's the "bottom turtle" in the proof, but it's easy to read and review.

      2 replies 1 retweet 34 likes
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    5. Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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      We then use SAW to map our s2n_handshake C code to something that can be symbolically executed for all possible inputs. Here's the SAW code that does the mapping: https://github.com/awslabs/s2n/blob/master/tests/saw/s2n_handshake_io.saw …

      2 replies 0 retweets 10 likes
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    6. Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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      So together, these actually check that our handshake code really does all TLS valid state transitions correctly, and only those. We caught bugs when we first added it too! Not a security issue, but it turns out we didn't support an obscure combination of OSCP and resumption.

      1 reply 1 retweet 13 likes
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    7. Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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      Now my favorite part ... how do we know that the verification itself actually works? We run it on every build, but what does that say?

      1 reply 0 retweets 7 likes
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    8. Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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      We *also* "verify the verifier" by forcing some negative test cases. We actually patch the code with known errors: https://github.com/awslabs/s2n/tree/master/tests/saw/failure_tests … and check that the formal verification fails! This is super important for checking formal verf btw, and often overlooked.

      2 replies 8 retweets 55 likes
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    9. Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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      We also do fuzz tests (https://github.com/awslabs/s2n/tree/master/tests/fuzz …) and we have integration tests with other implementations, to check for inter-op.

      2 replies 0 retweets 16 likes
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    10. Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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      Our goal with tooling is to ensure that we have failsafes beyond code-review. For these state machine type bugs, it actually takes quite a lot! There's a lot more code verifying it than implementing it. It's not surprising that these issues crop up in real world software.

      1 reply 3 retweets 30 likes
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      Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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      Anyway, that's it unless you want to AMA. I'll just ask @threadreaderapp to please unroll this thread!

      2:45 PM - 18 Oct 2018
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      • Mr. Prescott Robert Devereux Merritt Baer motownmutt Robin Kauffman 🅿🅴🆃🅴🆁 🅹 🅷🅸🅻🅻 Erik Nylund Andrew Strutt
      5 replies 0 retweets 28 likes
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        2. Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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          Replying to @BRIAN_____ @threadreaderapp

          The first tool I used to this with what TLA+, because I'd used it before, and we use it to verify state machines elsewhere at AWS, such as our implementations of PAXOS, the state machines at the core of the S3 Load Balancer, AWS HyperPlane, and Elastic Block Store.

          1 reply 0 retweets 2 likes
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        2.  Ⓜike «...» Ⓕlester‏ @mobileAgent 18 Oct 2018
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          Replying to @colmmacc @threadreaderapp

          Great read, thanks! Could you comment on the utility (or otherwise) of concolic testing?

          1 reply 0 retweets 0 likes
        3. Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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          Replying to @mobileAgent @threadreaderapp

          I had to look up what it is! Sounds super cool. We do use directed fuzzing, which has a similar goal.

          0 replies 0 retweets 1 like
        4. End of conversation
        1. New conversation
        2. Colm MacCárthaigh‏ @colmmacc 18 Oct 2018
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          Replying to @chadloder @threadreaderapp @UniOulu

          That's not automated, but thankfully it doesn't change much. There's only so many TLS extensions!

          0 replies 0 retweets 1 like
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        1. Thread Reader App‏ @threadreaderapp 18 Oct 2018
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          Replying to @colmmacc

          Bonjour there is your unroll: Thread by @colmmacc: "Thursday tweet thread time! This one is all about what we do in Amazon s2n to prevent security issues similar to this we […]" https://threadreaderapp.com/thread/1053031892734242817.html … Have a good day. 🤖

          0 replies 2 retweets 5 likes
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        2. Chris Morgan (latest model)‏ @mihalis68 18 Oct 2018
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          Replying to @colmmacc @threadreaderapp

          @threaderapp please unroll this thread!

          1 reply 0 retweets 0 likes
        3. Thread Reader App‏ @threadreaderapp 18 Oct 2018
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          Replying to @mihalis68

          Hallo you can read it here: Thread by @colmmacc: "Thursday tweet thread time! This one is all about what we do in Amazon s2n to prevent security issues similar to this we […]" https://threadreaderapp.com/thread/1053031892734242817.html … Enjoy :) 🤖

          0 replies 0 retweets 0 likes
        4. End of conversation
        1. Mr. Prescott‏ @prescott_steven 6 Nov 2018
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          Replying to @colmmacc @threadreaderapp

          Incredible thinking out of the box.

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