Built by our product and engineering team, in partnership with the University of Cambridge’s department of computer science ​and technology​, Secure Messaging is unlike traditional information-sharing platforms​. The technology behind Secure Messaging conceals the fact that messaging is taking place at all by making the communication indistinguishable from other data sent to and from the app by our millions of regular users. By using the Guardian app, other users are effectively providing “cover” and helping us to protect sources.

That is interesting. There is also secure drop, which is used by a lot of news organisations including The Guardian

https://securedrop.org/