Are there any messengers that use private key encryption?

[Whiteronin]

Currently, most messengers use public key encryption. This encryption uses mathematical principles, such as the fact that multiplying two large numbers together is much easier for computer than decomposing a large number into prime numbers (RSA). This type of encryption is more convenient than public key encryption, but it's not completely secure. Soon, quantum computers will become widespread, and they'll crack the RSA encryption. And that's not the only risk; mathematics is advancing now, and I believe it's entirely possible that someone will soon invent an algorithm for quickly decomposing a number into prime numbers (this would, by the way, finish Bitcoin). Some messengers use quantum-resistant encryption algorithms, but I'm afraid that they, too, could be cracked in the future.
Private key encryption is less convenient, but it is completely secure. I have written a program in Delphi, that can encrypt and decrypt any file if they share the same key—a very large file with a random bytes. If you transmit this file to the recipient only via a flash drive, without placing it anywhere online, this guarantees that no one will intercept your message. In the future, I'll probably write another program to transmit these key files remotely using public key quantum-resistant encryption (currently I've almost figured out  how the RSA encryption works). Does there exist a messenger, with which I can specify a path to my key file so it will encrypt my video calls with that key?

[tvbcof]

Nostr  is pretty aligned with Bitcoin cryptographic methods, and Bitcoin has a pretty strong interest in having it's cryptography be sound.  That is enough to spark my interest in and hope for the developments.

I looked into nostr a couple years ago, but the development seemed somewhat chaotic and non-professional.  Until recently, the few times I've tried to use a service based on it to be pretty much completely non-functional.  I did recently try some voice/vid framework (https://github.com/HiveTalk) which worked perfectly (where protonmail failed miserably), but that is probably in part because someone was running (and had not yet abandoned) a node with adequate resources provisioned.