#ix Beyond Citations | Is Signal secure?
The who’s who of US leadership appears to believe so, but are they right?
If one is an avid follower of international affairs, it is likely that the magazine The Atlantic will ring some bells. For a watcher of US politics in particular, The Atlantic would have a recall value closer to, if not equal, the New York Times or the Washington Post. But it is unlikely that one would have heard of Jeffrey Goldberg, the Editor-in-chief of The Atlantic. It is also unlikely that one would have come across any exclusive, big-breaking story by The Atlantic.
But thanks to an accident by the senior-most national security leadership team of the US, Jeffrey Goldberg got a ring-side view of the US plans to bomb Houthis in Yemen. Goldberg got a one-in-a-lifetime opportunity and he took it — his story on the ‘Houthi PC small group’ on Signal broke the internet. After the Trump administration downplayed the significance of the chat, and maintained that no classified information was shared there, Goldberg published another story — this time revealing the bombing timeline that he had previously withheld.
Below is the timeline that the US Defence Secretary Pete Hegseth shared:
Pete Hegseth
TEAM UPDATE:
TIME NOW (1144et): Weather is FAVORABLE. Just CONFIRMED w/CENTCOM we are a GO for mission launch.
1215et: F-18s LAUNCH (1st strike package)
1345: “Trigger Based” F-18 1st Strike Window Starts (Target Terrorist is @ his Known Location so SHOULD BE ON TIME) – also, Strike Drones Launch (MQ-9s)
1410: More F-18s LAUNCH (2nd strike package)
1415: Strike Drones on Target (THIS IS WHEN THE FIRST BOMBS WILL DEFINITELY DROP, pending earlier “Trigger Based” targets)
1536: F-18 2nd Strike Starts – also, first sea-based Tomahawks launched.
MORE TO FOLLOW (per timeline)
We are currently clean on OPSEC.
Godspeed to our Warriors.
The chat also revealed what the Trump administration really thinks about Europe beyond the optics. “I just hate bailing Europe out again” is how the US Vice President JD Vance thought of the US action in Yemen. Vance also expressed the first publicly known sign (since assuming VP office) that he can question Trump: ‘I am not sure the president is aware how inconsistent this is with his message on Europe right now.’ Pete Hegseth, not to be left behind, called European free-loading ‘PATHETIC’ in all caps!
The Signal chat leak has infused fresh energy into the disheartened Democrats who have mounted an attack on the Trump administration over the use of a non-government, non-classified application to share sensitive information. The debate is increasingly about semantics: for example, how the attack timeline is different from war plans. The Trump administration is taking the stand that while Michael Waltz, the National Security Advisor, made a mistake of adding Goldberg into the group, no classified information was shared. And that the focus should be on the success of the Houthi operation and not on Signal.
But questions are being asked about the prudence of using a non-governmental application for sharing sensitive military information before the actual bombing took place. If a dedicated hacker group (state or non-state) gained access to this information and passed on to Houthis, it would have put American soldiers involved in the operation at a serious risk.
The signaling from the Trump administration on Signal has been mixed. At one end they have said that it is approved for government use and comes pre-installed in government devices, at the other hand they have acknowledged that it could be defective and vulnerable.
The million-dollar question is whether Signal is secure enough to be used for non-public, non-classified but sensitive information sharing?
Wichelmann et al. in their 2021 conference paper provide some answers:
Wichelmann, J., Berndt, S., Pott, C., Eisenbarth, T. (2021). Help, My Signal has Bad Device!. In: Bilge, L., Cavallaro, L., Pellegrino, G., Neves, N. (eds) Detection of Intrusions and Malware, and Vulnerability Assessment. DIMVA 2021. Lecture Notes in Computer Science, vol 12756. Springer, Cham. https://doi.org/10.1007/978-3-030-80825-9_5 (pre-print version pdf available here: https://eprint.iacr.org/2021/626.pdf).
The authors acknowledge right at the outset what is widely believed to be the case: the Signal protocol is one of the best in the world from an encryption and security point of view. That is the reason WhatsApp and Skype use it too in addition to the Signal messenger app.
So much so for Signal’s brilliance. The authors then proceed to demonstrate that Signal does not guarantee post-compromise security in multi-device settings.
But what is post-compromise security? Authors explain:
Modern cryptographic protocols aim to achieve different security guarantees, depending on their use case. One of those guarantees is the security in case the long-term keys of a party are leaked. Two important notions dealing with this are forward secrecy and post-compromise security: Forward secrecy (typically achieved by the use of ephemeral keys) guarantees that previous communication is still confidential, even if the long-term keys of the parties are leaked…In contrast, post-compromise security guarantees that leakage of the long-term keys does not break the confidentiality of future communication…
The authors find a fault with the Sesame sub-protocol in the Signal protocol suite that is used for multi-device support. They demonstrate that ‘multi-device support of the Signal messenger can be abused to eavesdrop on all communication after a one-time credential breach.’
It is very much possible that Signal rectified the Sesame issue in the years since this paper has been published. But as the Pentagon memo warns, hackers can still use the linked devices feature of Signal to eavesdrop on conversations. It is another question about how much blame can be laid at Signal’s doors when malicious actors use phishing attacks to bypass end-to-end encryption.
Regardless, the 2021 paper discussed above does demonstrate that the Signal protocol — while arguably one of the best in the world — might still have vulnerabilities and chat rooms on Signal could be compromised by a dedicated actor.