jonatan on Nostr: What you are describing is a known-plaintext attack. Modern ciphers can even be ...

What you are describing is a known-plaintext attack. Modern ciphers can even be immune to chosen-plaintext attacks which is a much more potent attack than what you are describing.

I’m not qualified to judge the implementation of nostr DMs but if done correctly the could be secure

https://en.m.wikipedia.org/wiki/Known-plaintext_attack

https://en.m.wikipedia.org/wiki/Chosen-plaintext_attack