Instant Messaging (IM)
Real-time Text Transmission via Internet
Threema
More trustworthy than instant messengers from American or Chinese companies is Threema from Switzerland.
This messenger does not require any personal data! It needs only the Threema ID generated on your device!
- no registration with name, address or telephone number
- no SIM card required (works in a tablet without 4G/5G modem)
- Threema meets the requirements of the EU-GDPR
- Threema is approved for young people under 16 years of age
With Threema, you can chat and make phone calls with end-to-end encryption (optionally also with video, i.e. videophone).
Easy to operate User Interface
The ease of use also speaks for Threema: chatting works just like with other messaging services. The app is not overloaded and only has what you really need.
It is true that you cannot show your friends your status (state of mind, greeting, statement) and the recipients are not told what music you are listening to. But the sending status of the messages (delivered/read) is reliably displayed (can be switched off if desired). For most of us, this is much more important than knowing what music, audio books or videos are currently playing on our friend's phone or what he/she is currently thinking.
If you want to know the latter, it's a nice reason to start a conversation via chat or classic telephony. 
And if you want to send pictures to each other, you can do so very conveniently with any smartphone using the "Share" function.
In the reviews of the Apple App Store, one finds criticism that no large videos could be shared via Threema.
Videos are wonderful Trojan slingshots! I'd rather not get videos that might exploit a still unknown or new vulnerability of the operating system and thus make a nonsense of the smartphone's encryption!
Threema Apps are Open Source
Since 21.12.2020, Threema has published the source code of its apps, i.e. the apps are now «open source». This allows interested experts to conduct independent security audits at any time. In addition, external experts are regularly entrusted with subjecting Threema to comprehensive audits.
The source code of the server version is not disclosed. Threema considers the server source code to be the basis of its business and therefore does not want to publish it. Otherwise, anyone could copy the service and endanger Threema's business model as a competitor.
However, it is not a real disadvantage that Threema's server software is not open source. Thanks to «Security and Privacy by Design» without a central user account with decentralised storage of customer data exclusively on customer devices, Threema's servers only serve as relay stations anyway.
As an example of someone using source code for their own messaging service and competing with the original service, consider Wire.
Wire uses the Signal protocol (formerly Axolotl protocol) and was a German-Swiss company before their headquarters moved to San Francisco.
Reference Customers
Many companies trust Threema, including: ADAC, Bosch, Daimler, Rossmann, Erasmus University Rotterdam, Kantonsspital Winterthur, thyssenkrupp.
Threema is also used by the Swiss Federal Administration up to the level of the Federal Councillors for confidential communication.
Reference
Mike Kuketz: Messenger: Threema-Client nun Open-Source (German)
Threema: Information and graphics for press purposes
Dr.-Ing. Mario Heiderich: Pentest & Audit Report Threema Mobile Apps 10.2020 (PDF)
YouTube: TWiT Tech Podcast Network: Threema Goes Open-Source
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