XSF Discussion - 2021-02-11


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  151. flow emus, check https://wiki.xmpp.org/web/Kevin_Smith_Application_2020
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  165. Kev Yeah, just get the Board member to mail me at my isode account please.
  166. Kev I thought all of Board knew how to contact me by now, sorry.
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  170. emus Thanks, yeah I just checked your page
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  177. arc goffi: when you're around, I'd love to talk about xep-0355
  178. goffi arc: hi, I'm here
  179. arc When you drafted this, did you consider using xpath instead of namespace plus attributes?
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  182. arc One thing that you have written here that I had not considered is client defined services. It is an interesting idea, though I am curious how you envision this being implemented in the real world
  183. goffi initially there was only namespace, the attribute has been added because it was the only to distinguish MAM for chat from MAM for Pubsub. XPath would probably complicate (and servers would need to have a handy xpath implementation available).
  184. goffi Note that I would love to have xpath or at least simplified xpath in XMPP
  185. arc XPath is certainly more complicated. But is also more versatile. One of the problems with XPath is modern implementations are rare and hard to come by.
  186. Kev As a standards body we’re generally wary of xpath because of the burden it places on implementations.
  187. MattJ FWIW XPath is also being discussed as a solution for improving push notifications currently
  188. MattJ Maybe 2021 is the year to embrace XPath :)
  189. Zash Is "simplified xpath" a thing?
  190. MattJ XMPPath
  191. goffi I've written this years ago, so I need to refresh a bit my memory ^^. But the idea was basically that you if you want e.g. advanced PEP and your server doesn't offer that, you could have a third party implementation and ask yourself your server to redirect stanza there, without having to wait for admin or anything.
  192. Kev MattJ: Ah, interesting. Where’s the push notification discussion happening? I missed that if it was on list.
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  194. goffi Zash: I don't know if it's already a thing, but just keeping the base of path and attribute matching, without all the methods could be useful and easy to implement.
  195. Kev I’m coming to the conclusion that we need two distinct models for push notifications - one for e2e and one for non-e2e.
  196. Kev And trying to address both with the same mechanism is likely to be painful.
  197. Zash goffi, so just a path like `/{xmlns}name/...`?
  198. goffi Zash: for instance yes, maybe with attribute matching. I don't know if this would be enough for our needs though.
  199. arc Nobody wants to work on lower level XML software. It's not sexy. It is easily overlooked as a resume filler. Nobody will pay for it. So that boils down to students or retired developers who love XML and take it on as a challenge.
  200. Kev Teensy bit of hyperbole there :)
  201. Zash No true Scotsman wants to work on XML! /s
  202. Kev Zash: :D
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  204. arc Kev: is it? I mean certainly there are /some/ people working on it.
  205. Andrzej Kev, there was noting on the list about push notifications and xpath, but it was just an idea to allow client select what it wants to have in the push notifications payload (assuming it is encrypted) https://github.com/tigase/tigase-xeps/issues/4
  206. Andrzej or just filter what is expected to send push notification and what is not
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  208. Zash Including a stanza "skeleton" was suggested at some point as well, ie stripping all content and attributes, leaving only `<name xmlns="">` of each tag.
  209. Zash Uh, with my server dev hat on, plz no XSLT in the server
  210. Andrzej Zash, that was just an idea, something else allowing to get some data out of stanza and trigger push notifications would be good as well
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  214. arc I am certainly on board for discussing a limited XDM for use with a XMPP stream
  215. Ge0rG I think the idea of stanza skeletons was that the skeleton gives the push server all required info for deciding whether to make a silent or a noisy push notification, without leaking any actual user data
  216. Ge0rG So this is a different use case from "send encrypted message payload over push"
  217. arc Not sure I follow
  218. Zash Right, giving the client everything it needs vs giving the push gateway everything it needs.
  219. Andrzej after online FOSDEM and Matrix usage I'm pretty sure that usage of Push & Fetch is a bad thing compared to pushing encrypted data (almost 30% of battery used by Element fetching data)
  220. Andrzej I think that giving control to the client would allow them to evolve quicker with less changes on the server side
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  223. Ge0rG Andrzej: I'm not sure if FCM and APNS will be okay with carrying all your encrypted payloads.
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  225. Andrzej Why not? there is a limit of 4KB but it would work
  226. Ge0rG That said, what you fundamentally want in that case is to initiate/terminate the xmpp client session on your app server and have a custom protocol that pushes over APNS to your client
  227. Ge0rG because then you can optimize everything
  228. Ge0rG As long as you have the xmpp client connection terminated on the mobile device, it will have to reconnect to the server and to resume the 0198 session rather often
  229. Andrzej every 30-120s on iOS and most likely will be killed anyway
  230. Ge0rG Andrzej: only if you receive something that warrants an ack
  231. Andrzej that is why I prefer to have offline client and pushes with notifications on which user can act (ie. open app)
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  233. Ge0rG "Your chat program received an XML stanza. Open app to see if it was something you care about"
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  235. Andrzej as for APNS and encrypted push, they've created even example for doing that https://developer.apple.com/documentation/usernotifications/modifying_content_in_newly_delivered_notifications see Listing 1
  236. Andrzej Ge0rG, I was usable "content" in the notification and with encryption and XPath that could work quite well
  237. Ge0rG Andrzej: well, I suppose you could use that to send an encrypted blob to the app
  238. Andrzej Ge0rG, I want usable "content" in the notification and with encryption and XPath that could work quite well
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  240. Andrzej I'm already doing that in Tigase & Siskin and AFAIR in Prosody
  241. Ge0rG Andrzej: so what you want from XEP-0357 is to pass the full stanza to the app server?
  242. Andrzej no
  243. Zash Pass the parts you're interested in, encrypted, via the app server to the client?
  244. Andrzej to allow client send "XSLT" to tranform stanza in "encrypted payload" opaque to the app servers
  245. Ge0rG Andrzej: but the app server is under your control, so you can do arbitrary modifications ther
  246. Andrzej this transformation and encryption is done on the XMPP server side
  247. Ge0rG Andrzej: but the app server is under your control, so you can do arbitrary modifications there
  248. Andrzej yes, but XMPP server is leaking user data to the app server in your example
  249. Ge0rG Andrzej: that's a very significant effort for the xmpp server. What key should it use?
  250. Ge0rG also that means the xmpp server needs to have significant knowledge over the used client / app-server infrastructure
  251. Andrzej client with transformation would upload key, in my case AES128 key
  252. Ge0rG Why not an ec25519 key?
  253. Ge0rG You don't really need the server to be able to decrypt the payload, right ;)
  254. Andrzej right, but it is aware of it anyway, so it can decrypt it
  255. Ge0rG I am not sure if the trade-off of leaking the private data to the app server is a big thing.
  256. Andrzej I'm not sure which algorithm is faster, I've assumed AES128 is good in this case
  257. Ge0rG Well, encryption isn't the bottleneck; XSLT is
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  259. Zash XSLT seems like overkill.
  260. Andrzej As I've said, it does not have to be XSLT, but could be list of fields + XPath to fetch then
  261. Andrzej As I've said, it does not have to be XSLT, but could be list of fields + XPath to fetch them
  262. Kev > Uh, with my server dev hat on, plz no XSLT in the server M-Link has xslt to multiple reasons :| > just an idea to allow client select what it wants to have in the push notifications That seems sane at first thought.
  263. arc Andrzej: I tend to use XPath as a generic term for XDM myself so this is no way a criticism, I'm just wondering if you are talking about XPath proper
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  265. Andrzej I'm not sure, I was thinking (as XSLT cannot be used) to create some XEP that would specify filteiring data and fetching them from existing XMPP schema in to some structure, basically transforming stanza into something else
  266. Andrzej to filter and fetch data I was thinking about using XPath, but I could be wrong
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  269. Ge0rG Andrzej: what about a whitelist of xml namespaces to retain in the <message> element? would that suffice?
  270. Ge0rG something like https://hg.prosody.im/prosody-modules/file/64b7daa6c42c/mod_csi_battery_saver/mod_csi_battery_saver.lua#l105 but more formalized
  271. Andrzej as we have 4KB hard limit, I think that might be not enough to just filter elements
  272. Andrzej but if clients could decide what should be included that would be a step forward
  273. Zash Namespaces does seem like a plausible Good Enough, and is easy to implement.
  274. jonas’ 4kb after base64, right?
  275. Andrzej yes, correct, after base64
  276. jonas’ so in effect just about 1kB
  277. Andrzej not 3KB?
  278. Zash `base64(encrypt(strip(<message/>)))`?
  279. Zash Depending on how much we trust the app server to not be /too/ evil, we could stuff some compression in there too
  280. Andrzej Zash, I think this could be too big and in some cases it is more valuable to lose some of the message <body/> but deliver notification correctly
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  282. Zash How big are your messages?
  283. arc Xmpp does not implement full XML. Never did. It is a subset. So I don't see a problem with creating a subset of XDM
  284. Ge0rG So you end up encoding the limits of google and apple services into my server.
  285. arc XPath and xquery both use XDM
  286. jonas’ right, 3kB, not just 1
  287. jonas’ Zash: with omemo, messages can get big quickly
  288. Zash I was just thinking that, with omemo, the server can't do anything like ship half the <body>
  289. Andrzej is server is aware of clients OMEMO device id it could filter out keys in the notification and notification could still work
  290. jonas’ XDM seems to be xpath 2.0, which afair lacks implentations and is may be overkill
  291. Andrzej if server is aware of clients OMEMO device id it could filter out keys in the notification and notification could still work
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  294. arc Referring back to my earlier statement about how nobody wants to work on XML stuff 😆
  295. Zash You can have all of XPath as long as it's only `/{xmlns}name/{otherns}foo@bar/` and nothing else.
  296. mathieui Zash, and attribute matching!
  297. Zash Nope!
  298. arc At this point we're really talking about XPath 3.0 or quite possibly XPath 4.0
  299. Zash Prosody doesn't have that, so it doesn't exist!
  300. arc Given that everything in this community takes 10 years or more to come about
  301. jonas’ arc: who is "we"?
  302. arc We in this room right now, I would hope
  303. arc And this community as in XML community. Given that we are still stuck with XPath/xslt/etc in browsers now in 2021
  304. Zash I'd refer back to what Kev said
  305. arc XSLT is perfectly fine stop hating on it 😋
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  308. Andrzej arc, would exi work for transforming XML stanza into "payload" for the client?
  309. Andrzej maybe I should be thinking about EXI instead of XSLT
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  312. Zash I am, of course, referring to https://logs.xmpp.org/xsf/2021-02-11?p=h#2021-02-11-521589a72d0ff42a
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  318. arc No. EXI is just an alternative representation of XML data. It does make XDM/XPath/Xquery/XSLT faster when written with it in mind. But all processing is generally faster with the EXI because string compare is just that much slower
  319. Ge0rG I'm sure there is nothing wrong with executing attacker-provided XSLT on your XMPP server.
  320. arc I never said processing XSLT provided by the user.
  321. arc XSLT is a turing complete language. You can open a similar glaring security hole by running user provided python, or most languages for that matter.
  322. Zash (XEP-0060 has references to XSLT 😱️)
  323. Andrzej also (as it was mentioned that I would like to "encoding the limits of google and apple services into my server"), I think that transformation and encoding is OK and would not impose limits on your server and if client would pass "limit" of the payload which it can receive, then server would just respect request from the client
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  326. Zash So, size constraint. How about an ordered-by-priority list of payloads you're interested in? Server strips anything not in that list, then if it's still too large, strips the lowest priority payloads until it's small enough?
  327. Zash Where "list of payloads" could be xpath or just namespaces or somesuch, details.
  328. Andrzej that "could" work
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  331. arc Andrzej: this is a simplistic but valid way of thinking about EXI; within the root of a XMPP stream you typically only find about 6-10 elements; obviously your IQ, MESSAGE, and PRESENCE elements (capitalized only for clarity), stream: namespace stuff, and possibly session management.
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  333. arc Text domain XML would have the xml parser run multiple passes over the data stream to find delimiters (space, brackets, quotes, etc), break the stream up into events, possibly create a minidom out of those events, and then pass qnames (namespaces, prefixes, element names, attribute names, etc) to the application typically as string pointers into the stream buffer. But increasingly for memory safety, memcpy all those strings into new buffers too.
  334. arc And then no matter how the client or server handles stanza routing, it either ends up testing the qnames against a large list of qnames it's designed to pass to various functions, or it hashes the qnames and checks for a value on a hashmap. In every case there's thousands of machine instructions for every stanza.
  335. arc Going back to those, lets say always 15 or less possible elements found in a <stream:stream> root, EXI represents that as a binary number which typically is either bitpacked with only four bits used, or structured for later compression. In every case, you don't get "message", you just get the binary number 1. Which is much much simpler to parse.
  336. arc Ignoring the stuff you can read up on if you ever wanted to implement it, that is EXI in a nutshell. And that is why it is not just 25x or more faster than text XML processing, it also takes far less memory and can be much more easily squeezed onto a microcontroller.
  337. Andrzej ok, I get it now, thanks
  338. Kev Has anyone tried doing EXI for S2S?
  339. arc Yes.
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  342. Kev Hmm. Going from https://www.w3.org/XML/EXI/#Efficient_XML I find only one (commercial) EXI implementation for C/C++ that doesn’t claim to be alpha quality.
  343. Zash Someone did mention a lack of libraries earlier.
  344. Kev Indeed.
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  346. arc Transforming stanzas between exi grammars is not difficult provided your grammar mapping is set up correctly
  347. Kev arc: Was that responding to me? Because if so I don’t see how it relates :)
  348. Kev arc: Was that responding to my lack of library comment? Because if so I don’t see how it relates :)
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  352. arc Using it for s2s, you're typically going to be routing stanzas between grammars. So in the above example, a xmpp client might say a message=1, but you may route that stanza to a server using a grammar that says message=0
  353. Kev Ah, my previous question, gottit, thanks.
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  356. arc With text domain xmpp you often don't have to modify the stanza. And when you do, only in small specific ways like changing the from= attribute. But with EXI S2S is also grammar conversion.
  357. arc But going back to the xpath discussion, XDM 3.0 was certainly designed with EXI in mind though it is not referenced directly. EXI does not solve any problems in this area, it only makes things faster and with less bandwidth.
  358. Kev After a few minutes of looking, I’ve come to the conclusion that there’s no implementation for C/C++ that doesn’t claim to be alpha/seem abandoned other than one commercial Windows-only one. And, as you asserted earlier, I don’t want to be working on an EXI parser :)
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  360. arc There are more, and yes they are commercial. Thankfully no one has released a free one that is ready to use or I would start having trouble paying my rent.
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  364. arc Anyways all that is simply different problem. It doesn't help with identifying stanzas for routing to external software, whether you want to call that microservices or not.
  365. Ge0rG Re EXI I think we could have significant benefits on mobile / low bandwidth, if a client implementation creates a grammar of everything supported by the client, uploads it to the server in some secure way, and the server only ever uses elements within that grammar to the client, stripping everything unknown and dropping empty elements.
  366. Ge0rG Your client doesn't support CSNs? It's omitted from the grammar, server strips out the element from messages, empty messages get dropped on the server, battery wins
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  371. jonas’ except only proprietary implementations, so no chance there
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  390. Ge0rG jonas’: implementations of EXI?
  391. Ge0rG How hard can it be?
  392. jonas’ how hard can a binary xml parser be?
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  398. Kev I mean, you just take an XML parser and you change some strings to numbers, right? Job done ;)
  399. Zash That sounds fun, we could call it FunXMPP
  400. Kev Funkkit/
  401. Kev Funkkit?
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  410. Ge0rG it's not-fun, so let's call it NunXMPP. Will also make clear that it's neither fun nor sexxy.
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  450. Steve Kille f
  451. Steve Kille f
  452. Steve Kille ignore me
  453. Zash "push 'f' to pay respects"?
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  568. arc You do not need to even parse. All you really need is some basic data structures. EXI is extremely easy to implement a reader and writer for.
  569. neshtaxmpp has left
  570. arc The difficult part is generating grammar based on schema, then generating code based on grammar. And you'll see that for most cases, this is the part that is not provided
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  572. arc I have seen some foss implementations that work fine, but require that you provide a grammar file. Or provide all the low level data structures and the header, but leave the application api to build your own EXI decoder/encoder.
  573. moparisthebest what are the upsides? just that it's ever-so-slightly smaller?
  574. jonas’ moparisthebest, savings by EXI are more than just "ever so slightly". especially when base64’d stuff comes into play. my understanding is that you could transfer that in decoded form in EXI
  575. moparisthebest which saves what, 33% only in the case of base64'd stuff?
  576. jonas’ that’s considerable on GPRS
  577. jonas’ and remember that all of OMEMO is base64’d
  578. moparisthebest GPRS is dead though so who cares
  579. jonas’ you wish
  580. derdaniel has left
  581. moparisthebest actually I wish it wasn't, but that's beside the point :)
  582. moparisthebest in the USA 2G has been gone awhile, most carriers dropped 3G last month, the rest will next year
  583. neshtaxmpp has joined
  584. moparisthebest is all EXI offers "better compression" then ? and if so, how does it compare to "just compressing" XML, and does it suffer from the same security vulnerabilities "just compressing" XML does ?
  585. SamWhited moparisthebest: that doesn't mean 2G is dead, that means the major carriers dropped it and if you were in an area where it no longer exists you just don't have internet anymore unless you switch to a smaller local provider or use a slow wireless uplink. That's *more* of a reason to save bandwidth, not less.
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  588. SamWhited I make no comment on EXI in particular, just complaining because I'm always annoyed when I've lived somewhere where it's impossible to get fast internet and people say "what's the problem, everywhere has fast broadband now!"
  589. moparisthebest I think we might have discussed this before, and I think the conclusion was EXI is probably as vulnerable to CRIME/BREACH as gzip is ?
  590. moparisthebest SamWhited, yea I think it sucks, but it doesn't change the fact that in the USA (and soon everywhere else presumably), it's dead
  591. moparisthebest I think next year for canada iirc
  592. SamWhited I don't thik that's true though, it's just dead if you use AT&T or T-Mobile, otherwise I believe smaller providers are still using it because it's what they can afford to do easily. Even if it is dead, that doesn't mean "we don't need to worry about a 33% bandwidth savings" because it being dead doesn't mean everyone gets upgraded to 4G
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  596. moparisthebest right, I'm not saying EXI is good or bad, I'm asking what advantages it offers, how it compares to just running normal compression on XML etc
  597. neshtaxmpp has left
  598. SamWhited Maybe I misunderstood "which saves what, 33% only in the case of base64'd stuff? / GPRS is dead though so who cares"
  599. Zash I vaguely recall all of Sweden having some kind of GSM coverage at some point, but since recent-G networks, not so much. Not much profits in covering the Scandinavian mountain range in high-speed internet.
  600. SamWhited That sounds like "we don't have to save bandwidth, everyone has fast internet now"
  601. Zash AIUI you get rid of the parser and shuffle packed structs over the wire, instead of a text format that requires a proper parser.
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  603. moparisthebest that sounds like a proper security nightmare to me
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  605. Zash How's HTTP/2 doing? How's all the ... binary Google format I don't remember the name of anymore.
  606. moparisthebest so for sure EXI has downsides compared to compressed XML, like comparatively few libraries, no/much less security auditing, and such, does it have upsides?
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  609. Zash Compression of the zlib variety has memory usage issues and security .. ickyness.
  610. moparisthebest Zash, HPACK ? https://tools.ietf.org/html/rfc7541 https://blog.cloudflare.com/hpack-the-silent-killer-feature-of-http-2/
  611. Zash EXI supposedly doesn't have that
  612. moparisthebest how sure is anyone EXI doesn't have the same security problems ?
  613. Zash I'm sure it's just as good as ASN.1 & co! 😛
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  615. Zash moparisthebest, protocol buffers, was the thing I forgot the name of. I mentally included HPACK in "HTTP/2"
  616. SamWhited I don't remember much about EXI, but it negotiates the cmopression up front instead of constructing a dictionary that is reused across stanzas right? In that case I don't see how it could be possible for it to have the same issues as stream compression or TLS compression
  617. moparisthebest even HPACK acknowledges it's vulnerable to CRIME and friends https://tools.ietf.org/html/rfc7541#section-7.1.1
  618. Zash Can't you ... not do that with EXI?
  619. Zash Just do the binary packing stuff
  620. Zash CRIME & co comes from compressing user/attacker data in the same context as other stuff, so if you just don't do that, things should be better
  621. moparisthebest SamWhited, I realize current-xmpp-compression doesn't do this, but nothing stops you from designing a XEP such that each stanza is compressed separately right?
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  624. Zash moparisthebest: Correct, just need to write it down somewhere and get implementations to comply.
  625. SamWhited moparisthebest: sure, I've worked plces that implemented it that way in the pas
  626. SamWhited t
  627. moparisthebest so, that's *not* vulnerable to CRIME then ?
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  629. moparisthebest then the questions are: 1. is EXI vulnerable to CRIME ? 2. if not, how does it compare size-savings-wise to just doing the above with XML ?
  630. jonas’ I did some measurements on the impact of compressing stanzas individually some time ago, and it was "still worth it"
  631. SamWhited Right. Assuming you're not doing something weird with your stanzas where you're mixing secret data and non-secret data, but in the general sense that's right
  632. jonas’ I’m not sure if EXI contains any compression at all
  633. Zash moparisthebest, doing a full flush between each stanza? yes, that fixes CRIME (if I remember correctly what CRIME was about)
  634. jonas’ or if you have to use it
  635. moparisthebest in theory it should get rid of basically all base64 overhead right?
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  637. Zash IIRC you only need to do a full flush when the sender changes.
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  641. Zash Might be interesting things you could do with CSI integration
  642. neshtaxmpp has left
  643. SamWhited We just did a full flush on stanza boundaries at HipChat. Probably could have saved more by being clever, but it was easy and it helped a lot. Dropped our network traffic by a factor of 0.58 and dropped CPU utilization by a factor of 0.60
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  645. SamWhited (with ZLIB)
  646. Zash A big part of why we dropped compression completely from Prosody was memory usage tho.
  647. moparisthebest might even be better options today, it'd be interesting to see XMPP-wise how zstd and brotli compared
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  649. SamWhited Yah, I didn't write down or publish our memory usage, but I assume it went up a bit. Don't remember it though, had to go look up those two numbers. It's been a while.
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  652. Zash I'd be interested in fixed-dictionary compression
  653. moparisthebest iirc that's what EXI is doing ^ ?
  654. moparisthebest except it negotiates the dictionary, roughly
  655. Kev That’s more or less what EXI is isn’t it?
  656. Kev Heh, beaten :)
  657. Zash I though it was more like bit packing structs, not like zlib & co
  658. moparisthebest but that implies doing the same compression across all stanzas in a stream, which probably implies CRIME
  659. Zash Not if you don't allow backreferences into user data, only into the dictionary
  660. moparisthebest or possibly some other attack if you know the dictionary used, idk
  661. moparisthebest isn't "xml element names used in the stanza" sometimes "user data" ?
  662. Zash You don't learn anything about the previous stanza sent by someone else
  663. Zash If you say "hello", then an attacker saying "hello" right after would be smaller because it can reference that previous "hello"
  664. Zash But if you build a static dictionary, that everyone involved already know, you don't leak user data from that.
  665. Zash Static dictionary would be something full of angle brackets and protocol stuff, no user data.
  666. moparisthebest I don't know, maybe? https://tools.ietf.org/html/rfc7541#section-7.1.2
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  668. SamWhited I've thoguht about doing that with zstd a couple of times. It has a training mode where you can give it a sample set and it builds a dictionary from that you can reuse later. Never actually tried to see how well it does with a big set of XML though.
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  694. arc moparisthebest: is more than a little compression. With xmpp it can transform the overhead of a <message> stanza from around 100-200 bytes (depending on jid length) to around 10.
  695. moparisthebest arc, so is it vulnerable to CRIME, and how does that compare to simply compressing XML stanzas
  696. arc Where did you get that idea?
  697. moparisthebest that was a question, not a statement :) "is it"
  698. arc I don't believe so, no. Because it is not compression. It would use the same number of bits regardless of the length of the qnames, jids, etc.
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  701. arc EXI is a binary representation of XML. One of the functions is designed for is to then run it through a conventional compression such as DEFLATE
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  706. arc Another option is to bitpack.
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  709. moparisthebest is *that* vulnerable to CRIME
  710. arc https://www.w3.org/TR/exi-primer/ goes into the higher level nitty gritty if you want to read more.
  711. arc No because it is constant width.
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  720. arc The most you could probably learn from CRIME is encoding type. EXI supports four modes; byte packed, bit packed, precompressed, or compressed.
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  723. arc And the letter two are basically the same except the compression is deflate added on top of precompressed
  724. arc https://www.w3.org/TR/exi-primer/#compression explains how data in the stream is rearranged for better compression
  725. moparisthebest so how does it do on a super simple message stanza like https://paste.rs/Oe6.xml ?
  726. moparisthebest $ wc -c * 175 msg.xml 101 msg.xml.brot 146 msg.xml.gz 130 msg.xml.zst
  727. arc That will depend a lot on the mode
  728. Kev And the schema, presumably.
  729. moparisthebest brotli does surprisingly well here (no settings touched, just `brotli -c msg.xml > msg.xml.brot`)
  730. arc But you can basically ignore everything inside the XML brackets
  731. moparisthebest not the attribute values?
  732. arc Everything inside the XML brackets
  733. moparisthebest so, run some common examples maybe?
  734. arc There is an example in the primer
  735. moparisthebest of what, say a normal chat client might use for a mode/grammar/whatever
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  737. arc https://www.w3.org/TR/exi/#stringTable will take care of the jids
  738. arc <message becomes one byte (except in bit packed), SE-MESSAGE
  739. moparisthebest > The life cycle of a string table spans the processing of a single EXI stream.
  740. moparisthebest so that likely makes it vulnerable to CRIME then ?
  741. moparisthebest in fact, it certainly would right?
  742. moparisthebest if you put JIDs in there anyhow
  743. arc from= and to= are each AT, so one byte each since they are optional. The values come from the string value table
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  745. moparisthebest except they aren't because you can't do that and avoid CRIME
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  748. moparisthebest so add them back in
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  750. arc What is your obsession with CRIME? The string tables are a constant value. They are not susceptible
  751. moparisthebest because you can't just go "maybe it's not vulnerable to breaking the stream encryption ¯\_(ツ)_/¯"
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  754. moparisthebest it either is, or it is not
  755. arc The only thing you're going to get for information leak from the string table is the number of values in the table.
  756. moparisthebest attackers can manipulate the JIDs that go across the stream right ?
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  759. arc How?
  760. moparisthebest how would any JID except the sending one be in the dictionary up front anyhow ?
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  762. moparisthebest how will you know randomjid@randomdomain is going to message you?
  763. moparisthebest maybe EXI is only useful in closed deployments where you know the entire network up front ?
  764. moparisthebest (and, maybe, don't care about TLS providing CRIME-proof security, still unknown on this one)
  765. arc You're right the length of jid could be leaked.
  766. moparisthebest so if you cut that stanza down to essentially the bare minimum info that must be transmitted in it, and it's probably *too* cut down actually, you end up with something like https://paste.rs/aPt which is 86 bytes
  767. mathijs has left
  768. moparisthebest that means EXI would have to fall somewhere between 86 and brotli's 101 to be useful at all
  769. moparisthebest that's a small range for improvement
  770. Daniel i think that's a very simplistic example
  771. Daniel look at an omemo pre key bundle for example
  772. Daniel or generally anything with lots of nasted elements
  773. moparisthebest sure, I suspect brotli would improve even more with anything base64'd though
  774. Daniel plus 184 requests 333 requests
  775. Daniel and all the other stuff we regulary put into messages
  776. Daniel and it should be easier on the cpu
  777. moparisthebest honestly regular compression's ratio should get better the bigger the stanza is right? so a small one like this is probably least fair
  778. Daniel i have a deployment where we would like to use compression but can’t because it's too expensive
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  781. Daniel and if implemented correctly exi can even be faster than string parsing
  782. Daniel where as compression is always slower
  783. arc You can add brotli on top of EXI. It works very well for that purpose.
  784. moparisthebest I doubt that's always true (that compression is slower)
  785. SamWhited Daniel: are you sure about that? We had CPU usage drop on one machine because there were less TLS packets where most of the CPU was being taken up
  786. arc But again this is all based on the grammar.
  787. Daniel SamWhited, no
  788. SamWhited Worth measuring anyways if you haven't. If you're using a slow TLS cipher and a fast compression algorithm you might make some gains
  789. moparisthebest I understand EXI should be able to be better than general compression in a closed system where you know every stanza that will ever be passed
  790. moparisthebest the question is, can EXI be better than general compression for a chat client in the public federated XMPP network
  791. larma any compression system that works on reusing user input is has issues the like of CRIME and similar. EXI could be fine if no string lookup table was used. However schema-based EXI is complicated and schema-less EXI is far from optimal
  792. larma any compression system that works by reusing user input has issues the like of CRIME and similar. EXI could be fine if no string lookup table was used. However schema-based EXI is complicated and schema-less EXI is far from optimal
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  794. SamWhited I suspect the question is actually "assuming that EXI and GZIP flushing on stanza boundaries are both 'good enough', which one is easier to implement and deploy widely" (and the answer is probably normal stream compression), but of course I may be wrong.
  795. moparisthebest yes roughly SamWhited , I think it's rather obvious general compression and regular XML is easier no ?
  796. larma somthing like exi would still be more efficient than gzip. also why would you think flushing on stanza boundaries is enough?
  797. moparisthebest so it becomes 1. is EXI secure 2. is it better than that, and if so, better enough to justify the effort?
  798. Zash Could do something in the direction of HPACK and/or WhatsApps FunXMPP compression scheme...
  799. SamWhited moparisthebest: I suspect so, yes
  800. arc moparisthebest: again, EXI is not compression. It is a schema-aware binary representation of XML data. It is intended that you use compression on top of it.
  801. arc When you're not using compression on top of it, typically you use bit packed mode.
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  804. moparisthebest then you'd have to show EXI+compression is better in terms of size+effort vs XML+compression ?
  805. moparisthebest and additionally, ensure EXI doesn't introduce security bugs like CRIME
  806. flow larma> somthing like exi would still be more efficient than gzip. also why would you think flushing on stanza boundaries is enough? honest question: why would you think that it is not enough?
  807. arc Actually as I'm thinking about it, it would make a lot of sense to pre-populate the values string table with the JIDs in your roster. And if you are worried about CRIME, you can also specify that padding is used to make all jids transfer as a fixed width
  808. larma flow, attackers can control certain content of stanzas even if it's not them sending
  809. moparisthebest so then you can use than as an oracle to determine a user's roster
  810. arc How?
  811. flow larma, unfortunately I am appearantly missing pieces why this is relevant
  812. larma ah maybe there is something that I have in mind that nobody else has. If we ever do some kind of efficient compression on stanzas, I'd want to also make use of it inside SCE. Then it's no longer transport encryption only, but also about end-to-end-encryption relevant. And if you assume the server as an attacker, they can easily modify IDs and similar to modify certain parts of your message to exfiltrate others.
  813. larma ah maybe there is something that I have in mind that nobody else has. If we ever do some kind of efficient compression on stanzas, I'd want to also make use of it inside SCE. Then it's no longer transport encryption only, but also about end-to-end-encryption relevant. And if you assume the server as an attacker, they can easily modify IDs and similar to modify certain parts of your encrypted message to exfiltrate others.
  814. flow larma, thanks, I probably need to think about this a little more (and with more sleep)
  815. SamWhited I still think that doesn't seem worth considering, the server has to be trusted, that's the whole model that we have. Trying to change that and doing complicated partial stanza encryption just seems like a waste of time. Let's just settle on compression that's "good enough" and in the occasional system where there's some high-security environment where the server isn't trusted, don't use it.
  816. Zash Did anyone define "good enough"?
  817. flow larma, but it sounds like you are talking about an intermediate hop modifyin encrypted bytes, that shouldn't be possible, no?
  818. SamWhited Zash: "better than no compression?"
  819. arc I have to agree with Sam
  820. arc Here's the thing; XMPP is low enough bandwidth and no one really cares except IoT, and maybe mobile.
  821. emus (Different topic: Kev - there was a misunderstanding with Sam W., nevermind about the Twitter thing - I will just request the pinning via mail 😊️ )
  822. larma flow, no, certain parts are unencrypted, like message id. Also you very likely would want to be able to reply to unencrypted messages with encrypted messages without risking to actually leak the content of the encrypted message 😉
  823. SamWhited Yah, sorry, I didn't realize Twitter required me to have a separate personal account
  824. SamWhited I'm still happy to help on the comms team as necessary though, someone just tell me what needs doing and I'll see what I can do.
  825. flow larma, ahh I think I got it now: values that are to-be encrypted are determined by the server
  826. arc On IoT these days we mostly care about making xml easy enough to use that we can fit it on a microcontroller and utilize less cpu and transmit size so that the battery lasts longer
  827. flow that is certainly interesting. I never considered compressing the encrypted bytes prior base64 encoding them and the implications of doing so
  828. emus SamWhited if you read in the CommTeam MUC occasional or subscribe to the PR that should be fine. If you want, I can point you as reviewer there, too
  829. arc EXI seems extremely complicated, but CPUs can process it with a minimum amount of code. A typical EXI compiled binary static library is around 8k.
  830. flow FWIW, I believe EXI would also be able to avoid base64 for raw bytes (at least, that is what I remember being told ~5 years ago)
  831. SamWhited emus: can you invite me? I don't see the comms team muc listed on the website
  832. emus https://xmpp.org/newsletter.html here is a link if thats okay for you, I dont have you in my contact list so I guess thats quicker
  833. SamWhited thanks
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  835. arc And when you're dealing with a microcontroller costing under $1, that only has like 16k-64k flash for all the software, the size of the software matters a lot. And when the manufacturer is budgeting the mAh battery, they care a lot about keeping the microcontroller in sleep and moreso keeping the transmitter powered down as much as possible when the cpu is awake.
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  837. arc Because in the end, they don't care about anything else. They don't care about standards compliance. They don't care what technical solution is used. They don't care about CRIME or data security.
  838. arc They're making 10 million of these units, at minimum, so if they can save 10 cents that's a million dollar profit.
  839. arc They look at a big stack of money on one side, and a technician whining about standards compliance or user security on the other, they will choose the big stack of money every time.
  840. arc The only reason *some* of these manufacturers are using xml at all, and a very small number overall, is because exi can do the work with less code, less cpu, and less transmit power than message queues, which is what they would be using otherwise
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  842. moparisthebest right, I get why EXI is a win for bottom-of-the-barrel iot, still not sure if it's a win or not for generic federated xmpp chat client though
  843. arc Nobody is using it for general federated xmpp
  844. moparisthebest yes but I'm wondering if it's worth pursing it for that though
  845. Zash moparisthebest, ~1MB chunks of XML sent to my phone something something would be nice if it was smaller
  846. moparisthebest but maybe regular compression solves it for you, and that's easy
  847. arc Half the time they just want http anyway. Because the software is free and HTTP is lightweight. And because their in-house developers already know it.
  848. Zash but then most of that consists of people sending presence to all the same MUCs that I'm in
  849. arc Zash, I agree with you. And it would likely turn the 1MB chunk into 1kb.
  850. arc String tables are surprisingly efficient at jids
  851. Zash FWIW this is with CSI buffering up unimportant stuff into larger chunks, that's how it gets so large.
  852. moparisthebest arc, but "string tables" can't be used on public federated XMPP because they are vulnerable to CRIME, so they are out
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  854. arc I would say that you choose not to use it because you believe is impossible to implement with your security criteria in mind
  855. Zash the real question is, is CRIME really that bad?
  856. arc And if I really cared I would have worked on it a long time ago
  857. SamWhited yah, honestly, after SASL is complete I'm not sure how much I care about CRIME-like vulnerabilities.
  858. Zash IIRC you almost need to launch a DoS attack to get anything out of it, maybe just sprinkling some rate limits on top makes it all Good Enough
  859. arc I am certain that there is an orchard full of hanging fruit for xmpp security, nobody will go through the effort
  860. jonas’ CRIME also gets less and less relevant with e2ee
  861. jonas’ (post-SASL that is)
  862. Zash And good luck doing CRIME with SCRAM anyways
  863. arc I mean, all the software that uses libxml2 should just be considered insecure by that very nature
  864. moparisthebest makes me pretty nervous to pretend CRIME isn't a threat... attacks only get better, never worse
  865. arc Google was so certain of the fundamental insecurity of anything XML that they never even implemented encrypted S2S
  866. Zash Huh. Source?
  867. moparisthebest and actually I keep saying CRIME when BREACH is actually a better fit for what we are talking about https://en.wikipedia.org/wiki/BREACH
  868. arc Zash, source is the number of times I've had beer with Google developers
  869. Zash moparisthebest, C-ompression something something is easy to remember tho
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  872. arc In 2019 I had a drink with a few of the guys that previously worked on the gtalk/hangouts sre team, and I commented that it probably was a lot easier doing seculity with Go and it's brand new xml library. One of them shot his drink out his nose
  873. arc They apparently don't use XML internally anymore because of it
  874. moparisthebest well, google is well known for their poor decisions so :)
  875. moparisthebest imagine taking advice from a company that created and killed 83 chat systems in the last 15 years
  876. arc But that's the thing, not once did they create one that actually works the way they wanted it to
  877. arc And anything even remotely stable is just piled on mess over mess until they have to start over
  878. SamWhited They are spot on with that one though, XML is absolute garbage in terms of being able to do a secure implementation. Way to big of an attack surface for something that should just be a way to transfer a tree…
  879. SamWhited Not that Google's dev practices or product management practices are always great, but they know their security.
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  881. arc Protobuffs is wide open to attack too. They work around that with things like ucs4 and fixed length string entries
  882. SamWhited I'm not saying protobuf is perfect, I don't know much about them, just that they're not wrong about XML being bad.
  883. moparisthebest XML is the worst except for everything else
  884. SamWhited No, it's just worse than most other things. We're stuck with it for the base of XMPP, and I don't see anything else that works in a similar enough way to do a streaming protocol in as nice of a manner, but that doesn't mean it's not insecure garbage or that we shouldn't be *very* careful with it
  885. arc XML is not bad by design. Is bad because nobody cares.
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  889. SamWhited I don't think that's true. Designs like hacking namespaces on and mixing them with attributes, adding things like proc insts, etc. are just bad.
  890. arc I'm not saying that's not bad. I'm saying the XML suffers the same type of abandonment that led to heartbleed
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  892. SamWhited I mean, I'm sure nobody caring is a problem too, but I'd also say XML is just bad by design and therefore we need to be *very* careful about its use and not dismiss other peoples concerns about it so redily.
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  894. SamWhited I just get mad every time somebody does the "it's just <> vs {}, why is HN so trendy?" or whatever in this room when realistically whatever they're complaining about is probably a serious problem that we should be addressing instead of being dismissive.
  895. moparisthebest but the "json is better than xml" people don't have any answers for all the ways in which json is worse
  896. moparisthebest and you can s/json/anything/ there too
  897. Zash My langsec friend said something some time that I remember as "xml is okay, it's not made of length-prefixes and stuff"
  898. SamWhited Sure, but that's not the problem, the problem is that we pretend that means XML is good somehow and then Go has the issue where namespaces can be manipulated and it's like the third time I've seen that in an XML decoder and somehow we just say "no, XML is fine, let's use more of it" every damn time
  899. Kev The fundamental problem is that XML got so much stuff *right* that we’re stuck with the stuff it didn’t.
  900. moparisthebest "bad libraries exist" is a thing
  901. SamWhited "Every library is consistently bad in the same ways" is the actual problem.
  902. arc Oh I'm not dismissing anything. I'm just worn down by an industry that doesn't care until it nearly destroys them. I lead the Python xml-sig that produces code responsible for billion dollar industries. Those companies don't care. If the problem were made bluntly clear to them, they wouldn't fund anything. They would direct their technical teams to migrate to another language
  903. moparisthebest binary formats never have parsing vulnerabilities https://duckduckgo.com/?q=asn.1+parsing+vulnerabilities
  904. SamWhited But I dunno, I just think XML is garbage in general outside of the security realm too and got next to nothing right. I accept it's the only thing we could use for a system like XMPP, but IMO we should literally never use more of it ever regardless of whether it's one of the parts that has consistent security issues or not
  905. Zash asn1, the xml before xml, let's go back to it!
  906. Zash If only for the CRITICAL bit
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  908. Zash SamWhited, face it, everything we do is garbage held together with duct tape. That we haven't nuked/pandemic'd/burned ourselves into extinction yet is quite amazing. Probably because we invented duct tape 😀
  909. SamWhited you're not wrong
  910. arc That and because we are hanging just above the low hanging fruit. Like vulnerabilities in cryptocurrency exchanges that allow people to steal millions of dollars in untraceable funds
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  912. Zash At least it's our garbage. Our own! Our ... precious!
  913. Zash pets pile of angle brackets.
  914. arc Sure people could attack xmpp. I imagine many of us could, knowing what we know. But there is no direct profit from doing so, and any scheme to profit from such an attack would be too complicated to pull off
  915. moparisthebest I think this can be summarized as "computers are bad, formats don't matter" https://duckduckgo.com/?q=json+parsing+vulnerabilities https://duckduckgo.com/?q=xml+parsing+vulnerabilities
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  918. Zash https://xkcd.com/2030/ comes to mind
  919. SamWhited That's exactly the problem, "our entire field is bad" doesn't mean "so we should give up and use whatever we want and ignore the problems with specific formats because another one might also have problems or there might be an issue at some other layer of the stack"
  920. moparisthebest but, switching formats doesn't address the problem, you should just address the problem instead
  921. Zash "our entire field" has existed for like half a century and usually doesn't directly kill people ... unless you count artillery trajectory calculations ... oh no
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  923. SamWhited I didn't say we should switch formats, I said XML is what we've got and it's probably the only thing that works for XMPP but we shouldn't be dismissive when people point out problems with it and we shouldn't add more of it.
  924. Zash SamWhited, and we shouldn't do what everyone else does and wrap it in JSON!
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  927. Zash Oh wait, oh no, https://xmpp.org/extensions/xep-0295.html
  928. SamWhited I mean, yah, we should definitely not implement XEP-02395 :)
  929. SamWhited err, 0295
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  932. arc No Sam. We should fix it. Here I'll put it on my giant stack of things I care about that will never earn a cent, will not improve my chances of getting hired, would take the rest of all of our lifetimes, and leave us living on the street.
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  934. Kev Although 295 was obviously a joke, I note that the first suggested encoding pretty much works, and actually avoids some of the security issues of XML ;)
  935. Zash The one that looks a bit like JSON-LD?
  936. arc I do really care about these things. I'm not completely jaded. I think a lot of people really care about these things. I'm pointing out that we need to solve some lower-level fundamental problems first.
  937. Kev Zash? Does it? Yes, anyway. The one that isn’t entirely stupid.
  938. Kev When I wrote it I didn’t think of it in terms of having benefit over the XML representation, but it *does* eliminate whole classes of security issues.
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