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this post was submitted on 23 Sep 2023
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People vaguely disparage signal on social media a lot, what's the evidence for signal having a backdoor?
The fact that they store encryption keys on their servers in the cloud, relying on SGX lock boxes to prevent trivial exploitation of those keys.
In information security, as with intelligence work, it's about capabilities not intent.
Signal has the capability, to brute force the SGX enclosures, or even use trivial code signed by Intel to simply export the keys from SGX enclosures, which means all of the encryption keys stored in signals cloud, which is all of them, could be compromised. That is a capability they have.
SGX has had multiple exploits, especially side channel attacks through timing, and other metadata in the CPU. Intel is a US corporation, and their subject to national security letters, so they could be compelled to release their SGX signing keys..
All the Lego pieces are there for signal to have a back door. It's about capabilities. I'm not saying they have a back door, but the pieces are there for one.
If you recall a few years ago, there's a big hullabaloo about signal storing encryption keys in the cloud behind four digit pins.. this is why people are so angry about it. It means we have to trust the central servers, which is antithetical to the capability model that we talked about.
That being said we are reasonably sure the signal client code is secure. So if you disable pin codes and signal, your encryption key is still sent to signal cloud, but it is signed with a cryptographically secure 128-bit something code. So that's fine. But if everybody you're talking to hasn't disabled the pin, then the other side of your conversation is still exploitable.
TLDR: signals great if your threat model does not include five eyes intelligence services, and if your threat model does include five eyes intelligence services you should use something else. Not by intent, but by capabilities.
Thanks very much for the breakdown. I was totally unaware of the keys being stored in the cloud, that seems like a terrible idea for a privacy based messaging system.
Are there more secure alternatives?
Oh. Well yea, the keys not relating to my conversations does flip everything on its head again. So back on its feet?
Are there other keys for your conversations in signal?
Do you use signal?
Okay, thanks for taking the time to write all of this down and explain it to me, I feel like I understand signal a lot better now.
And I feel better knowing that the keys to the conversation are regularly replaced in the app and that there are plenty of steps to take to make it more secure anyway.
Man, that was a really interesting reply hah, I've read through it like three times.. I'll be checking out the links you sent also.
Here's a point in favour:
https://signal.org/blog/cellebrite-vulnerabilities/
Okay thanks
https://signal.miraheze.org/wiki/Secure_Value_Recovery
The master key is stored. At the very least somebody with the master key could be able to engage your contacts, without getting a new key warning.
Yes. But signal is the gold standard, it's going to be hard to get your contacts onto any other platform.
https://www.privacyguides.org/en/real-time-communication/
Reviews the options nicely, I use briar, it's rough around the edges. But it does the job.
I've been meaning to try simple x, but I haven't given it a go yet.
You can follow the privacy guides guide on hardening signal, it's useful. Net net the easiest thing to do is disable your PIN, and ask any friends you're speaking with to disable their PIN.
https://blog.privacyguides.org/2022/07/07/signal-configuration-and-hardening/#signal-pin
Simplex is working pretty well for me. One of the people I chat with has an apple device so briar wasn't an option, otherwise that's probably what we would be using.
Okay, thanks. I'll read both of those articles and for now disable my pin on signal and talk to my contacts.
Matrix has absolute shit adoption, but is open source and pretty secure. Then there is always Telegram.
Do you still have to consciously enable encryption in Telegram? That was the gripe people had with it for a while. Chats weren’t encrypted by default.
Yes, you still do for E2EE.
Okay cool. Thanks. Oh I didn't even know telegram had encryption capability
Almost every chat platform uses encryption by default, including telegram. If you are talking about E2EE, you have to enable that manually for each chat.
Matrix is really awesome and I hope it becomes the gold standard. However, if I were a Snowden, I would pick signal over matrix for the simple reason that signal doesn't store your conversations on the server. Matrix does. Those conversations are encrypted client side with a key the server doesn't have, but they are still stored centrally. That has advantages and disadvantages. It is much better for usability, because you can log in from any device and you see all of your conversations in one place. Unlike signal, there are no primary and linked devices, you can run matrix on desktop, laptop, phone, tablet, or straight from a web browser. When logging in from a new device, you need your username, password, and to either authenticate the session from another device, or manually put in your encryption key to decode the chats. That also means there is no need for backup or restore of anything other than your encryption key. For that reason, I am more frequently pushing people to install matrix than signal these days.
However if security is more important than usability, signal wins, if only because there is never a question of storing anything on any server. Start a chat with somebody, make the messages disappearing, and you can be pretty sure that as long as neither of your devices are captured while the chat is in progress it will never be seen by anybody.
This breakdown makes me much more hesitant to ever use Signal over Matrix. Signal is storing the keys themselves, where as Matrix is storing messages that can’t be decrypted and no keys. If the keys on Signal’s servers are ever stolen, you can kiss all of your message privacy goodbye. If a Matrix server is hacked, the user can’t do anything with the messages because they’re encrypted and no keys are stored.
You also have the option to host your own Matrix server and have more control—something that is not an option with Signal.