Signal is at least open source, so the veracity of it's encryption claims can be verified (and have been).
It is almost certainly the most secure chat application out there.
When you use Signal, your data is stored encrypted on your devices. The only information that is stored on the Signal servers for each account is the phone number you registered with, the date and time you joined the service, and the date you last logged on.
This is different from apps like WhatsApp, which actually collects your metadata for their own use (and passes it along to Facebook too). This is also different from apps like Wire messenger that encrypt your data but store some of it on their servers.
Is Signal messenger really safe?
The short answer is yes. Signal messenger is really safe.
The long answer is, complicated. What do you mean by safe? Is Signal the most secure and private messenger app in existence? Probably. Can I guarantee that the NSA can't crack the encryption? No. The cryptographic community says that the encryption is secure. But can anyone guarantee it with 100% certainty? No. Will quantum computers be able to break the encryption? Theoretically, yes, at some point in the future. But as far as we know, no sufficiently powerful quantum computer exists so we can't say for sure one will be able to do the job.
Is Signal more secure than WhatsApp?
While it isn't obvious at first glance, Signal is definitely more secure than WhatsApp. Both products use secure end-to-end encryption for the content of their messages. They are both equally secure as far as that goes. In fact, WhatsApp's end-to-end encryption uses the Signal protocol to encrypts its data too.
But Signal encrypts your metadata, while WhatsApp logs as much of your metadata as possible, and passes it along to Facebook. While metadata doesn't expose the contents of your messages, it does include information on who you talked to, when you connected, and much more.
https://securitytech.org/secure-encrypt ... pp/signal/