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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. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 10 Mar 2019
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      I believe the recent efforts to bring Forward Secrecy to TLS 1.3 0-RTT data are far, far into the realm of diminishing returns. Thing is, I don't think FS is that valuable in itself. Thread.

      3 replies 43 retweets 125 likes
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    2. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 10 Mar 2019
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      When PFS cipher suites first came along they were a big leap forward in the security of TLS, for two reasons: 1) they provided Forward Secrecy (if you are compromised tomorrow, today's connection is safe) relatively to very long-lived key material—certificates that lasted years

      1 reply 0 retweets 10 likes
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    3. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 10 Mar 2019
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      2) less obviously, they provided what we now call Post-Compromise Security (if you are compromised today, tomorrow's connection is safe) against passive attackers If all an attacker has is your certificate key, with PFS it has to mount a MitM to eavesdrop, which does not scale!

      1 reply 1 retweet 10 likes
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    4. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 10 Mar 2019
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      That second property, PCS, is critical in disrupting dragnet surveillance, in particular in today's era of geographically distributed CDNs. If you can take a small key from a machine in country X and use it to _passively_ decrypt all traffic to a website globally, it's Bad News.

      1 reply 0 retweets 15 likes
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    5. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 10 Mar 2019
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      So PFS cipher suites were good, but then Session Tickets were implemented in the worst way possible and messed it all up again. Now there is again a key of unspecified lifetime with passive decryption capabilities. I wrote about it some time ago.https://blog.filippo.io/we-need-to-talk-about-session-tickets/ …

      1 reply 18 retweets 42 likes
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    6. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 10 Mar 2019
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      Anyway, TLS 1.3 does it all right instead: 1) it caps the Session Ticket Encryption Key lifetime to 7 days and 2) it allows a Diffie-Hellman exchange on resumption, bringing back Post-Compromise Security. If all you have is the STEK, you again need to mount a noisy MitM. Yay!

      1 reply 0 retweets 18 likes
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    7. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 10 Mar 2019
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      Unfortunately, you have to encrypt the 0-RTT data before the DH exchange, so early data gets no Forward Secrecy relatively to the STEK. Which brings us to the new schemes to use puncturable encryption (which don't get me wrong, is super cool!) to provide it.

      1 reply 0 retweets 12 likes
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    8. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 10 Mar 2019
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      The idea is that you forget the pieces of the key you used to decrypt a certain session ticket after using them. The problem is that this does not provide any Post-Compromise Security: the attacker that stole the whole key yesterday is not going to forget any of it.

      1 reply 0 retweets 12 likes
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    9. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 10 Mar 2019
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      IMHO, the scenario where an attacker quietly extracts a key from a single machine to then passively decrypt traffic is much more likely than the one where the attacker breaks in to decrypt yesterday's traffic. In particular because the lifespan of STEKs is now capped at 7 days.

      2 replies 3 retweets 12 likes
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    10. Filippo Valsorda  🇮🇹‏Verified account @FiloSottile 10 Mar 2019
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      (Moreover, if you can sync state between your servers to do the puncturing, you might as well do Session IDs instead of Session Tickets, so there's no STEK to begin with, and you get real PFS and some PCS—that is, the attacker has to keep exfiltrating a lot of keys, noisily.)

      2 replies 0 retweets 8 likes
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      Colm MacCárthaigh‏ @colmmacc 11 Mar 2019
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      Replying to @FiloSottile

      Puncturing doesn't really work if you use a distributed store; you need atomic delete-and-retrieve, so you'd have a large slow transaction span. The stores can be sharded easily though, and the benefit Vs plain session-ids is less memory usage, at the cost of more memory access.

      1:58 PM - 11 Mar 2019
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      • Patrick McDowell
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