How to encrypt messages/files on the site?

Question

Encryption

Hello. The next question is - you need to do something like encrypting messages on the site between users. To ensure that only the participants in the correspondence have access to information.
Messages of textual content, without pictures (I don’t know if this affects the essence of the issue).
What implementation methods can you think of?
There are sites like privatlab where the functionality of creating notes with a password is implemented, and it seems like messages are encrypted, maybe this will work or is it from a completely different opera?
And finally, how much would such a solution cost?

Answer 1

1. Either users exchange a password with each other in advance, without using your site.
2. Or they use asymmetric keys, which must also be generated outside of your site.
Well and further - standard things. You can read https://ru.wikipedia.org/wiki/PGP

Answer 2

This is impossible.
1. If messages will be transmitted through the site, then users must somehow exchange keys before the message exchange begins.
2. If the site writes out the keys, it will also be able to read all the messages.
The main problem, which is now impossible to solve in any way, is key exchange without intermediaries .
If PGP and so on are advised, then reading the collected encrypted messages is only a matter of time, and the security of this, with today's capabilities, is lower than none.

Answer 3

PGP has long been invented for this very purpose.
If you need "something like encryption", then even XOR is enough for you.
It costs 0 dollars.