What happens if we take this idea and instead encrypt the messages using PGP. If bluetooth wasn't so crazy on power consumption you could delivery peer to peer SMS's. It would probably take a while though for delivery to happen if you weren't walking around syncing with other nodes.
We are considering as a next step having signed messages so you know at least two messages came from the same person. We want to maintain anonymity to protect posters, but it'd be interesting to know that posts were at least coming from the same person. I'm not sure this is what you're talking about though, sounds like you're more meaning person to person private messaging right?
This is neat. But. If this took off, and was actually being used by people...
...and the authorities, or any other attacker, wanted to disrupt the system -- they could just put all sorts of devices in the area distributing an incredibly high volume of spammy messages. As you circulated and downloaded messages from other nearby devices over bluetooth -- the legit messages from good actors would be overwhelmed by the spam messages, taking up all your storage space and making it hard to notice the legit messages.
I was thinking about this, but then read this thread, and it occurs to me there may be some solution involving crypto, whitelisting certain signatures as 'known good actors', or even a web of trust thing. But yeah, that also could compromize the desired anonymity.
And it's also probably true not to bother designing for a hypothetical problem/attack, the actual problem/attack will be subtly different. Still, I see a lot of these systems that are _really neat_ tricks, but seem to me like they would break down if they actually became popular, they work only as neat tricks.
But yeah, I also really like the idea of private person-to-person (or person-to-known-group) encrypted messages -- they could even be distributed over participating third parties devices right? Author walks by person X, who's device picks it up but can't actually read it, and later hands it off to person Y, one of the intended recipients, who can read it. I'm not sure if that would end up actually being useful or not, but it would be NEAT.
[I'm the co-creator of this project] I think that because this is a "human network", you wouldn't get those kinds of problems. People won't be synching with random strangers in the street, they will be synching with people they know and trust. Data will spread via "six degrees of separation". Think of it is just a more convenient form of USB thumb-drive sneakernets.
I think PGP signing could be a great way to prove authorship on publicly available messages (in those cases where rather than anonymity one does want to definitively know who wrote a message).
EDIT:
If you are not already familiar with it, take a look at the PGP web of trust idea to get a better feel for the "proven authorship" use case. Some linux distros use PGP keys to prove authorship of software. http://en.wikipedia.org/wiki/Web_of_trust.
Considering the power consumption, bandwidth, etc., 802.11 Ad-hoc networks might be a great choice for this concept. Plus there's already OLSR[1] for handling multi-hop packets. Unfortunately currently mobile devices are only equipped with one 802.11 adapter and it can be either ad-hoc mode or Wifi-client at one time.
I was talking a little bit about this in a comment down below, but android just added adhoc wifi networking (in v4.1 maybe?) so it's not too widespread. iOS has had it for a while, but the app is currently android only, which seems to be the most appropriate OS for the project anyway.
Either way it'd be great to support it for those power reasons you mention and for how much faster it would be to sync.
The problem is that a wireless (802.11) interface can only join one SSID, and work in one of the modes (ad-hoc, wifi, AP) at one time. If like current phones on which there's only one wireless interface, when people are connected to Wifi, they won't be able to use ad-hoc.
I've been hoping there's a company coming out with a phone/tablet with two 802.11 interfaces, so I can stay on Wifi and at the same time join an ad-hoc network :-)
I'm not exactly sure how the adhoc stuff works on android but there's a specific api for it which is usually used for local gaming. In any case the use case for this app is that the Internet is shut down in your ___location so you probably don't want to connect to it anyway.
I'm not sure how far Android goes to sandbox you, but you might be able to write an app that runs in the background to detect/manage/switch-between network interfaces depending on what the intent of the user is at that moment. It'd be a handy little tool if it worked.
Bluetooth LE has very low energy (heh) consumption and a 50m range. A short walk in public should be enough to collect the most recent messages, if not just standing by a window on a busy street.
