Threema

History

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.

In Spring 2014, operations have been transferred to the newly created Threema GmbH.

In December 2014, Apple listed Threema as the most-sold app of 2014 at the German App Store.

Features

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 numbers if the user allows the app to synchronize their address book. 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 users. Using this feature, the users can make sure they have the correct public key from their chat partners, which provides security against a Man-in-the-middle attack. Threema knows three levels of verification (certainty 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 send text messages, multimedia, locations, voice messages and files of any type (up to 20 MB per file). It is also possible to create polls in personal or group chats.

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.

Architecture

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.

Reception

In February 2014, German consumer organisation Stiftung Warentest evaluated several data-protection aspects of Threema, WhatsApp, Telegram, BlackBerry Messenger and Line. It considered the security of the data transmission between clients, the services' terms of use, the transparency of the service providers, the availability of the source code and the apps' overall availability. Threema was the only app rated as 'non-critical' (unkritisch) in relation to data protection, but lost marks due to its closed-source nature.

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.

In October 2014, Threema won the "connect App Awards 2014" for being the best app of the year.

As of November 2015, Threema has a score of 6 out of 7 points on the 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).