Bitcoin Forum

I think that one of the problems Bitcoin will be facing over the long term is the aftermath of some disastrous events. I'm talking about events leading to internet blackouts or power cuts. These events include but are not limited to strong cyber attacks, fuel shortage (see Lebanon), earthquakes and so on.

If power or the internet goes down, it's safe to assume that Bitcoin becomes practically worthless/useless in an instant for anyone who's living the aftermaths. We're talking about situations like the current one Lebanon is facing. Had this happened in a much larger nation like the US, I think Bitcoin would've confronted a huge sell-off and loss of interest/trust.

We currently have Blockstream's Satellite which can be either a DIY setup or you could buy the equipment (when it's in stock anyway) off their website AFAIK. I read every now and then a bit about it, but I never had much interest in it. The reason? It's seemingly difficult for a beginner to set it up, it isn't the most portable thing you can have and it's not affordable for the regular guy. Can't see the regular Bitcoiner to own a Satellite setup.

On the other hand, there is GoTenna with their TxTenna. This one is portable, it's much cheaper but as far as I know, the company behind it doesn't care much about the regular GoTennas anymore since they took the military equipment more seriously. When you have a new company providing the regular Joes a way to go off-grid and independent from ISPs or network carriers, you as a military force have to bait them and pull them away from the average Joes .. anyway.

There must be a way to create a device that is affordable, portable and allows you at the same time to be your own bank without depending on ISPs or network carriers no more. I might be very wrong, but I think ham radios can be used to send e-mails. Not sure if there are any ham radios that you can use without a license in some countries (e.g. the US), but perhaps Bitcoin transactions could be transmitted through radio frequencies? There were some kind of radios that you could transmit over a range of up to .. 200 miles (or kms) as well? Is there absolutely no way this can be done?

We have so much technology to use and Bitcoin is imo still just at the beginning. Building a rechargeable device that allows for fully offline Bitcoin txs to happen would make it about invincible even in disastrous events. Is this a bad/impossible-to-realise idea? What do you think?

Cool topic! I'm a big proponent of a new, independent network, similar to how the Internet itself was first created way back. Independent and more resilient than the current infrastructure.
Keep in mind: a network just for Bitcoin poses a few issues. For one, everyone will know if you use this network, that you use it for Bitcoin. Furthermore, it can be more easily banned or prohibited. It would be better if this new network was more generic and e.g. we also had messengers operating through it.

Online banking or traditional banks in general are as reliant on electricity and internet as well, though. I am pretty confident even a brick-and-mortar bank doesn't work without internet connectivity. Did people lose all trust in their bank when they couldn't go there due to blackout? I haven't encountered this so far.

That's correct; you can even run a complete TCP/IP stack over ham radio (https://www.febo.com/hamdocs/intronos.html). However, it requires a license (at least in all the countries I know of), very specialized hardware and no encryption is allowed. That's the biggest culprit in my opinion. No TLS! No bueno, in my honest opinion.


Yes, it depends on the frequency and thus on antenna size though. For example very low frequency signals travel insanely far, but have a terrible data rate and require antennas literally the height of someone's home.

It's certainly possible to build and I'm very interested in the topic. One feasible way is to create a 2.4GHz mesh network using directional antennas. It should be possible by repurposing 2.4GHz routers with a custom firmware and attaching directional / parabolic antennas to them, of course positioned outside. The ranges as far as I know are around 10-20km, so you'd need a few people in every city running one such thing to get a good coverage. That's why I say mesh network.

Here's an unrelated video that shows what off-the-shelf hardware can do in the 2.4GHz band:
https://youtu.be/15JmpVAuzZY

Immidiately after any of the examples you provide, the percentage of barter transactions tends to go way up. There tends to be less use for actual money, and more use for tangible goods used to survive, such as food, fuel, and water.

I don't think any kind of "mesh" network is going to work very well when there are many users involved, or when it needs to cover the entire globe (or even large parts of the world). A mesh network greatly increases the chances of a Sybil attack and makes detecting such an attack much more difficult. There are also privacy implications to using radios to use bitcoin, as it would become trivial for anyone in your area to know you are using bitcoin; this is compared to only your ISP knowing you are using bitcoin currently.

The word is "unusable" not useless.

There are two problems here, first if you can't move your bitcoins you can't sell them off. Second is that if there is such an aftermath you can't use your fiat either so there is no point in selling bitcoin to get another unusable thing!

In many ways, this is centralized because the satellite is owned by a company (located in US!) and they can decide what services they provide and to whom.

Exactly. In my opinion if we can't use something with the tools we already have in our possession it is not going to be a widely used thing.

The problem is range, it is too short. According to google it is between 3 to 28 kilometres (2 to 18 miles).
Besides any "disastrous event" could take out radio towers too making any kind of longer distance communication impossible.

I believe they use the towers (as booster) which is basically a middle man.

Of course! The more you could do on the network, the better. If you look at GoTenna's slow loss of ground in this area of development (I'm talking about the development and existence in the open, retail markets and not military who already own a ton of unknown tech anyway), you would further understand that such a device would be likely treated as a threat to the existing banking system. I'm actually in fact surprised that TxTenna hasn't met a backlash from the governments (although I think the fact that GoTenna doesn't maintain their products anymore could be taken as an indirect backlash...)

I actually do think people got much more scared of banks whenever they had problems with them. Think India when their banknotes suddenly got taken out of circulation, or think any disastrous event/crisis where the people couldn't take their own money out of their own accounts. In such events, had Bitcoiners had a device as mentioned in the OP, you can only imagine how much ground BTC would earn in front of banks. Over-the-air transactions in an offline environment. It's like solving one of the biggest flaws of the banking system.

I've been looking for its name, but I still got no clue. There was one particular way of communication through radio that was sending the signals up to the sky and then back to Earth rather than horizontally, which extended its range significantly and eliminated the issue of waves being blocked by certain obstacles.

Interesting. Wouldn't it be possible for such a device specifically for BTC & messaging (perhaps messaging on blockchain) to be produced en masse with a very low cost (<$100, perhaps even <$50)? With Ledger's success, I can only imagine how insane it'd be to have a massive mesh network like that specifically run for Bitcoin. The only issue I see is, if people don't get incentivized for using the mesh network, they will probably not keep them up-and-running 24/7. In the event of blackouts that would likely change, but still.

What about a mesh device with a built-in lincense-requiring radio transmitter/receiver? As far as I know, during disasters ham radio licenses for example are not necessary anymore. Not sure if massive blackouts or power cuts trigger this as well.

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I agree with you about the barter part - but I think it's safe to assume that the levels of barter use go significantly up in these kind of events specifically due to the instant distrust in fiat/banking system. If Bitcoin was there to be used even fully offline, this would change.

About the security & privacy of such an idea, I don't know much so thanks for underlining these issues. It's definitely not the most reliable thing, although perhaps there is one key solution we just haven't thought about yet.

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I think cash would still work, unless people stop trusting fiat at all due to the likely extremely high sudden price changes (black markets & barter) and due to the loss of purchasing power. And if that happens but Bitcoin can still be used, then I think it's a winning situation for BTC. The point isn't selling it for fiat but rather using it as a currency instead.

Answered this above, still can't find its name. It transmits radio waves vertically rather than horizontally. Not very sure about using towers as boosters.

Of course cash will still work but the problem is that you should have had cash already before the "event" otherwise considering the fact that almost everyone stores their fiat in banks and the banking system can also be affected by the same "event" we can assume people's access to their fiat is cut off.
I think it was Bangladesh a couple of years ago when people formed long queues in front of banks to get their cash out and the banks didn't allow them. Life practically stopped and it showed people don't really store cash.

It's nice seeing that there's always some sort of enthusiasm for a new invention. The intelligent part of this innovative technology is that it incentivizes you to think likewise.

I think that's fearmongering.

Everything is built to operate online; the whole world moves (and has already moved) their businesses to the internet. There are stores which only exist in the internet. Part of the education system is done from distance. The communication etc. Saying that one of the problems Bitcoin will face is this kind of disastrous event is trembling the least.

Lots of other events will occur in such scenario which will affect the economy much worse.

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BTW, lightning doesn't always require internet connection (https://bitcointalk.org/index.php?topic=5158920.msg58075119#msg58075119).

The term you are looking for is skywave propagation, as opposed to ground wave propagation.

I would agree with the feeling that mesh networks are not really a workable long-term solution. In the event of a city or country wide black out or similar, then your mesh network still requires someone to be connected to the internet to relay information back and forth to the rest of the network. It's one thing to trust that person to broadcast a transaction for you, but it's another altogether for an entire town or city to be relying on one person's internet connected node to relay all the information through the mesh network. I suppose you could partly mitigate this by also running your own node via Blockstream Satellite.

It is not required to have just "one person" in charge of connecting the mesh to the rest of the world, multiple paths could exist supported by a smart gateway protocol. I'd suggest such a mesh based approach using mobile devices as a default topology even for day-to-day low bandwidth use cases and not just extreme conditions.

As others have said, if things get that bad that we have no power and no internet then not being able to get to your BTC is the least of your problems.
The run on banks for cash, is one thing, but if you want to buy something unless the merchant also has some way of converting the BTC that you could get them through whatever method to something they want (cash / gold / whatever) then they will probably not take it.

As for satellite, with starlink coming online and kuiper a few years off and there was another one whos name I don't remember although they will be controlled by big corporate entities there should be enough different ones out there to keep prices low and bandwidth up.

Ham radios and such will never be able to keep up and if you think that blockstream satellite is expensive, look at the price of a good ham rig and antenna.

-Dave

Sorry, I think that the solution is missing something important and it becomes kind of worthless as it is.
Imho complete blackout of the entire internet and electricity is not really possible for reasons like shortages. Or, let's say, the chances for that are way too small to matter.
Imho the only actual problem is massive solar flares/coronal mass ejections/geomagnetic storms which can indeed shut down... maybe everything. But in such a case even the satellites aren't safe.

I don't know what would be the correct solution, but we seem to try to address problem with lower chance to occur and it may be better to focus on the bigger problem; if that's handled, the smaller one will also be solved.

Sure, but what is the likelihood of such a scenario? In such a case where there is a city wide black out, but enough internet coverage via cellular or satellite for multiple connections throughout the city in question, then how necessary is a mesh network? If some people can still get online via cellular or satellite, then surely most people can still get online? And the few who can't are much more likely to simply use someone else's connection or some public hotspot than they are to set up a mesh network. If there was a mesh network already functioning, then sure, but there is pretty much zero incentive for people to set one up or maintain it for some very hypothetical situation in which not trading bitcoin would be the least of your concerns.

There's no thing such as full/absolute decentralization.


Maybe because it's not popular? For comparison, their github repository (https://github.com/MuleTools/txTenna (https://github.com/MuleTools/txTenna)) only have 71 stars, while blockstream satellite (https://github.com/Blockstream/satellite (https://github.com/Blockstream/satellite)) have 822 stars.

Not necessarily true, actually one could list dozens of scenarios in which your argument doesn't hold.



It is not just about trading Bitcoin, in the bigger picture, it is about preserving tcp/ip connectivity exploiting its essential inherent decentralized nature and resistance against single points of failure. As of your lack of incentive argument, I suppose it is an open issue that needs further innovations and more elegant ideas.

It is possible to transmit thousands of kilometers without boosters of any kind; I don't know that something like this is even used much in ham radio. Usually, HF signals can be bounced at the ionosphere and travel around the whole globe without any need of something like that.

Yes, everyone can just buy one such antenna for around $50 and an existing router with custom firmare & build a mesh network. They need UPS though for it to make any sense in a blackout so it will be a bit more than $100 and a good firmware + lots and lots of participants. Keep in mind there are only around ~10.000 BTC nodes worldwide. You'd need more than that in each country to have a working mesh net.

Ham radio laws are very tight. I don't think a blackout immediately allows you to use ham radio. Furthermore, in the stress of a real disaster, nobody wants to first sit down and setup their ham mesh Bitcoin node stuff. It has to be up and running before, be all configured and connected beforehand, realistically. And that's 100% not allowed without ham license on ham radio frequencies. (like HF bands which cover larger distances)

I agree, it would be sick to have another independent and more resilient network in general; not just for Bitcoin. But if it's pushed and mainly used for Bitcoin, it would surely help BTC reputation :D

I very much agree with this; however, IF Bitcoin could show that it works even when literally everything else doesn't, would be a quite sick feat! However, I don't think it's a downside for Bitcoin that it's (right now) fully reliant on internet; since everything else is as well. So it's kind of a level playing field.

There are multiple research studies on this topic already; especially a few years back, when Android was more open :sigh: ::) and it was possible to do cool things with its 2.4GHz antenna easily. Not sure what's the state of the art right now, but I haven't heard much about it since. The biggest issue is that it requires even more participants than if using home routers and directional antennas (which is the best idea for me so far).

Unfortunately, satellite internet is a quite bad idea in general, for a variety of reasons. A few weeks ago, I stumbled upon this video, which points out lots of serious issues with Starlink and the whole idea in general. In short, it makes very little sense for the majority of people, in fact the technology can't even feasibly serve as many people as Musk claims and thus it never will. It is and will stay a niche market due to price and technological reasons (max. base stations per satellite, satellites need to be replaced from time to time => more cost etc).
DEBUNKING STARLINK (https://www.youtube.com/watch?v=2vuMzGhc1cg)

Why do you think people start to distrust the banking system after a natural disaster? I would argue that barter transactions increase because people have less use for anything that can be described as a store of value -- people need actual goods/services that allow them to survive.
If you are in a rural or even a suburban area, with a mesh network, it would take a very small number of dishonest nodes to pull off a Sybil attack that affects a large portion of the entire network, as all traffic is flowing through so few nodes. There is little reason to run a node in a sparsely populated area because it will connect to so few people.

My understanding is that the blockstream satellite will broadcast transactions to anyone whose "antenna" is pointed in the appropriate direction, and has no way of discriminating against a specific user.

Using a blockstream satellite alone will not allow someone to broadcast a transaction, which would be important in a power/internet outage. 

We should not be surprised to see some temporary and fabricated electricity shutdown like some insane individuals announced it months ago, with purpose of crashing global economy,
but I don't expect this to happen globally in all countries in the same time, so Bitcoin transactions should not stop, especially in US that have many states with separate independent systems.
There is one cool project from Brazil for creating bitcoin stations for listening and broadcasting of transactions, but you would still need electricity for operating radio, so maybe physical Bitcoins will be used much more than now, and not only as collectible items.
https://satoshi.radio.br/wp/

I find it interesting that they plan to make it available also as a DYI station. It would be nice for those having their own solar panels, for example.
Of course, the approval can be a problem...

Wow, there are so many more answers than I expected. Thanks to you all for the replies.

So I think the scenario in which a nation-wide blackout occurs is not that far from reality. It is already being speculated that a quite dark winter is on the way, and I agree with dbkit98: there are some sick individuals who are looking for a global reset, and it's not the reset itself that is to be worried about but the way it's going to happen. "You will own nothing, and you will be happy". They said it themselves, and we're looking at a number of leaders who don't like Bitcoin even now, even today.

A massive blackout or a regulated Internet v2.0 might mean Bitcoin's disappearance or turning it into a massive surveillance tool. While thinking about this mesh as a "survival/emergency" tool (and I think there would actually in fact be a quite big market for it, considering how large the "preppers" community is), I also saw it as a way of ensuring that even if someone wants to take Bitcoin down indirectly through disastrous scenarios, it would still thrive at least partially within our community.

Of course, there will be some of you here who will contradict me or think that this is just being paranoid or so, but I personally think that the current financial system isn't the happiest with Bitcoin's presence and there is nothing that can make it stronger than proving it is not just something virtual/temporary but something that you can rely on even in such events.

So far, I think the Satoshi Radio from Brazil is the closest I can get to what I was thinking about. Will definitely research more about how it works, although I think that unless this capability of receiving/broadcasting txs is embedded into a device specifically created for offline over-the-air Bitcoin transactions, it would be useless in the events previously mentioned (how many people would use the OTA Bitcoin receiver/transmitter .. let alone the Satoshi Radio?).

But besides radio waves, is there no other technology using waves or something that could be used over very long distances without a necessary license/SIM card? ???

As mentioned a number of times already, if you need cash to survive, and there's no electricity, well, you can't convert your BTC to cash.

If there's not enough electricity to run anything normal, you wont have enough to run a world wide bitcoin network for everyone to use.
You will not find the world suddenly turn to BTC, they'll turn to something physical, like cash or gold (yep good idea to keep some gold ...)

Nationwide blackout, well, maybe in the 3rd world, or under authoritarian rule, but in the west, and a lot of europe, not gonna happen.
It's like worry about the world being destroyed by an asteroid ... why didn't you mentioned that as well :)

Fun to be one of those people with 24/7 solar power/battery at the end of the world :)

I wouldn't be so sure about that. Things in 1st world countries are hunky dory as long as everything is going according to expectations, otherwise one thing goes wrong and everything starts crumbling. The 2008 economy crisis, 2019 COVID, etc are examples. Just recently UK is experiencing a fuel crisis and is starting to look more like Lebanon minus the power outage (so far)!

As I pointed out ... you've left out including asteroids destroying the earth ...

The end of the world is coming, run for your lives ... hmm I think too many people have been bored and watching too many movies ...

The UK is experiencing fuel delivery issues mostly because brexit forced 1/3 of their truck drivers out of the country.
You don't get to pass a law that makes it difficult / impossible to have enough workers to do something and then complain when it does not get done.

It's not like stacking boxes, there are strict requirements and training to haul fuel. It's not like you can just grab anyone off the street to do it.

-Dave

Well I used real events that have happened and are happening in real world while you used an example of an event that has a minuscule chance of happening and only happens in movies...

True, but the reason for it is not important, last month 3 million homes in same UK were dark. Even the example being UK wasn't important. For weeks parts of Louisiana (1.2 million homes) remained dark in the past month. Currently China seems to be experiencing a lot of power cuts. And a lot more...
The point was that power outages (and other disasters) happens everywhere, whether due to natural disasters or incompetency. Imagining some countries are safe just because some ranking calls them first world is naive.

Which is why the topic started here is a very important one that needs to be discussed and we should find a good solution before the disaster happens on a bigger scale.

In any of these situations with homes losing power, did they also lose internet connectivity via cellular or satellite signals?

If people still have cellular or satellite connectivity, then bitcoin can still function just fine. They can use their phones to make transactions and run light wallets, and charge up from power banks, solar panels, batteries, whatever. If people lose all internet connectivity, then bitcoin is probably far very down on their list of things they are concerned about. Almost everyone will be more interested in re-establishing wider internet connectivity (which will obviously include bitcoin) for things like communication and managing supply chains than they would about setting up some mesh network which is only designed to allow bitcoin to work.

Why not making it analogical? Give it a phisical, material form.
Some ways of doing that are opendimes and paper-wallets. It's not like the internet is gonna offline forever, so would just need to use the latter as a kind of 'paper currency' with the coverage in bitcoin while things dont go back to normal.

There are two main problems with that; verification and trust.

If I hand you a paper wallet, you have no way of verifying the balance of the paper wallet is what I say it is without access to a synced node. Without internet access, you are going to need a mesh network or blockstream satellite as discussed previously in this thread. You also have absolutely no way of knowing if I have retained a copy of the paper wallet or the private key for myself, or maybe I've handed out the same paper wallet to five other people, none of whom have been able to spend it in the meantime since the internet is inaccessible.

Opendimes would solve the problem of trust (provided you trust that Opendimes are reliable and the company which produces them are honest), but they don't solve the verification problem. Without access to a node or a block explorer, then you cannot verify the balance of the Opendime. Perhaps if I had loaded up my Opendimes well before we collectively lost internet access you could check the balances on your node which is no longer synced with the network but was synced beyond the block which contained my deposit transactions. Either way, its an imperfect solution.

I can say that they are honest and legit.
I had one die with funds on it. Sent it to them, they did their magic and sent the funds to an address I provided.
No issues at all. They could have said sorry can't help you.
They could have said it's not repairable and sat on the private key for years and then swept it.
But, they just got it working and sent me the BTC

A bit OT, but still a thought. The OpenDime that is on my keyring getting knocked around for YEARS still reads with no issue.
The one that died was in an anti-static bag and 100% untouched except for the one time it was put into a PC to get the address to load it.
All electronics can die, don't store anything in a product like an OpenDime you cannot afford to loose, since if they had not been able to repair it I would have lost the funds.
Had they gone out of business I may have had to pay an independent circuit board repair place more then the value of the BTC that was on it.

-Dave

This is definitely an invalid argument.

Their reputation is worth (probably) a lot more than your lost money. Same applies to every other custodial services. Just because they didn't cheat you on many of your deposits doesn't mean they won't. If they're anonymous, the more the money you've deposited, the more they're incentivized to steal them.

Another point that I would make would be:
If Alice and Bob both have internet access, if Alice wants to buy fuel from Bob, it is very easy for Bob to know with certainty if a transaction has been confirmed, and if the transaction has been well propagated throughout the network. Bob can do this in two ways, he can connect to a diverse set of nodes that is random, and is unknown to Alice, and he can also use one or more offsite nodes he controls in a geographically diverse area that is also well connected.

If Bob only has access to a mesh network, Alice will know exactly which nodes he is connected to based on their location. He does not really have any way of knowing if the transaction made its way through the mesh network to the on-ramp to the internet and into the bitcoin network and if the transaction has been well propagated (if the transaction is valid, but contains low fees based on the current fee market, it may not propagate). This makes it very difficult for Bob to assess if he can safely accept Alice's transaction. If Bob discloses to Alice prior to the transaction that he will need to wait for 10 or 20 confirmations before accepting the transaction, Alice will probably not be interested in paying with bitcoin.

It's not a custodial service. It's a hardware usb dongle with an address on it. They didn't have to fix it. They didn't have to fix it for free.
It's just good customer service. It's open source. https://github.com/opendime/opendime

Anyway, back OT.
It still falls back to the fact that no matter what Alice and Bob are doing in a trade, or how they have access it comes back to the world is more then Alice and Bob.
If Alice and the rest of the world has good internet and Bob does not, Bob can come up with all these convoluted ways to get his BTC to Alice. And Alice can spend it how she likes with the rest of the connected world. If we are in the situation of where nobody has good internet since the world has collapsed. Then BTC / crypto / your credit cards / your cash / your gold and silver is all going to we worthless anyway. There is so much tech everywhere, that if it stops working for and extended period of time most of the world is going to collapse.

Having a good plan to make sure you can spend you BTC if you happen to be living in a cabin in the woods with no internet is good, if that happens to be your choice.
But if we are in a tech collapse situation, then it's not going to matter.

-Dave

I agree on the verification problem.

I don't agree on the trust problem. I don't think that we should trust that much any entity. After all, this is what Bitcoin is about: trust only math and yourself.
Maybe I'm a bit paranoid/over-zealous, but I think that you can understand my point. If they have the private keys "somewhere safe" and nobody can really complain about them (no internet for most), what stops them "claim" some of that money (if they do have access to internet)?

The final conclusion is the same:

I agree with everything else you have written, but can you elaborate on why Bob would require 10 or 20 confirmations in such a case? I'm not sure I follow your reasoning.

Lets say Bob does not know if Alice's transaction has propagated through the mesh network, does not know if it pays a reasonable fee, does not know if it has been propagated through the main bitcoin network, and does not know its position in the mempool (and also does not know if there are any competing double spends or other issues). All that becomes significantly less important after 1 confirmation, since that is proof of good propagation and reasonable fees. After a handful of confirmations, it would be just as secure as any other transaction, no? Why is Bob waiting for 20 confirmations in this case? What additional security is he gaining from waiting for 20 rather than for 6? Are you assuming that Alice knows all the nodes Bob is connected to, all those nodes are malicious, and all those nodes are colluding with Alice?

They don't, or at least, they claim that they don't. Opendimes are provided to the customer without any private key, and you must feed it 256 bits of data before you can first use it. The Opendime concatenates those 256 bits with a 128 bit serial number and a 32 bit nonce, before double hashing with SHA256 to generate a private key. Once you have unsealed an Opendime, you can verify that the private key was indeed produced this way (provided you know the 256 bits of data it was fed by the initial owner). You can read this here: https://opendime.com/faq#provability

Having said all that, I'm sure that it is a very small number of Opendimes which are checked by the end user, so they could still be releasing some with a private key known to them and just hoping that those ones aren't checked. It's unlikely, but possible.

The security of n confirmations relies on the assumption that you are not being subjected to a sybil attack. If your transaction has 6 confirmations on the actual chain with the most total work, unless the attacker controls 51% of the network hashrate, the transaction for all intents and purposes is not going to get double-spent. However, if you are connected to a single malicious node that sends you 6 blocks that in sub reflect a transaction having 6 confirmations if these 6 blocks do not reflect the sum total of the most total work, your transaction could already be double-spent.

If your node was previously connected to the internet, and you know you were not being subjected to a sybil attack because your node was connected to a diverse set of nodes, you can be reasonably certain you are not being subjected to a sybil if you immediately start receiving blocks every approximately 10 minutes. However, in an event as described in the OP, it is likely that many of the miners are likely to be without power, so it is likely the average block time will be well above one every 10 minutes, and therefore it will be difficult to know if you are being subjected to a sybil. The cost of serving n blocks to someone while executing a sybil is approximately the expected mining revenue of generating n blocks, so the additional security from asking for 10 confirmations over 6 confirmations is that any sybil attack would be more expensive to execute.

You might argue that Alice is only one person buying a few hundred dollars worth of supplies. However, it would be possible to execute a sybil attack on a large geographic area, and it would be possible that Alice was part of a larger syndicate of organized crime that is taking advantage of the current situation.

Aren't you find your sayings a little paranoid? Alright, let's assume that Alice has been encircled from dishonest nodes who're ran by the attacker. In order for the attacker to make those 6 confirmations look like they've come from the honest miners, they have to gain access to a LOT of computational power.

For instance, if each block is generated every 10 minutes averagely and they want to make it look like a bad-luck scenario (100 minutes each block) instead of an obvious sybil attack, they still need to accumulate a 10% of the total hash rate.

And note that there's no way for the attacker to be 100% sure that Alice is connected only to dishonest nodes, other than compromising her computer. And I'm asking myself, who and why would wanna do this?

The scenario in the OP is that electricity and internet connectivity are both down, and that some kind of mesh network is being used to communicate transactions and found blocks. With a mesh network, an attacker physically present in the same location as his victim will know which nodes his victim is connected to because he will be connected to the same nodes.

The point I was trying to make in my last post is that in the scenario described in the OP, as someone receiving bitcoin, you really don't have a good handle on the risks involved in accepting transactions. It might not be unreasonable for 90% of miners to be offline for some time in this type of scenario, so maybe there are several days with 100-minute blocks.

IMO the more likely risk is that accepting a transaction via a mesh network with no internet connectivity is that, due to network failures, the transaction never makes it out of the mesh network, or if it does, that the transaction is not paying a sufficiently high fee that internet-connected nodes relay it. There wouldn't necessarily be malicious intent on the part of the person sending you bitcoin, but the end result would be that you would still be unable to spend your coin.

Actually, a good step that anyone could do really, would be to write a piece of software for Raspberry Pi which uses a GPIO pin with an antenna to transmit in HF range and replicate what 'Satoshi Radio' built. It is illegal in most countries, but if you're allowed to, you don't care, or you just want to 'prep' and only use it with a proper antenna once there's actually a catastrophe event, you could build one and have it on the ready in case you'll need it.
If your idea gains traction and people want this as a pre-built, you will surely be able to buy some Pis, have some enclosures made and make it all look neat & sell it as a finished product. That's how many companies get a product out which is not too hard to make since they use off-the-shelf parts - like the MyNode for example.

A good place to start would be the following article.

Well on a physical level, there are 3 main means of communication: wires, radio waves (electromagnetic waves of any kind) and light (morse alphabet for example); though light is actually an electromagnetic wave as well, just different wavelength really.

Was looking for something else and found this:
https://lora-alliance.org/
Not the fastest thing out there but it will move limited amounts of data. So you could probably hack something together after the end of the world.
But, I still think it's a thought experiment only, never going to be put into live use.

-Dave