https://help.sendbird.com/hc/en-us/articles/235854108-Are-yo...
https://aurorabbit.github.io/emojicrypt.com/
It encrypts messages with a passphrase using scrypt, AES-GCM, and sha256 for an HMAC.
Each v1 protocol message includes a salt/iv, scrypt N&r, HMAC, and whether or not to lower-case the password. All messages encrypted with the proof-of-concept will be decrypt-able by all future clients, as there's room for 13 more protocol versions.
The code is in complete disarray, and I wouldn't want anyone to sift through it in it's current state[1], but the proof-of-concept is finished, and life came up, so it's been sitting since.
I have a list of features[2] I want to add in the issues tracker :)
[1] if you do want to sift through it don't miss https://github.com/aurorabbit/libemojicrypt. I separated the repos without following through on that reorganization.
"Customers can choose to further encrypt the messages prior to sending and after receiving for end- to-end encryption as needed (but this will cause moderation tools and search features to malfunction)."
Reddit may prefer to not provide end-to-end encryption due to reasons, say, keyword marketing.