Evaluation of a Safe Messenger

2023-01-09 13:04:27

Evaluation of a Safe Messenger

Threema is a Swiss encrypted messaging utility. It has greater than 10 million customers and greater than 7000 on-premise prospects. Distinguished customers of Threema embody the Swiss Authorities and the Swiss Military, in addition to the present Chancellor of Germany, Olaf Scholz. Threema has been extensively marketed as a safe various to different messengers.

In our work, we current seven assaults in opposition to the cryptographic protocols utilized by Threema, in three distinct risk fashions. All of the assaults are accompanied by proof-of-concept implementations that exhibit their feasibility in apply.



Safe messaging functions have the essential purpose of permitting pairs and teams of customers to speak securely. Fascinating properties embody confidentiality, integrity, origin authentication, and prevention of message reordering, replay, insertion and deletion assaults.

State-of-the-art functions also needs to provide ahead safety and post-compromise safety. Roughly talking, ahead safety implies that knowledge exchanged previous to a compromise stays safe, whereas post-compromise safety implies that safety may be recovered after an assault has taken place (supplied the adversary doesn’t at all times stay lively within the communication exchanges).

All of this ought to be finished in an end-to-end (E2E) safe method, that means that solely the supposed recipients ought to be capable of learn and ship messages. Reaching E2E safety minimises the belief required within the server that’s usually utilized by such functions to retailer and ahead messages.

Even previous to our work, it was identified that Threema options neither ahead safety nor post-compromise safety. We present that a number of of the opposite fascinating safety properties additionally fail to carry. Our work highlights a number of the difficulties confronted by builders in creating safe messaging methods. As well as, we draw classes for builders of safe protocols extra typically.

We uncover seven assaults, throughout three totally different risk fashions:

  • Community Attacker risk mannequin: The usual attacker for safe community protocols. The adversary has management of the communication channels between events (together with the customers and the server).
  • Compromised Server risk mannequin: A mannequin during which the server has been compromised and is appearing maliciously. That is in accordance with the potential of a safety company or superior hacking group attempting to acquire entry to a big and concentrated quantity of delicate knowledge. We count on a messaging utility that claims to offer end-to-end safety to have the ability to defend in opposition to this sort of attacker.

  • Compelled entry risk mannequin: A mannequin during which the adversary has entry to the machine of the sufferer. Safety in opposition to this sort of attacker is related within the case of border searches, when protesters are detained by police forces and looked for incriminating proof, or within the setting of intimate companion violence. We assume that the attacker has managed to persuade the person to unlock their cellphone. Even on this setting, we count on a point of safety to be achieved.

Assault 1 (Community Attacker)

Ephemeral Key Compromise Impersonation

Ephemeral keys are utilized by the Threema shoppers and server to create contemporary keys for utilization in Threema’s bespoke client-to-server (C2S) protocol. In idea, this could make totally different periods impartial from one another (i.e. compromising one session shouldn’t affect the safety of previous or future periods).

In distinction, we present that compromising a single consumer ephemeral key permits an attacker to impersonate that consumer to the server endlessly. Moreover, we uncover improper reuse of ephemeral keys within the C2S protocol each on the consumer and on the server, with vital safety impacts.

Assault 2 (Community Attacker)

Vouch Field Forgery

The Threema C2S protocol makes use of a particular worth referred to as a vouch field as a way to authenticate the person to the server. The vouch field is created by encrypting the person’s ephemeral key with its long-term key (mixed with the server’s long-term key), binding them collectively.

By utilizing a cross-protocol interplay between the C2S protocol and Threema’s end-to-end (E2E) protocol, we present that the attacker can trick a person into creating a legitimate vouch field and sending it to the attacker. This enables the attacker to impersonate the consumer to the server endlessly.

This assault implies that, underneath some circumstances, a person would possibly compromise his or her personal account by merely sending a message to a different person.

Assault 3 (Compromised Server)

Message Reordering and Deletion

The Threema E2E protocol doesn’t present any approach for a recipient to test the order during which messages had been despatched, nor for a sender to test whether or not a message has been delivered. Though a timestamp is hooked up to every message, it’s not integrity-protected and a malicious server can overwrite the timestamp with a pretend one. The Threema utility merely shows messages to a person within the order during which they had been obtained.

Taken together, these options enable a malicious server to ahead messages from one person to a different in any desired order and to not ship messages of its selection, all with out being detected.

The influence of this assault is that the semantics of the dialog between two customers may be altered by an attacker, regardless of it not with the ability to learn the end-to-end encrypted messages.

Assault 4 (Compromised Server)

Replay and Reflection Assaults

Threema has an express mechanism to forestall replay and reflection assaults, based mostly on remembering which messages had been despatched and obtained. Every message has a singular worth related to it, referred to as the nonce, which is used as a part of the encryption course of. If a specific nonce is seen twice, the applying merely discards the message.

By way of efficiency, this strategy requires extra storage to recollect the nonces. Moreover, it creates an issue when transferring an account between units.

This is a matter on the Android model of the applying, since there isn’t any user-friendly technique to switch the nonce database to the brand new machine. When reinstalling the applying, the nonce database can also be deleted. This makes the person susceptible to having previous messages replayed and mirrored in opposition to them.

Assault 5 (Compromised Server)

Kompromat Assault

The next assault was patched in a earlier model of the applying. We select to incorporate it because it highlights basic weaknesses of the protocol: whereas the person problem may be mitigated, a brand new protocol is required to sort out the issue at its root.

A malicious server can trick the consumer into utilizing the identical key whereas speaking to the server throughout the preliminary registration protocol and whereas speaking to different customers within the E2E protocol.

By doing so, the server can idiot any person Alice into unwittingly encrypting a message of the server’s selecting that may be delivered to a distinct person Bob. Such a message may include compromising materials (‘Kompromat’), e.g. libellous content material or a false confession to a criminal offense.

It is a second occasion of a cross-protocol assault, this time between the registration protocol and the E2E protocol.

See Also

Assault 6 (Compelled Entry)

Cloning by way of Threema ID Export

Threema offers a mechanism that enables the person to switch their account to different units, by copying over the person’s long-term non-public key. The hot button is encrypted through the use of a password chosen on the spot.

Whereas handy for customers, this opens up the likelihood for an attacker to clone the account of a sufferer person in a matter of some seconds, assuming that the sufferer’s machine is left unlocked. Because the safety of the whole utility depends on sustaining the secrecy of customers’ long-term non-public keys, this characteristic fully breaks safety within the compelled entry setting. Moreover, an astute attacker can mount this assault undetectably, so the sufferer would don’t have any technique to discover that their account has been cloned and that their messages are being learn.

Assault 7 (Compelled Entry)

Compression Facet-Channel

Along with the important thing export characteristic, Threema offers its personal cloud-based backup system within the type of Threema Protected. The person chooses a password and a key’s derived from it. The person knowledge is first compressed after which encrypted utilizing the derived key. The backup consists of, amongst different issues, the person’s long-term non-public key and the usernames of the person’s contacts.

As showcased within the CRIME assault on TLS, this compress-then-encrypt paradigm is susceptible to assault if the attacker has partial management of the information being protected. Within the context of Threema, an attacker can management their very own username, and we present that that is ample to make it doable to extract the sufferer person’s long-term key. The assault requires the power to power many backups to be finished. On Android, a brand new backup try is made every time the applying is restarted and the final backup failed; which means it suffices for an attacker to make use of the debugging instruments to restart the applying repeatedly. This allows the extraction of the sufferer person’s non-public key in a couple of hours.

We disclosed our findings to the Threema growth staff on the third of October 2022, together with doable mitigations for the assaults. Quickly after, we met with Threema representatives to debate our work and its public disclosure. On that event, we agreed on an preliminary batch of mitigations to be launched in This autumn of 2022, adopted by the general public disclosure and last mitigations to be launched in Q1 of 2023. In December 2022, we agreed on the ninth of January 2023 because the date of public disclosure.

On the twenty ninth of November 2022, Threema launched a brand new protocol, Ibex, as a way to additional mitigate our assaults. The Ibex protocol goals to offer ahead safety for the E2E layer in Threema. We have now not audited this new protocol.

We consider that all the vulnerabilities we found have been mitigated by Threema’s current patches. Which means, right now, the safety points we discovered now not pose any risk to Threema prospects, together with OnPrem cases which have been stored up-to-date. However, a number of the vulnerabilities we found might have been current in Threema for a very long time.

There are a number of broader classes to remove from our evaluation of Threema.

Our work exhibits that it is not simple to evaluate the safety claims made by builders of functions that depend on bespoke cryptographic protocols. Notably, previous independent audits of Threema didn’t evaluation the cryptographic core of the applying. Such an evaluation ought to be a minimal requirement for any safe messenger, particularly one being utilized in delicate environments. Ideally, any utility utilizing novel cryptographic protocols ought to include its personal formal safety analyses (within the type of safety proofs) as a way to present robust safety assurances. Such an evaluation will help to cut back uncertainty about whether or not additional severe cryptographic vulnerabilities nonetheless exist in Threema.

Our evaluation additionally exhibits that there are some ideas well-known within the analysis group which haven’t totally made their approach into the group of builders utilizing cryptography. We thus draw three classes, which we predict could also be helpful for such builders:

  1. Utilizing fashionable, safe libraries for cryptographic primitives doesn’t, by itself, result in a safe protocol design: libraries comparable to NaCl or libsignal may be misused whereas constructing extra advanced protocols and builders have to be cautious to not be lulled right into a false sense of safety. Whereas the mantra do not roll your individual crypto is now extensively identified, it ought to be prolonged to do not roll your individual cryptographic protocol (assuming one already exists that meets the developer’s necessities). Within the case of Threema, the bespoke C2S protocol may very well be changed by TLS.
  2. Watch out for cross-protocol interactions: even when a protocol by itself is taken into account safe, there isn’t any a priori assure that it will likely be safe when composed with different protocols. Cross-protocol interactions can undermine the unique safety ensures, as we’ve got proven with the vouch field forgery and Kompromat assaults. Such dangerous interactions may be prevented by following the key separation precept which states {that a} system ought to use totally different keys for various functions.
  3. Proactive, not reactive safety: our incapacity to search out an assault on a protocol doesn’t suggest it’s safe. New assaults may very well be discovered at any second and identified assaults solely get stronger over time if left unaddressed. Typically, safe methods and protocols observe a design-release-break-patch course of (a reactive strategy). That is inconvenient for customers and sometimes requires the upkeep of backwards compatibility. Builders ought to as a substitute undertake a proactive strategy, the place the system or protocol is formally analyzed throughout the design stage.

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