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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. Patrick Toomey‏ @patricktoomey Jun 8
      Replying to @lvh

      Patrick Toomey Retweeted lvh

      Pretty good thread . Thanks for the chat @lvh.https://twitter.com/lvh/status/1005085612607787009 …

      Patrick Toomey added,

      lvh @lvh
      Replying to @liamdaws @tqbf @patricktoomey
      That sounds like a signature. Ed25519 it.
      2 replies 0 retweets 2 likes
    2. thaidn‏ @XorNinja Jun 8
      Replying to @patricktoomey @lvh

      No, don't use Ed25519. Use a probabilistic signature scheme that derives nonces from private key, message and some randomness.

      2 replies 0 retweets 0 likes
    3. lvh‏ @lvh Jun 8
      Replying to @XorNinja @patricktoomey

      Such as...?

      1 reply 0 retweets 0 likes
    4. thaidn‏ @XorNinja Jun 8
      Replying to @lvh @patricktoomey

      XEdDSA from @trevp__

      2 replies 0 retweets 0 likes
    5. Brian Smith‏ @BRIAN_____ Jun 8
      Replying to @XorNinja @lvh and

      Incidentally, I thought the XEdDSA specification was going to be updated to move the random value so that the private key and the message weren't in the same hash block, but https://signal.org/docs/specifications/xeddsa/ … doesn't seem to have that change. I wonder if that is actually the latest version.

      1 reply 0 retweets 1 like
    6. Brian Smith‏ @BRIAN_____ Jun 8
      Replying to @BRIAN_____ @XorNinja and

      If you care about fault injection attacks then I don't know that any variant of Ed25519 is good to use, because Ed25519 is usually implemented using x-coordinate only multiplication so you can't verify that the result is on the curve. (IDK if that's necessary or sufficient yet.)

      2 replies 0 retweets 0 likes
    7. Henry de Valence‏ @hdevalence Jun 8
      Replying to @BRIAN_____ @XorNinja and

      What? Ed25519 points are encoded using the (affine) y coordinate, and most implementations (including yours) uses the extended coordinates of Hisil, Wong, Carter, and Dawson, not Montgomery x-line arithmetic.

      1 reply 0 retweets 2 likes
    8. Brian Smith‏ @BRIAN_____ Jun 8
      Replying to @hdevalence @XorNinja and

      Is this the normal way it's done? I thought it was normal to try to implement x25519 and then implement Ed25519 as a hack on top of it. If the hacky way isn't normal then it's probably less of a concern.

      1 reply 0 retweets 0 likes
    9. Brian Mastenbrook‏ @bmastenbrook Jun 8
      Replying to @BRIAN_____ @hdevalence and

      How would one do that? AFAIK for signing you can't use x-only Montgomery arithmetic because encoded Ed25519 points specify a single point, not a pair of points; and for verification, you can't use a differential addition formula with Shamir's trick.

      1 reply 0 retweets 1 like
    10. thaidn‏ @XorNinja Jun 8
      Replying to @bmastenbrook @BRIAN_____ and

      +1 to what @bmastenbrook said. @BRIAN_____, people usually convert public keys from X25519 to Ed25519, to use a single key pair for both encryption and signature (I never am comfortable with this idea)

      2 replies 0 retweets 5 likes
      Brian Smith‏ @BRIAN_____ Jun 8
      Replying to @XorNinja @bmastenbrook and

      OK, let's jump ahead to the thing I want to verify: For these kinds of fault attacks, what is the value of checking that the result is on the curve? If the bit flipping is actually modifying the private key scalar itself then it doesn't help AFAICT, but how about otherwise?

      7:24 PM - 8 Jun 2018
      1 reply 0 retweets 0 likes
        1. New conversation
        2. thaidn‏ @XorNinja Jun 8
          Replying to @BRIAN_____ @bmastenbrook and

          EdDSA signing requires two scalar mults, one to compute the public key (which can be cached) and another to compute R. If adversary can cause any faults in these computations, and also learn the correct value of the public key or R, they can compute the private key

          1 reply 0 retweets 3 likes
        3. thaidn‏ @XorNinja Jun 8
          Replying to @XorNinja @BRIAN_____ and

          Point-on-curve check can detect certain faults, but it doesn't help if the adversary can actually flip a single bit of any scalar

          1 reply 0 retweets 1 like
        4. thaidn‏ @XorNinja Jun 8
          Replying to @XorNinja @BRIAN_____ and

          Something to think about: suppose there's a deterministic carry mispropagation bug that can be triggered with 1% of the scalar values. Is it possible to extract the private key?

          1 reply 0 retweets 1 like
        5. thaidn‏ @XorNinja Jun 8
          Replying to @XorNinja @BRIAN_____ and

          Point-on-curve check can detect mispropagation bugs, but I'm not sure whether these bugs leak the Ed25519 private key. Since the bugs are deterministic, the adversary cannot obtain two different R values for the same nonce and message. @jurajsomorovsky thoughts?

          3 replies 0 retweets 0 likes
        6. Brian Smith‏ @BRIAN_____ Jun 8
          Replying to @XorNinja @bmastenbrook and

          I think this is probably something beyond what's reasonable to solve on Twitter. What I'm really wondering is what are the near-zero-cost countermeasures that can reduce the likelihood of a Rowhammer-like attack the most, ideally without randomization the scalar mult.

          2 replies 0 retweets 1 like
        7. thaidn‏ @XorNinja Jun 8
          Replying to @BRIAN_____ @bmastenbrook and

          We spent a lot of time on this question, and concluded that randomization is safest. Other options: 1/ sign then verify (expensive, detect many, but not all faults) 2/ sign twice and compare sigs (cheaper than 1/, but detect fewer faults)

          1 reply 0 retweets 1 like
        8. Brian Smith‏ @BRIAN_____ Jun 11
          Replying to @XorNinja @bmastenbrook and

          3/ Design the protocol to take signatures out of the critical path w.r.t. performance and then do #1. Anyway, how did you randomize the nonce? I'm doing it the BN_generate_dsa_nonce way for now, e.g. https://github.com/briansmith/ring/pull/662 … for ECDSA.

          1 reply 0 retweets 0 likes
        9. Brian Mastenbrook‏ @bmastenbrook Jun 11
          Replying to @BRIAN_____ @XorNinja

          FWIW my take on this was to provide the private key and message as additional input to the HMAC-DRBG, which ends up being pretty similar to the RFC6979 path:https://github.com/naniteproject/sweet-b/blob/master/src/sb_sw_lib.c#L1330 …

          1 reply 0 retweets 1 like
        10. 9 more replies

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