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CasCremers's profile
Cas Cremers
Cas Cremers
Cas Cremers
@CasCremers

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Cas Cremers

@CasCremers

Professor of Computer Science, Faculty at CISPA Helmholtz Center for Information Security

Saarbrücken, Germany
Joined February 2016

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    1. Cas Cremers‏ @CasCremers Jan 14
      • Report Tweet

      Cas Cremers Retweeted NSA/CSS

      It looks like it exploits what Vaudenay warned against in 2004 : "Digital Signature Schemes with Domain Parameters" ( https://lasec.epfl.ch/pub/lasec/doc/Vau04b.pdf … )https://twitter.com/NSAGov/status/1217152211056238593 …

      Cas Cremers added,

      NSA/CSSVerified account @NSAGov
      This #PatchTuesday you are strongly encouraged to implement the recently released CVE-2020-0601 patch immediately. https://media.defense.gov/2020/Jan/14/2002234275/-1/-1/0/CSA-WINDOWS-10-CRYPT-LIB-20190114.PDF … pic.twitter.com/log6OU93cV
      3 replies 71 retweets 175 likes
      Show this thread
    2. Cas Cremers‏ @CasCremers Jan 14
      • Report Tweet

      Pre-patch, a cert can be used if the hash of the public key matches one in the cert root, after which the parameters of the new cert get used, enabling Vaudenay's attack. The patch requires the curve parameters to be the same, preventing this attack.

      2 replies 3 retweets 24 likes
      Show this thread
    3. Cas Cremers‏ @CasCremers Jan 14
      • Report Tweet

      This is ultimately similar to Pornin and Stern https://www.bolet.org/~pornin/2005-acns-pornin+stern.pdf … (2005). (Which we built on for https://ia.cr/2019/779  )

      1 reply 2 retweets 23 likes
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    4. Cas Cremers‏ @CasCremers Jan 15
      • Report Tweet

      Based on what I know now, I think the attack fits into a single tweet, including references:

      1 reply 2 retweets 11 likes
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    5. Cas Cremers‏ @CasCremers Jan 15
      • Report Tweet

      1. Find an ecc root cert C with pk 2. Apply Vaudenay|(Pornin&Stern) 2004 get C' with sk',params' for that pk 3. Create a normal code signing cert C'' with key pair (pk'',sk'') and sign software with sk'' 4. Sign C'' with sk' 5. Present software,C'',C' to windows' sigcheck64.exe

      1 reply 34 retweets 95 likes
      Show this thread
    6. Cas Cremers‏ @CasCremers Jan 15
      • Report Tweet

      @kennyog @Dennis__Jackson @matthew_d_green @hyperelliptic

      2 replies 0 retweets 3 likes
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    7. Cas Cremers‏ @CasCremers Jan 15
      • Report Tweet

      Can someone please confirm/deny if this degenerate version works? (It is still Vaudenay 2004 but with d' the identity) @kennwhite @saleemrash1d It would be easier to detect in logs of course.

      1 reply 1 retweet 5 likes
      Show this thread
      Cas Cremers‏ @CasCremers Jan 15
      • Report Tweet

      1. Find an ecc root cert C 2. Create C' with the same public key and curve but set the generator to the public key of C 3. Create a normal signing cert C'' with key pair (pk'',sk'') and sign software/cert with sk'' 4. Sign C'' with sk=1 5. Ship software/cert with C'' and C'

      10:14 AM - 15 Jan 2020
      • 68 Retweets
      • 156 Likes
      • Michael Diaz Joachim Strömbergson undefined Ilya Paul M Gerhardt Ralf Sasse Hector Martin 💿 JohanS
      2 replies 68 retweets 156 likes
        1. New conversation
        2. Cas Cremers‏ @CasCremers Jan 15
          • Report Tweet

          This degenerate case was just confirmed to work by @reaperhulk , thank you! (@kennyog @kennwhite )

          2 replies 7 retweets 31 likes
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        3. Cas Cremers‏ @CasCremers Jan 16
          • Report Tweet

          @kennwhite Curve Validation Errorpic.twitter.com/8XEQc9JYwf

          1 reply 0 retweets 9 likes
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        4. Cas Cremers‏ @CasCremers Jan 17
          • Report Tweet

          When comparing a received cert to cached root certs, windows only compared the public keys, but not the parameters, and would therefore assume that a received fake root cert C' with different parameters was the same as a cached root cert C, using C' to verify the cert chain.

          2 replies 3 retweets 5 likes
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        5. Cas Cremers‏ @CasCremers Jan 18
          • Report Tweet

          By choosing the right parameters for C', you can know the private key for C' -- even when you don't know the private key for C -- as Vaudenay noted in 2004.

          0 replies 0 retweets 2 likes
          Show this thread
        6. End of conversation
        1. New conversation
        2. 3-5 30-50x elves in a trench coat‏ @leftpaddotpy Jan 16
          • Report Tweet
          Replying to @CasCremers

          for context: A defined curve has an order n [prime number] and a generator point G [point] privKey ≡ random number [integer] pubKey ≡ privKey * generator [scalar * point = point] the part I don't quite get yet is where the generator/"curve" is stored and if it is modifiable

          1 reply 0 retweets 2 likes
        3. 3-5 30-50x elves in a trench coat‏ @leftpaddotpy Jan 16
          • Report Tweet
          Replying to @leftpaddotpy @CasCremers

          because I thought the "curve" was a fixed thing, so which part is stored in the cert if any? can you have custom ones and it will still accept them? source on terms:https://cryptobook.nakov.com/digital-signatures/ecdsa-sign-verify-messages …

          0 replies 0 retweets 3 likes
        4. End of conversation

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