Viber "not signed"

I wanted to install Viber on Leap 15.3. I read this Thread on this forum and started to Install. Everything started fine, but, I 've got the message:

Package header is not signed!

viber- (Plain RPM files cache): Signature verification failed [6-File is unsigned]
**Abort, retry, ignore? [a/r/i] (a): **
Problem occurred during or after installation or removal of packages:
Installation has been aborted as directed.

Is there any way to find signification file for Viber in rpm package? Is it secure toi install/use this unsigned Viber?

Thank you for answering my question.

The question is:
Is it secure to download an rpm and install it by Hand.

zypper has not the key for that rpm to validate, so you have to decide of installing it by typing i (ignore) on the question…