I need to send passwords to other people. Is there an app (cross-OS if possible) that allows me to send them in a secure way ? Maybe a simple private/public keys system.
GPG (free replacement for PGP) has clients for Windows, macOS, Linux, android, iphone... virtually every operating system used in the last two decades. Just search the OS's software repositories / app store / the web, etc.
If your friends want to create & upload their own keys, you can send them messages only they can read. Or if you can share only one password with them, you can "conventionally encrypt" messages for them to decrypt with that password.
WinZip can make an encrypted file. However, the customer needs a password to open the file.
But the system doesn't need a password it just needs a password hash. So the customer, if provided with an application, could make their own password and then send in the password hash. Then when the password is input into the system, the system just checks to see if the input produces the correct hash.
The customers could make their own password and password hash with KBH Password System or with KBH Crypto Clearing Application. However, the system would need to be running the same application to check the password input against the password hash. I suppose that the applications could be available as dll's for use by the system. Well, the first input into the applications is a set parameter for this purpose, the second input is the customer's password, and the output is the password hash. The parameter and the hash are relatively public while the password is secret.