jdev - 2022-04-28


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  296. Martin From the Ox XEP 373: > The <signcrypt/> and <crypt/> elements SHOULD furthermore contain a 'rpad' element which text content is a random-length random-content padding. Are there any best practices about the length range for rpads?
  297. Martin Mine is in the range of 20 to 49 chars, but as long as it is random I think I could just use a 0 to 10 chars rpad and don't add unnecessary bloat.
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  349. flow Martin, depends on your paranoia level
  350. flow For example, you could calculate the length of the pad so that a certain minimum total length is guranteed
  351. flow For example, you could calculate the length of the pad so that a certain minimum total length is guaranteed
  352. Zash beware statistics!
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  354. Martin > For example, you could calculate the length of the pad so that a certain minimum total length is guaranteed Yes, but is 20 to 40 chars better than 0 to 20 chars? I don't think so, but I'm no cryptography expert.
  355. Zash I feel like the thing is to pad up to the next multiple of X, but best ask some cryptogopher about how to safely use padding
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  358. flow Martin, the idea is that you take the actualy payload length into account when calculating rpad's length
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  364. flow There are near endless possiblities how to determine rpad, and given that most cryptographic messaging systems don't even have a thing like rpad, it is potentially not super important, but still nice to have
  365. flow especially in IM communcation where the length of the reponse may provide some insights to an outside observer
  366. Zash What's it for here?
  367. Zash AIUI you can counteract padding meant to hide the length of a message using statistics
  368. flow primary to conceal the length of the plaintext
  369. flow Zash, I am happy about some pointers to reserach in that direction
  370. Zash "yes".length+rand(10) > "no".length+rand(10) given enough samples, that kind of thing
  371. Zash I've got no pointers, sorry. RFCs for TLS &c might have references
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  376. Zash Wikipedia! 🙂
  377. flow the one true truth
  378. flow :)
  379. thomaslewis Couldn’t you randomize the argument to rand()? 🤔
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  381. pep. Then one would "just" have to run stats with the same method? :p
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  388. thomaslewis Well, if the padding is of random length and content, it would make statistical analysis near meaningless, no?
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  395. jonas’ incorrect
  396. jonas’ the point of statistics is to get signal out of noise :)
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  404. Martin So rpadlength=messagelength%100 would make sense?
  405. Martin Instead of using random length.
  406. Zash Martin, that's what I think is the sensible thing.
  407. jonas’ Martin, there is a `100-` missing in that, but yes.
  408. jonas’ `rpadlength = 100 - (messagelength%100)`
  409. Zash that reduces the available entropy, right?
  410. Martin Why? Using modulo 100 would make messagelength + rpadlength always a multiple of 100 or am I wrong?
  411. Zash throws some math and statistics words around
  412. jonas’ Martin, you're wrong
  413. Martin Oh yeah
  414. jonas’ len = 120, 120 % 100 = 20, 120 + 20 != 200
  415. Zash if you pad or crop all messages to the same size, that would leak the least data, right?
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  417. jonas’ yes
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  419. jonas’ requires you to pad them all to the maximum stanza size though, otherwise you lose data, obviously :)
  420. Zash indeed
  421. Zash hence the pad to multiple of X
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  423. jonas’ I'm still trying to figure out how much entropy that gives you
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  426. Zash depends on ... distribution of message sizes, no?
  427. jonas’ possibly
  428. jonas’ also possibly on the maximum message siz
  429. jonas’ also possibly on the maximum message size
  430. jonas’ another way to think about it: it conceals log_2(modulus) bits of the real message length
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  432. jonas’ (the logic of which is obvious, if you assume modulus = 2^n (i.e. a power of two); the message length L is a k-bit number. if you pad to a multiple of 2^n, the new message length is L' = m*2^n, with L+2^n >= L' >= L. multiplication by 2^n is identical to left shift by n bits, hence the lowest n bits of the message length are zero, hence the lowest n bits are concealed).
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  441. Martin 😳
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