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Comment by 4bpp | Hacker News Reader

Even if Signal's assertion that they do not keep metadata is in fact trustworthy, a network-scale adversary (such as a nation-state) could easily figure out that you called someone and who you called by looking at the data exchanged between your phone (which they know) and the Signal servers, and looking what other connections started and ended at the same time as yours and had a similar bandwidth usage profile.

(How many Signal phone calls are even started in any given short window of time, like 1 second?)

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Correct me if I'm wrong, but I believe the signal servers are hosted on google cloud, using encrypted traffic to obfuscate the point of exit for a signal user.

This is why in the past, governments were unable to block signal without blocking google or all google cloud customers themselves.

Perhaps this would be another way to obfuscate the collection of metadata with techniques similar to anti-fingerprinting in Tor. An app like Signal could sync the time across all parties and send/receive all data in predetermined packet sizes and at predetermined delivery times across vast enough segments of their network. This would in theory make it a lot harder to infer who are the partners in a specific conversation based on timestamps and traffic volume.

The mechanism for picking the interval and packet size is debatable as long as the entropy of these values is low enough (does not allow for uniquely identifiable values). Maybe randomly from a list of predetermined values, taking into account some priority you set in advance, the size of the message, or the network conditions.

If every client/server on the network sent and received the data in synced identically looking bursts it might raise the bar for successful metadata analysis. Any chance Signal or any other app already do this?

Is approach is going to kill battery life because the phone has to be always online to receive those times transmission.

Most modern CPUs/SoCs spend most of their time (ideally) asleep or trying to go to sleep, and then waking at predefined intervals to transmit data. As long as this interval could be synchronized to some multiple of the transmission interval I don't think this would negatively impact battery life.

The techniques you're talking about, and others, have also been used in anonymous remailers[1] to resist traffic analysis[2].

The mixminion[3] remailer design document[4] goes in to details on those mitigations.

Of course, real time conversations require certain constraints that email messages don't have, so some of the mitigations techniques used in remailers might not be practical here.

Signal's designers must have been well aware of Tor and anonymous remailers, so if traffic analysis mitigation techniques were not used by Signal it was a deliberate design decision on their part.

[1] - https://en.wikipedia.org/wiki/Anonymous_remailer

[2] - https://en.wikipedia.org/wiki/Traffic_analysis

[3] - https://en.wikipedia.org/wiki/Mixminion

[4] - https://www.mixminion.net/minion-design.pdf

If they know about your physical device you are in a bad situation, but you would be in a worse situation if you used a phone line that they knew about. If we're moving the goal posts then yes, there are many ways to compromise a connection, but in no situation can I see that it would be beneficial to use a landline or a regular cellphone over a smartphone with the ability to encrypt and authenticate. So why is the question how to "make a secret phone call" if a phone call is an inferior technology to begin with? If the threshold for usage was high and required specialized technical knowledge, then sure, but you can hardly say that about Signal or VPN applications.

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