Signal is a completely legal application. Its main purpose is to establish secure communications between two or more users. It fulfills this premise by letting you send text messages, images, videos, and other types of information. It also lets you make voice and video calls. All these transmissions are encrypted and can only be accessed by the people involved in a conversation or group.
This messaging platform, and its official client, is focused on privacy. It does not collect user data and therefore does not analyze it. Nor is it possible to intercept communications, thanks to its end-to-end encryption system. It also includes message self-destruction tools and prevents screenshots of conversations. Due to its characteristics, is Signal an environment conducive to illegal activities? The answer is simple: no more so than any other instant messaging service. Although WhatsApp collects much more user data, it also encrypts point-to-point communications. And, Telegram, offers secret chats with the same level of security. In general, users of all these applications make lawful use of them.
In short, you can use Signal without worrying about your country's legislation. Actually, it is a very interesting alternative if you want to get away from the control exercised by large corporations, such as Facebook, on users and their data.