[lxgr]

> iMessage uses RSA instead of Diffie-Hellman. This means there is no forward secrecy.

It's (probably, to my knowledge) true that iMessage does not have forward secrecy, but that does not follow from it using RSA:

You can have forward secrecy using RSA (e.g. by exchanging ephemeral RSA encryption keys and RSA-signing these using identity keys), and vice versa you may also not have forward secrecy with (static) Diffie-Hellman.

Ephemeral keys (and/or periodic key rotation) are what yields forward secrecy, not any particular encryption, signature, or key agreement scheme.

> * Uses Diffie Hellman + Kyber, not RSA

iMessage apparently uses ECIES in newer versions of iOS instead of RSA: https://support.apple.com/lt-lt/guide/security/sec70e68c949/...