This article has an unclear citation style.November 2015) (Learn how and when to remove this template message)(
|Initial release||December 2012|
4.1.3 (March 11, 2019 ) [±] 
2.3.1 (July 2, 2018 ) [±] 
|Written in||Objective-C (iOS), Java (Android), C, .NET (Windows Phone)|
|Operating system||iOS, Android, Windows Phone|
|Available in||English, German, French, Spanish, Italian, Russian, Brazilian Portuguese, Polish, Rumantsch Grischun|
|Type||Encrypted instant messaging|
Threema is a proprietary, end-to-end encrypted instant messaging application for iOS, Android and Windows Phone. In addition to text messaging, users can make voice calls send multimedia, locations, voice messages and files.
Threema is developed by the Swiss company Threema GmbH. The servers are located in Switzerland and the development is based in the Zürich metropolitan area. As of April 2017, Threema had 4.5 million users.
Threema was founded in December 2012 by Manuel Kasper. The company was initially called Kasper Systems GmbH. Martin Blatter and Silvan Engeler were later recruited to develop an Android application that was released in early 2013.
In Summer 2013, the Snowden leaks helped create an interest in Threema, boosting the user numbers to the hundreds of thousands. When Facebook took over Whatsapp in February 2014, Threema got 200,000 new users, doubling its userbase in 24 hours. Around 80% percent of those new users came from Germany. By March 2014 Threema had 1.2 million users.
Threema uses a user ID, created after the initial app launch by a random generator, instead of requiring a linked email address or phone number to send messages. It is possible to find other users by phone number or e-mail address if the user allows the app to synchronize their address book. Linking a phone number or e-mail address to a Threema ID is optional. Hence, the service can be used anonymously. Users can verify the identity of their Threema contacts by scanning their QR code, when they meet physically. The QR code contains the public key of the user, which is cryptographically tied to the ID and will not change during the lifetime of the identity. Using this feature, the users can make sure they have the correct public key from their chat partners, which provides additional security against a Man-in-the-middle attack. Threema knows three levels of verification (trust levels of the contact's identity). The verification level of each contact is displayed in the Threema application as dots next to the corresponding contact.
Users can make voice calls and send text messages, multimedia, locations, voice messages and files of any type (up to 50 MB per file). It is also possible to create polls in personal or group chats. With Threema Web, a client for web browsers, Threema can be used from other devices like desktop computers. Threema optionally supports Android Wear smartwatch and Android Auto.
On March 20, 2015, Threema released a gateway for companies. Similar to an SMS gateway, businesses can use it to send messages to their users who have Threema installed. The code for the Threema Gateway SDK is open for developers and available on GitHub.
On May 25, 2016, Threema Work, a corporate version of Threema, was released. Threema Work offers extended administration and deployment capabilities.
Since Threema's servers are located in Switzerland, they are subject to the Swiss federal law on data protection. The data center is ISO/IEC 27001-certified. Linking a phone number and/or e-mail address to a Threema ID is optional; when doing so, only checksum values (SHA-256 HMAC with a static key) of the e-mail address and/or phone number are sent to the server. Due to the small number of possible digit combinations of a telephone number, the phone number associated with a checksum could be determined by brute force. The transmitted data is TLS-secured. The address book data is kept only in the volatile memory of the server and is deleted immediately after synchronizing contacts. If a user chooses to link a phone number or e-mail address with their Threema ID, they can remove the phone number or e-mail address at any time. Should a user ever lose their device (and their private key), they can revoke their Threema ID if a revocation password for that ID has been set.
Groups are solely managed on users’ devices and group messages are sent to each recipient as an individual message, encrypted with the respective public key. Thus, group compositions are not exposed to the server.
The entire communication via Threema is end-to-end encrypted. During the initial setup, the application generates a key pair and sends the public key to the server while keeping the private key on the user's device. The application then encrypts all messages and files that are sent to other Threema users with their respective public keys. Once a message is delivered successfully, it is immediately deleted from the servers.
The encryption process used by Threema is based on the open-source library NaCl library. Threema uses asymmetric ECC-based encryption, with 256-bit strength. Threema offers a "Validation Logging" feature that makes it possible to confirm that messages are end-to-end encrypted using the NaCl Networking and Cryptography library. In August 2015, Threema was subjected to an external security audit. Researchers from cnlab confirmed that Threema allows secure end-to-end encryption, and claimed that they were unable to identify any weaknesses in the implementation. Cnlab researchers also confirmed that Threema provides anonymity to its users and handles contacts and other user data as advertised.
Along with Cryptocat and Surespot, Threema was ranked first in a study evaluating the security and usability of instant messaging encryption software, conducted by the German PSW Group in June 2014.[unreliable source?]
As of November 2015[update], Threema has a score of 6 out of 7 points on the - now withdrawn - Electronic Frontier Foundation's "Secure Messaging Scorecard". It has received points for having communications encrypted in transit, having communications encrypted with keys the provider doesn't have access to (i.e. having end-to-end encryption), making it possible for users to independently verify their correspondent's identities, having past communications secure if the keys are stolen (i.e. implementing forward secrecy), having its security design well-documented and having completed an independent security audit. It is missing a point because its source code is not open to independent review (i.e. it is not open-source).
On 9 March 2017 Threema, among other instant messengers, was included into the Register of organizers of information dissemination in the Internet operated by the Federal Service for Supervision of Communications, Information Technology and Mass Media of the Russian Federation. According to the Russian Federal Law of 6 July 2016 No. 374-FZ "On Amending the Federal Law on Counteracting Terrorism and Other Legislative Acts of the Russian Federation in terms of Additional Measures on Counteracting Terrorism and Maintaining Public Safety" from 1 July 2018 organizers of information dissemination will be obliged to retain in the territory of the Russian Federation the Internet users' text messages, voice information, images, sounds, video and other electronic messages for up to six months following the end of their reception, transmission, delivery and (or) processing (i.e. shall retain the content of communication). Organizers of information dissemination shall disclose this information to authorized state agencies that perform investigative activities or ensure national security. In a response, a Threema spokesperson publicly stated: "We operate under Swiss law and are neither allowed nor willing to provide any information about our users to foreign authorities." 
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