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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. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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      To decrypt, we check the MAC first by generating it again and making sure they are the same, and then we decrypt the output. Internally there are differences between how HMAC, GCM and Poly1305 generate these signatures, but you don't need to worry about that.

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    2. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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      Today, this combination of operations is wrapped up in function we call "AEAD" which means Authenticated Encryption with Additional Data, and it does all of this is a mostly-foolproof way for you. Basically: AEAD(key, IV, plaintext, additional_data) -> IV_encrypted_data_MAC

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    3. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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      The "additional data" is just any other data you might want to "prove" the sender has, but not send; like say some meta-data that establishes a permission. It's often left empty.

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    4. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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      Now you can still screw up with AEAD. If you re-use the same IV, that's bad!! There are attempts to make this better, my colleague Shay has been working on a cool scheme called SIV, and it adds a measure of protection against that too.

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    5. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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      If you do use unique IVs, modern encryption is really robust. In general, you could publish some encrypted text in the New York Times, and no-one will be able to crack it. This Is true even if /some/ of the text is known. For example ...

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    6. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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      In internet protocols a lot of the text is known, a HTTP server always responds the same way and the first few bytes are known and totally guessable. This doesn't matter at all - doesn't help an attacker figure anything else out even one bit. We've come a long way from WWII.

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    7. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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      But there are attacks that do work! If you're sending this data over a network, and someone can see the timing and size of message. This opens us up to traffic analysis.pic.twitter.com/8qeI9A3Ozp

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    8. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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      Let's look at length first. O.k. so the length is obviously not hidden. That's fine if you're trying to protect your password or credit card number in the middle of a response. No big deal. But it does mean that someone might be able to fingerprint the content you're sending.

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    9. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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      Simple example: if you send a gif over a messaging app, if the size of that gif is unique, someone in the middle can probably guess what gif you just sent. There are more sophisticated versions of this for Google Maps, Netflix, WikiPedia, and so on.

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    10. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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      The way we protect against this is to "pad" messages, to make large numbers of messages appear to be the same size no matter what. Military grade network encryption actually pads all traffic all the time, so it's always the same!

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      Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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      Another problem with length is that if you're using compression, and let attackers control any of the content on a page that a user sees, that can let the attackers figure out even small secrets. Look up the "CRIME" attacks. It's awesome, and scary.

      12:54 PM - 1 Mar 2019
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      • brian Jimmy ༻ᵏᵘᵐᵃᵛⁱˢ༄༜ smallstep Rob Burke Carson Kelly Clarkson (Since You Been Gone) Brian Broom
      3 replies 0 retweets 7 likes
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        2. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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          I said the other problem is timing. Obviously the timing of each message is public, but is that a big deal? It can be! For example, if you send a message for every use keystroke, it's trivial to figure out what they're typing through timing analysis. WOW.

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        3. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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          Another example is VOIP. If your call app only sends data when people are speaking, but not during the silences, that's still enough to guess about 70% of English-language speech. Just from the silences! Scary cool.

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        4. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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          These examples underling: even when you use encryption algorithms and schemes we've been perfecting for about 80 years, there's still some gaps you can walk into and break the security. Which is why this stuff is worth knowing!

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        5. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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          Anyway, that's the level I'm going to stick at for now, but we've covered a lot of ground. If you've finished this thread, thank you! But also you should now have some kind of better understanding of what's going on, and what to be wary of. Feel free to AMA.

          3 replies 0 retweets 31 likes
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        6. Colm MacCárthaigh‏ @colmmacc 1 Mar 2019
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          Oh the truth table for XOR is wrong. I guess it's more of a lies table. Should be: a | b | c 0 | 0 | 0 1 | 0 | 1 0 | 1 | 1 1 | 1 | 0

          5 replies 0 retweets 36 likes
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        7. End of conversation
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        2. Jonathan Hope‏ @jhope30203 7 Mar 2019
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          Replying to @colmmacc

          This thread is amazing, if even for just this tweet. Colm, quick question while I search for the answer online: is the CRIME vulnerability viable because of the use of compression on messages that include cipher texts, or because TLS encrypts compressed data?

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        3. Colm MacCárthaigh‏ @colmmacc 7 Mar 2019
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          Replying to @jhope30203

          CRIME is viable when you compress attacker-controlled "guess" data AND victim-valuable secret data. Both in the plaintext that's compressed, then encrypted. If the attackers guess overlaps with the secret, the overall size will be smaller, and the size isn't hidden by encryption.

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