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 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 I haven't check your repo yet, although you are using sodium, right?