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marcan42's profile
Hector Martin
Hector Martin
Hector Martin
@marcan42

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Hector Martin

@marcan42

If it ain't broke, I'll fix it! · 壊れてねぇのに直すぞ!日本語でもOK! · He/him.

Tokyo, Japan
marcan.st
Joined May 2009

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    1. Hector Martin‏ @marcan42 Jan 16
      • Report Tweet

      To clarify the Windows crypto fail: The problem isn't in signature validation. The problem is the *root store/cache*. CryptoAPI considers an (attacker-supplied) root CA to be in the trust store if its public key and serial match a cert in the root store, Ignoring curve params.

      12 replies 220 retweets 549 likes
      Show this thread
      Hector Martin‏ @marcan42 Jan 16
      • Report Tweet

      So it's not that Windows uses the wrong curve parameters or anything like that, it's that at some point the key used to index into a validated cert cache is (serial, pub) when it should be (serial, pub, params). As they say, one of the hardest problems in CS is caching.

      5:41 AM - 16 Jan 2020
      • 39 Retweets
      • 180 Likes
      • Wayne Werner Ionuț Ambrosie SwitHak Paul Préfontaine Alan Shen Ben Higgins Jeffrey “crypto means hidden” Paul _yossi_ Jared Maxwell
      10 replies 39 retweets 180 likes
        1. New conversation
        2. Dan Kaminsky‏Verified account @dakami Jan 16
          • Report Tweet
          Replying to @marcan42

          Presumably, only for ECC...ah, because only ECC has params significant/agile in this manner?

          1 reply 0 retweets 1 like
        3. Hector Martin‏ @marcan42 Jan 16
          • Report Tweet
          Replying to @dakami

          Yup.

          1 reply 0 retweets 1 like
        4. 7 more replies
        1. New conversation
        2. Rohit Mothe‏ @rohitwas Jan 17
          • Report Tweet
          Replying to @marcan42

          Maybe I'm missing something but based on my tests it seems that even the serial doesn't need to be the same? Just a public key match seems enough to trigger it

          1 reply 0 retweets 0 likes
        3. Hector Martin‏ @marcan42 Jan 17
          • Report Tweet
          Replying to @rohitwas

          Could be, the PoC I saw was explicitly cloning the serial so I assumed that much was needed.

          1 reply 0 retweets 2 likes
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        2. Kevin Hill‏ @CyborgTribe Jan 16
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          Replying to @marcan42

          Is it easier to find params that generate an arbitrary pub key than to find a pub key given the params? Aren't both roughly just at hard? What am I missing?

          1 reply 0 retweets 0 likes
        3. Hector Martin‏ @marcan42 Jan 16
          • Report Tweet
          Replying to @CyborgTribe

          What you're trying to find is the private key given the public key. You cannot find the original private key for the original params, but you can trivially craft parameters in such a way to make a private key of 1 "happen" to correspond to the original public key.

          0 replies 1 retweet 4 likes
        4. End of conversation
        1. New conversation
        2. Tavian Barnes‏ @tavianator Jan 16
          • Report Tweet
          Replying to @marcan42

          Why are the params not part of the serial?

          1 reply 0 retweets 0 likes
        3. Hector Martin‏ @marcan42 Jan 16
          • Report Tweet
          Replying to @tavianator

          What do the params have to do with the serial? The serial is just an arbitrary serial number.

          1 reply 0 retweets 0 likes
        4. 1 more reply
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        2. Conrado Gouvea‏ @conradoplg Jan 17
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          Replying to @marcan42 @sleevi_

          I wonder why does it even touch the supplied CA certificate. Shouldn't it simply get the CA certificate from the trusted store, keyed by the signer Subject/KeyID listed in the child certificate?

          1 reply 1 retweet 0 likes
        3. James Forshaw‏ @tiraniddo Jan 17
          • Report Tweet
          Replying to @conradoplg @marcan42 @sleevi_

          I'd suspect it's because the chain verification is distinct from whether the chain is trusted by policy. Therefore you supply the full chain with the bogus cert, which checks out okay. Then the trust is checked, it compares AuthKeyIDs and find a matching trusted root. Job done!

          1 reply 1 retweet 4 likes
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