jdev - 2026-06-08

How do you announce entity capabilities without the operator being able to tamper with them and do a downgrade attack

Can I send them as apart of a SCE message?

? where is a downgrade attack with capabilities?

I'm assuming it's trivial for the operator to take the caps (sent by a user, not the server) and omit a few features

yes, but to make this an attack, you would need to show that there is a process that depends on cap features for security features

Well I wanna make a new XEP based off of OMEMO Media Sharing that completely encrypts the filename (including extension), but have it still be compatible with the old XEP in case the client doesn't support it

i would say you should design it so you dont need entity caps

because nobody will query it

Well nobody would implement it if it doesn't have backwards compatibility (as it would be reduced UX for clients that don't support it)

if you cannot make it backwards compatible without entity caps, then it will not be backwards compatible

Like having to download a file named ahebejfwbwnskw.bin, decrypt and rename it to picture.png ✏

> if you cannot make it backwards compatible without entity caps, then it will not be backwards compatible Well that's gonna be difficult ↺

either you have full stanza encryption, as in OMEMO2 or OpenPGP, then this problem is solved

Full stanza encryption is not the problem really

It's having to maintain compatibility between a client who supports this and a client who only supports regular AESGCM://

who supports what?

what do you try to do?

Send a link similar to AESGCM:// except the original filename is stored at the end of the link

you can do this with aesgcm also

And the filename within the HTTP URL is completely scrambled, including file extension

simply add another parameter

Yeah but not file extension

either way, entity caps will not help you there

its only for a device, and you dont know who downloads a message in the future and what capabilities they have

i dont know, this is already a solved problem with full stanza encryption

How would a regular AESGCM client parse this: aesgcm://xmpp_server/files/sjwdhdsiwiw.zzz#[key and iv];originafilename.png

for example

ah you want to put it in the fragment, yeah thats not going to work

> i dont know, this is already a solved problem with full stanza encryption Which part of it? ↺

all of it

full stanza encryption means everything is encrypted

you can send file as8as8d8as9d9ads9ad.jpg, and tell the client in a tag that its picture.jpg

https://xmpp.org/extensions/xep-0447.html#file-sharing

here see the <name> element

you can even define the extension there

> you can send file as8as8d8as9d9ads9ad.jpg, and tell the client in a tag that its picture.jpg But can I send file with the extension scrambled? ↺

Specifically?

Hm

Maybe SFS could work yeah