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colmmacc's profile
Colm MacCárthaigh
Colm MacCárthaigh
Colm MacCárthaigh
@colmmacc

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Colm MacCárthaigh

@colmmacc

AWS, Apache, Crypto, Irish Music, Haiku, Photography

Seattle
notesfromthesound.com
Joined April 2008

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    1. Matthew Green‏ @matthew_d_green Apr 10
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      I feel like the crypto in the COVID tracing apps is so simple that it doesn’t need much explanation. But I also see plenty of people nervous about the privacy these things provide. Is it worth explaining?

      24 replies 22 retweets 151 likes
      Show this thread
    2. Colm MacCárthaigh‏ @colmmacc Apr 10
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      Replying to @matthew_d_green

      I think there's some more to it that is going unappreciated. The Google/Apple scheme looks optimized for a hybrid between filter distribution (first pass) and exact-match confirmation (second pass) in a way that reduces bandwidth a lot.

      1 reply 0 retweets 4 likes
    3. Colm MacCárthaigh‏ @colmmacc Apr 10
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      Replying to @colmmacc @matthew_d_green

      Because things are derived deterministically by time, the app can fetch a filter, or a set of exact infected ids (whichever is smaller), that covers the time periods it knows it had contacts. And then see if there are any matches.

      1 reply 0 retweets 1 like
    4. thaidn‏ @XorNinja Apr 10
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      Replying to @colmmacc @matthew_d_green

      Yes, but some people might consider fetching IDs by time a bug, not a feature. It allows Alice to know that Bob was infected because they met alone 3PM yesterday.

      1 reply 0 retweets 4 likes
    5. Colm MacCárthaigh‏ @colmmacc Apr 10
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      Replying to @XorNinja @matthew_d_green

      Good point, but the same applies to days anyway. In practice, which is better might hinge on whether the mobile providers are willing to zero-rate the daily dataset download.

      1 reply 0 retweets 3 likes
    6. thaidn‏ @XorNinja Apr 10
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      Replying to @colmmacc @matthew_d_green

      Days are *slightly* better because Alice may also meet Mallory. Downloads can happen over WiFi at night, like how photos are being backed up. I agree less data is better.

      1 reply 0 retweets 3 likes
    7. Colm MacCárthaigh‏ @colmmacc Apr 10
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      Replying to @XorNinja @matthew_d_green

      If I crack the downloaded dataset (which at least has to be in memory on the phone), can't I narrow down the time to a 10 minute window anyway?

      1 reply 0 retweets 0 likes
    8. thaidn‏ @XorNinja Apr 10
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      Replying to @colmmacc @matthew_d_green

      Yes, you can do that with the current scheme because pseudo ID X are derived from time T and key K. Knowing K and X, you can brute-force T. I hope that this time-based derivation will go away.

      1 reply 0 retweets 1 like
    9. Colm MacCárthaigh‏ @colmmacc Apr 10
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      Replying to @XorNinja @matthew_d_green

      If you're going to that volume of data, doing 63-bit ECDH between bluetooth senders is doable. Distribute a cuckoo filter to find the matches. That removes correlation and passive attacks. Though it does take bi-directional bluetooth ID receipt instead of unidirectional.

      1 reply 0 retweets 1 like
    10. thaidn‏ @XorNinja Apr 10
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      Replying to @colmmacc @matthew_d_green

      There was a discussion about using blinded bloom filter to reduce download size. It was rejected because it enables the server to mount active targeted attacks

      2 replies 0 retweets 3 likes
      Colm MacCárthaigh‏ @colmmacc Apr 10
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      Replying to @XorNinja @matthew_d_green

      What if you make it two stage? bloom filter to determine if it's worth downloading a dataset of exact matches. The dataset can be sharded arbitrarily by the first N bits of matching IDs. Of course don't let N get too big.

      6:26 PM - 10 Apr 2020
      0 replies 0 retweets 0 likes

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