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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. 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
    2. 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
    3. 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
    4. 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?

      1 reply 0 retweets 0 likes
    5. 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
    6. 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
    7. 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
    8. 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
    9. 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
    10. Henry de Valence‏ @hdevalence Jun 8
      Replying to @BRIAN_____ @XorNinja and

      The thing I don’t understand in this discussion is why randomizing the nonce (by inserting random data into the hash) is insufficient?

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

      Ideally the library implementing EdDSA would implement countermeasures that work regardless of the message content, and which are 100% compatible with the standard.

      8:47 PM - 8 Jun 2018
      • 2 Likes
      • Ryan Sleevi Deirdre Connolly
      1 reply 0 retweets 2 likes
        1. New conversation
        2. Henry de Valence‏ @hdevalence Jun 8
          Replying to @BRIAN_____ @XorNinja and

          Doesn’t Trevor’s suggestion meet those properties? The randomness is independent of the message, and only used to generate the nonce, which is known only to the signer.

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

          The standard says the signatures are deterministic so you have to generate the same signature given the same inputs.

          1 reply 0 retweets 1 like
        4. Henry de Valence‏ @hdevalence Jun 8
          Replying to @BRIAN_____ @XorNinja and

          I see. Do you know of any higher-level protocols/systems that rely on this property?

          1 reply 0 retweets 0 likes
        5. juraj somorovsky‏ @jurajsomorovsky Jun 9
          Replying to @hdevalence @BRIAN_____ and

          You can take a look at our paper, https://eprint.iacr.org/2017/1014.pdf , Section 7. There we analyze different protocols and their usage of EdDSA. tl;dr: Most of the protocols generate fresh random values which are signed along with the message, so you cannot enforce signature "re-computation"

          1 reply 0 retweets 0 likes
        6. Brian Smith‏ @BRIAN_____ Jun 9
          Replying to @jurajsomorovsky @hdevalence and

          If you can flip buts with Rowhammer then depending on PRNG implementation you might be able to reset PRNG state to replay the same output twice in a row, e.g. many ChaCah20-based PRNGs are used. This would create problems both for randomized EdDSA and for apps w/ nonces in msgs.

          1 reply 0 retweets 0 likes
        7. Brian Smith‏ @BRIAN_____ Jun 9
          Replying to @BRIAN_____ @jurajsomorovsky and

          Further, if you can flip a bit with Rowhammer then you could flip the "TLS handshake is complete" flag and cause the app to accept unauthenticated/unencrypted input and/or write unencrypted output. https://bugzilla.mozilla.org/show_bug.cgi?id=919877 … shows such a magic bit exists/existed in real life.

          1 reply 3 retweets 6 likes
        8. Brian Smith‏ @BRIAN_____ Jun 9
          Replying to @BRIAN_____ @jurajsomorovsky and

          In general, I am skeptical that we need crypto-specific mitigations for Rowhammer and similar bugs, because it seems likely there are always other magic bits that could be flipped to cause the same or worse damage.

          1 reply 0 retweets 2 likes
        9. juraj somorovsky‏ @jurajsomorovsky Jun 9
          Replying to @BRIAN_____ @hdevalence and

          This is true, you could e.g. flip a specific bit to get admin rights. The small motivation for our rowhammer eddsa attacks was that in certain scenarios you could flip ANY bit in a large message to be signed and obtain the key

          1 reply 0 retweets 3 likes
        10. 3 more replies

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