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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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      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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    2. 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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    3. 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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    4. 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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    5. 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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    6. 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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    7. 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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    8. 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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    9. 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!

      5 replies 0 retweets 28 likes
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      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!

      3:42 PM - 18 Oct 2018
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        2. Thread Reader App‏ @threadreaderapp 18 Oct 2018
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          Replying to @chadloder

          Thank to YOU! ✅ If you want you can support me: https://threadreaderapp.com/account/premium 

          0 replies 0 retweets 0 likes
        3. End of conversation

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