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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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    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.

    5:38 AM - 16 Jan 2020
    • 220 Retweets
    • 549 Likes
    • Saw-mon & Natalie CSIRT-SC Scott Schryver Ing. Yamila Levalle Chris Hammond james ay Kai Engert, ⛈️🐦,🔥🦊,🐧,🇪🇺 marco slaviero AlterLabs
    12 replies 220 retweets 549 likes
      1. 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.

        10 replies 39 retweets 180 likes
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      2. Geir-Arne‏ @MrG_A Jan 17
        • Report Tweet
        Replying to @marcan42

        How difficult is it to exploit? Ie. how much processing power does it take to create the key pair with an identical public key? Or perhaos those are already for sale?

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

        It takes the same amount of time as to generate a key normally. So microseconds. Maybe milliseconds.

        1 reply 0 retweets 3 likes
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      2. Elichai Turkel‏ @Elichai2 Jan 17
        • Report Tweet
        Replying to @marcan42

        I disagree with your assessment. Yes it could've been fixed by storing cache correctly, but that's like a compiler dev saying "well what you did is UB". He's right but also wrong, crypto should be designed to be resilient. TLS and Crypto32 should've never allowed custom curves.

        1 reply 1 retweet 4 likes
      3. Hector Martin‏ @marcan42 Jan 17
        • Report Tweet
        Replying to @Elichai2

        The bug is in not treating EC params as part of certificate identity. However, you *are* correct that supporting custom curves at all is a bug, because RFC5480 explicitly forbids that. I assume they support it because ANSI X9.62 does, because banks or something?

        1 reply 0 retweets 10 likes
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      2. Kevin Marquette‏ @KevinMarquette Jan 17
        • Report Tweet
        Replying to @marcan42

        So the attacker provides a cert that gets cached. If that cert has a pub key and serial that matches one the user already has in his local root store, the attacker cert is trusted and used? Is crafting a cert like that possible? I'm assuming yes, or did I miss something?

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

        The real cert gets cached, then the attacker cert masquerades as the cached cert and gets trusted. Yes, doing this is trivial and there is code on GitHub already. It's standard OpenSSL commands and a few lines of Python.

        1 reply 1 retweet 6 likes
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      2. Hector Martin‏ @marcan42 Jan 17
        • Report Tweet
        Replying to @byuu_san

        Are you tempting me? :P

        0 replies 0 retweets 16 likes
      3. 4 more replies
      1. Daniel B‏ @danixdefcon5 Jan 16
        • Report Tweet
        Replying to @marcan42

        Finally! Thanks! This is the first time I get details on this. I was unsure how exactly the “fake cert” was able to impersonate a real root CA!

        0 replies 0 retweets 10 likes
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