here is my progress so far: https://github.com/eapl-gemugami/twtxt-direct-message-php
The encryption part seems to work, if I decrypt it the message with OpenSSL.
I think it can help you for some key parts not well explained in OpenSSL documentation.

@andros@twtxt.andros.dev reading your spec I wrote a few notes here: https://github.com/eapl-gemugami/twtxt-direct-message-php/blob/main/direct_message_spec.md

@arne@uplegger.eu I haven’t check your repo yet, although you are using sodium, right?

@eapl.me@eapl.me Nope, I switched to the openssl library in PHP. But our rubberducking 🦆 seems to be working. Your find https://crypto.stackexchange.com/a/79855 for the IV generation may be the breakthrough …

@eapl.me@eapl.me @andros@twtxt.andros.dev Eureka! It works! https://github.com/upputter/testing-twtxt-dm
PBKDF2_KEY_SIZE = 48 was the turning point! My dirty little crypt.class.php can en- and decrypt, accoridng to the OpenSSL standard and options used in https://twtxt.dev/exts/direct-message.html

@arne@uplegger.eu Amazing!!! 😋