Keychat literally uses the Signal protocol. It's almost the same thing.
Discussion
It uses the same "double ratchet" encryption protocol, but with its own implementation. Provided Keychat runs a competent implementation, which I believe they do, it should be as secure as Signal/Molly Messenger. But the encryption protocol isn't the only component you have to consider when judging the security of a messaging platform. How the protocol is implemented is equally as important.
I've used Keychat a bunch of times and I'm really impressed with what they've created. They've created a competent and reliable system with verifiable security. Unfortunately, it seems as though it hasn't been widely adopted as most nostr users seem content to continue using NIP-04 DMs. NIP-17 adoption is apparently a distant 2nd place. As a security advocate, I wish more people would incorporate Keychat into their regular nostr usage. We all benefit when more people use strong communication encryption.
I don’t use Signal, but I find “almost the same” —not exactly the same. What would you do if your nsec were compromised? Additionally, Signal offers features like Sealed Sender to protect metadata. How does Keychat handle potential metadata leaks? In other words Keychat uses fundamentally different from Signal's architecture.
Keychat seals not only the sender but also the recipient.
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Interesting 🤔 Bob's almost one-time receiving address] - you say “almost” can you elaborate?
P.S. I still think in order to use nostr in a “private way” relay specs should be changed as well as key management.
If Alice and Bob chat using Keychat, when Alice sends a message and Bob replies, both of their receiving addresses are single-use. If Alice sends three consecutive messages and Bob then replies with one message, Bob's receiving address will have been used three times.
In summary, as long as the other party responds, the receiving address will be updated.
Not having identity is better approach than having it. Your public key still can be traced across the network if not managed carefully. The goal is the minimal digital footprint right ?
Why did SimpleX Chat previously only have 1-time invitation links, and now added the SimpleX contact address? What exactly is the SimpleX contact address?
You think with a broken nostr key pair you outsmart this guys? 
You don’t understand why Simplex Chat added the contact address.
I’m discussing specific point with you, but you blindly trust Simplex Chat. Do you really understand Simplex Chat?
There is no need to continue the discussion.
Yo already told you the flow the chat you are building ! The beginning is of the trace starts from the public and private key creation ! Then it goes to the relays !
I’m not saying that simplex servers are good and they are about to censor and you can run your own one and same applies to the relays but the public key trace is 💩
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