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BRIAN_____'s profile
Brian Smith
Brian Smith
Brian Smith
@BRIAN_____

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Brian Smith

@BRIAN_____

Code farmer. Security, crypto, performance, networking, usability. Rust, C++, C, Haskell, DSLs, etc. *ring*, webpki, crypto-bench, mozilla::pkix.

Honolulu & San Francisco
briansmith.org
Joined April 2008

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    1. Colin Percival‏ @cperciva Nov 2

      I've been getting a few questions about the recent "PortSmash" vulnerability announcement. Short answer: This is not something you need to worry about. If your code is vulnerable to it, you were already vulnerable to other (easier) attacks.

      2 replies 40 retweets 60 likes
      Show this thread
    2. Colin Percival‏ @cperciva Nov 2

      While it's great to see that someone put together exploit code for this, it's not a new discovery: I described this attack in 2005 when I first exposed the dangers of shared resources in Intel Hyperthreading.

      1 reply 2 retweets 14 likes
      Show this thread
    3. Colin Percival‏ @cperciva Nov 2

      I didn't write exploit code at the time because this attack only works when the sequence of instructions bring executed depends on sensitive information; and if that's the case you're already leaking information in many other ways (code cache, data cache, branch prediction...).

      1 reply 0 retweets 9 likes
      Show this thread
    4. Colin Percival‏ @cperciva Nov 2

      The defence against PortSmash is exactly the same as the defence against microarchitectural side channel attacks from 2005: Make sure that the cryptographic key you're using does not affect the sequence of instructions or memory accesses performed by your code.

      2 replies 1 retweet 12 likes
      Show this thread
    5. Colm MacCárthaigh‏ @colmmacc Nov 2
      Replying to @cperciva

      Not just the key, the secret data too! This is the right advice, but it's very unsatisfying. It's still too hard to write and verify this kind of code, and sometimes coding mistakes there are bigger problems than the side channels they were designed to fix.

      1 reply 0 retweets 1 like
    6. Colin Percival‏ @cperciva Nov 2
      Replying to @colmmacc

      Keys are generally more sensitive than data; but yes, it should all be kept secure. I've been asking compiler authors for 13 years to give us better tools, e.g. to mark variables as "cannot be used in control flow or address computation". Alas, no progress yet...

      2 replies 1 retweet 2 likes
    7. Colin Percival‏ @cperciva Nov 2
      Replying to @cperciva @colmmacc

      Maybe Amazon can find some smart compiler people and get them to do this? It would help everybody's security. ;-)

      2 replies 0 retweets 0 likes
    8. Brian Smith‏ @BRIAN_____ Nov 2
      Replying to @cperciva @colmmacc

      It would be nice to have such a tool, but it isn't essential. If you talk to the developers of many crypto/security projects that have secret-dependent branches, they are well aware of (almost all of) them. Just fixing them is a relatively low priority until PoC is published.

      1 reply 0 retweets 0 likes
    9. Colm MacCárthaigh‏ @colmmacc Nov 2
      Replying to @BRIAN_____ @cperciva

      LuckyMinus20 changed my mind on that. The branch free code fixes for Lucky13 went through a lot of review, loads of eyes, and patched a vulnerability. But then it unintentionally regressed within 18 months.

      1 reply 0 retweets 0 likes
      Brian Smith‏ @BRIAN_____ Nov 2
      Replying to @colmmacc @cperciva

      How would a `secret` modifier have helped With LuckyMinus20? IIUC, the LuckyMinus20 was a logic problem, not a side-channel.

      12:37 PM - 2 Nov 2018
      1 reply 0 retweets 0 likes
        1. New conversation
        2. Colm MacCárthaigh‏ @colmmacc Nov 2
          Replying to @BRIAN_____ @cperciva

          The LuckyMinus20 was a logic error, and worse than the Lucky13 side-channel (IMO), at least for TLS (not DTLS). That's what I mean when I say it's too hard to write this kind of code without risking worse problems.

          2 replies 0 retweets 1 like
        3. Colm MacCárthaigh‏ @colmmacc Nov 2
          Replying to @colmmacc @BRIAN_____ @cperciva

          http://www0.cs.ucl.ac.uk/staff/b.cook/VSSTE18_sidetrail.pdf … can catch the LuckyMinus20 regression, we actually used it as a target case. It needs more than the secret modifier, it also needs to know the entry and exit points, but since the logic error is data-dependent, it can catch it too.

          1 reply 0 retweets 2 likes
        4. Brian Smith‏ @BRIAN_____ Nov 2
          Replying to @colmmacc @cperciva

          I wonder if Lucky13/LuckyMinus20 is an anomaly in how tricky it is, though. Most of the timing side-channels I know about (especially in OpenSSL) are much more straightforward than that, in terms of implementation.

          1 reply 0 retweets 1 like
        5. Brian Smith‏ @BRIAN_____ Nov 2
          Replying to @BRIAN_____ @colmmacc @cperciva

          Also, the constant-time "fix" for Lucky13/LuckyMinus20 is really only a partial mitigation. To the extent it is a serious issue for an application, one should avoid CBC cipher suites in TLS and more generally avoid bad uses of CBC mode like that. (The *ring* approach, so far.)

          1 reply 0 retweets 1 like
        6. Brian Smith‏ @BRIAN_____ Nov 2
          Replying to @BRIAN_____ @colmmacc @cperciva

          (The application should avoid the CBC cipher suites completely because generally it can't assume the peer it is communicating with even bothered to fix Lucky13 or BEAST or any other issue related to CBC cipher suites in TLS.)

          1 reply 1 retweet 1 like
        7. Colm MacCárthaigh‏ @colmmacc Nov 2
          Replying to @BRIAN_____ @cperciva

          We're about to take CBC out of s2n's default set, it's finally, finally, small enough a percentage of traffic to be viable. Curiously RC4 and 3DES each went much faster.

          0 replies 0 retweets 2 likes
        8. End of conversation

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