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__agwa's profile
Andrew Ayer
Andrew Ayer
Andrew Ayer
@__agwa

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Andrew Ayer

@__agwa

Bootstrapped founder of @SSLMate, where I make SSL certificates easier and do #webpki and #CertificateTransparency stuff.

Cambridge, MA + SF Bay Area
agwa.name
Joined November 2011

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    1. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 30 Jan 2019
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      It looks like the ACME working group built a generic client-authenticated replay-resistant protocol on top of HTTPS as a side product. https://tools.ietf.org/html/draft-ietf-acme-acme-18 …

      1 reply 2 retweets 24 likes
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    2. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 30 Jan 2019
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      Although I suspect the signed out-of-band challenges are critical to the resistance against CDN MitM. Not that most protocols would care.

      1 reply 2 retweets 4 likes
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    3. Andrew Ayer‏ @__agwa 31 Jan 2019
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      Replying to @FiloSottile

      I'm afraid I don't follow... the challenges aren't signed and don't have anything to do with CDN MitM resistance.

      1 reply 0 retweets 1 like
    4. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 31 Jan 2019
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      Replying to @__agwa

      Without that side channel the CDN could just make its own account key, and MitM everything, right? I assume the challenges are still somehow tied to the account key?

      1 reply 0 retweets 1 like
    5. Andrew Ayer‏ @__agwa 31 Jan 2019
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      Replying to @FiloSottile

      The challenges do contain the account key fingerprint. It's an important distinction, because early drafts used signed challenges rather than a fingerprint due to a mistaken belief that it was necessary for CDN MitM resistance, and this was extremely bad: https://mailarchive.ietf.org/arch/msg/acme/F71iz6qq1o_QPVhJCV4dqWf-4Yc …

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      Andrew Ayer‏ @__agwa 31 Jan 2019
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      Replying to @__agwa @FiloSottile

      The structure of the challenges do stop a malicious CDN from getting unauthorized certificates, but they also stop anyone else from getting unauthorized certificates, so I don't really think of them as a CDN-specific security measure.

      11:20 AM - 31 Jan 2019
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      • 🏡 Matt Holt Royce Williams
      1 reply 0 retweets 2 likes
        1. New conversation
        2. Andrew Ayer‏ @__agwa 31 Jan 2019
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          Replying to @__agwa @FiloSottile

          Personally, I think the CDN MitM resistance thing is a big (and nearly fatal) distraction from ACME's core security model: a CSR is signed by an ACME account key, and the fingerprint of the ACME account key is placed in DNS/.well-known/etc. so therefore the CSR is legit.

          1 reply 0 retweets 0 likes
        3. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 31 Jan 2019
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          Replying to @__agwa

          Right, all I’m saying is that if you reuse the ACME “transport layer” for something that doesn’t have challenges, you don’t get that protection.

          0 replies 0 retweets 0 likes
        4. End of conversation

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