Underpass is different from every other encrypted chat app because it does not use a third-party service. It's just two devices and a password. Peer-to-peer. There's no signup. No account. No phone number, email, contacts, or any personal information requested, not even your real name.
You can chat securely over the internet or on a LAN, such as in an office, school, coffee shop, or home. All of your data is encrypted before it's sent over the network, and it can't be decrypted except by the other device using Underpass. Industry-standard AES-128 encryption algorithm.