From the Whitepaper: "Due to the inherently asynchronous nature of mobile messengers, providing reliable Forward Secrecy on the end-to-end layer is difficult. Key negotiation for a new chat session would require the other party to be online before the first message can be sent."
That's not a problem for voice calls because voice calls inherently require both participants to be online.
Though I am curious why Signal's approach [0] wouldn't work for Threema.
That's not a problem for voice calls because voice calls inherently require both participants to be online.
Though I am curious why Signal's approach [0] wouldn't work for Threema.
[0]: https://www.signal.org/blog/asynchronous-security/