TIL Briar does "offline messaging". This is news to me, though I've never used it.
That said, I am curious to hear more about the offline messaging. If it only is able to exchange when the two people who are trying to communicate with eachother are directly nearby it isn't so much a mesh network, right? A mesh network would be able to route across other nodes to get to its destination. Does Briar do that? The "How it works" page doesn't really seem to answer much, so I am assuming not.
I think a combination of LoRa, bluetooth, and WiFi might be the alternative. I've seen videos of LoRA functioning below the noise floor (perfect for evading RF triangulation), and at 200km (perfect for reaching past physical borders). The major weakness is line of sight (and availability), but bluetooth and WiFi can help there.
Communication protocols that incorporate spread spectrum (code/direct sequence for GPS and chirp spread spectrum for LoRa) get a "processing gain" at the stage of the receiver where the signal is despreaded. The resulting signal will have an SNR roughly equivalent to a narrow band (non spread spectrum) signal with otherwise the same parameters. You will have a generally equivalent bit error rate for the same SNR.
It's also possible to receive non spread-spectrum signals below the noise floor, if you can observe it over longer time and get additional "processing gain" that way
Additionally, it is a bad idea to use spread spectrum as a means of concealment because if the adversary is physically near enough, your signal will show up above the noise floor. Due to the inverse square law etc, you have a narrow zone of enough power to be received by your remote recipient, but not enough power for closer adversaries to detect you. You are also reliant on the unlikely situation of an adversary without more advanced RF hardware with lower noise receivers.
Other posters have pointed out that this is incorrect, but I wanted to give a bit of intuition as to how signals can be received when they are below the noise floor.
First, as a definition, below the noise floor means that the power of my signal at any given time is smaller than the power of the ambient noise in my channel, and usually this implies that you're only interested in a particular segment of frequency spectrum (e.g. within the 10MHz band centered at 1.8GHz). If we were doing a simple frequency-shift keying or amplitude-modulated signal, once the noise power exceeds the signal power, there is basically no hope of recovering anything useful, as those are both demodulation schemes that rely upon obtaining instantaneous estimates of the frequency or amplitude of the signal of interest.
However, spread-spectrum methods make a time/frequency tradeoff, where the signal of interest is "spread" across multiple points in time and frequency. A very simple example of this is to say "if I want to transmit a 1, instead of transmitting one cycle of a sinusoid at 18.GHz, I will transmit 10 cycles". Then, at the decoder stage, you average across 10 cycles of your carrier in order to detect whether a signal was sent or not. By doing this averaging across time, you get a 10x gain versus the noise which is expected to cancel itself out as often as not.
True spread-spectrum techniques are more advanced than this, they actually use wave shapes that are more complicated than just a sinusoid to make it easier to detect when they start and stop (whereas with a sinusoid there's a fair amount of ambiguity if you shift one period to the left or right) but the fundamental idea of averaging across time is the same.
Through this mechanism we are able to rescue out signals from far below the noise floor, although it reduces your maximum transmission rate. When dealing with digital radio systems we can even rescue out signals from below our quantization floor, although not too much lower, as eventually you lose the ability to average out a signal that is fluctuating by significantly less than a single bit.
Whenever I talk about making tradeoffs in transmission speed to aid in reception, I am reminded of the ELF systems in submarines [0]. While they did not use spread-spectrum techniques, (they just jumped between two frequencies, 76Hz and 80Hz) they still correlated across time to boost up their effective SNR.
[0] https://en.wikipedia.org/wiki/Communication_with_submarines#...
> If the RSSI is below the noise floor, it is impossible to demodulate the signal. However, LoRa can demodulate signals that are below the noise floor.
Not exactly. Briar uses bluetooth or wifi when peers are close, but also tor (over standard internet) when not, so it's possible to use it at wide scale.
I would think that a meshnet only over contacts would have too many holes to really be helpful in the hypothetical protest setting that's being described. Definitely a cool way to do it! Do you happen to know if it's XMPP or something, or it's own protocol?
There are still holes of course, connecting only to contacts limits the spread of messages but ensures you don't leak too much information if your device is compromised
Briar is awesome. Ive used it to talk to my wife when we are seated away from each other in planes or trains and cant get up, via bluetooth due to lack of service, wifi, or because of airplane mode.
I continue to be frustrated by having to use the plane's satellite internet connection (not always free) to message somebody sitting two rows away from me, so this would be great.
Seriously though, if you are going to take a phone to a protest, buy an Android used at a bodega and don't put anything personal on it. Expect to lose it.
I've bought a fair number of test devices. Nobody at a corner shop or mall phone repair kiosk has ever ID'ed me or kept track of the IMEI of a device I bought.
Interesting. My understanding was that IMEI could be traced to what vendor it was sold to, then they could pull the purchases and either see the card used to buy it, or find the video when the transaction occurred. Guess it depends on whether there is a method to pin which the exact phone without needing to scrub the purchases records.
You are significantly overestimating the level of recordkeeping by random sellers of used and/or ultra-cheap phones. Manufacturers, major retailers, and carriers may keep this information, but bodegas and street vendors certainly do not.
There are so many links in that chain that need to line up, from the manufacturer keeping track of it to the distribution system to keep track of what batch goes where to the vendor keeping track of what phone IMEI is sold when or to who. Even if all those link up you need to get at the video within the rotation time for their video storage or link to their financial transaction data.
I would not be surprised if it is still as easy to evade as shown in the show (and as easy to get wrong).
New burner phones, probably, but one could exit from a flea market or 2nd hand shop with a €100 bagful of phones not linked to anyone, at least until one puts personal data inside them or creates a potential association by using the same phones along personal ones on the same WiFi/cell tower/position or calling the same numbers.
US is one of the few nations where you can buy a phone and sim card without exposing your ID. You can even wear a full face mask when you do it if you're paranoid.
It's annoying, but open source projects tend to prefer more open platforms. I assume that many Briar users use a deGoogled custom ROM instead of the stock Android ROM and a privacy focused app store like F-Droid.
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That said, I am curious to hear more about the offline messaging. If it only is able to exchange when the two people who are trying to communicate with eachother are directly nearby it isn't so much a mesh network, right? A mesh network would be able to route across other nodes to get to its destination. Does Briar do that? The "How it works" page doesn't really seem to answer much, so I am assuming not.