I assumed that not only the entire app but also the entire client device had been audited. This was a client-side attack, not Meta momentarily adding itself to the trusted-device list. I’m confident it was a client-side attack because it would be impossible to hide even a momentary change in keys from the client without modifying the client app to conceal such a change.
Even in an “insecure” app without air-gapped systems or manual encryption, creating a backdoor to access plaintext messages is still very difficult if the app is well audited, open source, and encrypts messages with the recipient’s public key or a symmetric key before sending ciphertext to a third-party server.
If you trust the client-side implementation and the mathematics behind the symmetric and asymmetric algorithms, messages remains secure even if the centralized server is compromised. The client-side implementation can be verified by inspecting the source code if the app is open source and the device is trusted (for example, there is no ring-zero vulnerability).
The key exchange itself remains somewhat vulnerable if there is no other secure channel to verify that the correct public keys were exchanged. However, once the public keys have been correctly exchanged, the communication is secure.
Is there a specific “undress” button? I tried looking for proof that it exists but couldn’t find any (my searching skills clearly need work). Could you please share a screenshot or point me to a place where I can confirm that it exists?
That data might be easily accessible, but that was a choice Root made. I think that it is a safe assumption that Root knew most vigilantes keep their identity secret and, assuming a German background, had read Section 202 of the StGB and other relevant laws and court rulings. As such, Root most likely did this despite knowing their identity is at risk. It is likely they did this publicly specifically to inspire others, though I haven’t looked at all the details and there might be a different reason.
Nothing in this comment constitutes legal advice.
Yes, it is visible when a new trusted device is added. The QR code you scan to link a device contains a one-time public key for that device (ECC is used partly to fit the public key more easily into a QR code). Signal on the phone then sends a lot of information, including the identity keys, to the new device. The new device uses these identity keys to communicate. Note that the transfer of identity keys is fully encrypted, with encryption and decryption taking place on the clients. This can, of course, be bypassed if someone you’re talking to has their security key compromised, but the same risk exists if the recipient takes a screenshot or photographs their device’s screen.
Edit: The security key refers to the one-time key pair generated to initiate the transfer of identity keys and chat history. It can be compromised if someone accidentally scans a QR code and transfers their identity keys to an untrusted device.