A currency can arise from any fungible, tradable, generally-durable commodity that the bulk of society decides to accept as being worthwhile. In prisons cigarettes often fill that role; in pre-agrarian hunter gatherer groups it's often little baubles like shells. Government isn't necessary for this, but social contract is. You need to have enough faith that your neighbor will accept the currency to accept it yourself.
Cryptocurrencies have the advantage that they can be spent worldwide, instantaneously, with little friction, while most commodities require that you be physically present to exchange them. They're also significantly easier to store and more resistant to garden-variety theft, as long as they're secured by passphrases. But they exist in the uncanny valley of requiring electricity and Internet. It is fairly unlikely that the government collapses and yet you still retain always-on electricity and Internet. What will you use to pay the electric company, and how do you enforce the contract that they keep providing you power?
There's perhaps an interesting role for a digital cryptocurrency that does not require always-on connectivity, eg. something that can be carried over SneakerNet for local transactions, and then automatically syncs up with the global system when the community regains Internet connectivity. In actual conflict zones today, people seem to get intermittent electrical & Internet connectivity, eg. the electricity will come on for an hour a day with rolling blackouts, and then you try to get everything charged in that one hour. And there are enough solar installations out there that several communities are likely to retain significant power even in the event of a societal collapse. But current crypto's consensus mechanisms aren't built for intermittent connectivity: you'd need to build and design something that is eventually consistent, where a weeks-long fork does not result in two competing cryptocurrencies.
Cryptocurrencies have the advantage that they can be spent worldwide, instantaneously, with little friction, while most commodities require that you be physically present to exchange them. They're also significantly easier to store and more resistant to garden-variety theft, as long as they're secured by passphrases. But they exist in the uncanny valley of requiring electricity and Internet. It is fairly unlikely that the government collapses and yet you still retain always-on electricity and Internet. What will you use to pay the electric company, and how do you enforce the contract that they keep providing you power?
There's perhaps an interesting role for a digital cryptocurrency that does not require always-on connectivity, eg. something that can be carried over SneakerNet for local transactions, and then automatically syncs up with the global system when the community regains Internet connectivity. In actual conflict zones today, people seem to get intermittent electrical & Internet connectivity, eg. the electricity will come on for an hour a day with rolling blackouts, and then you try to get everything charged in that one hour. And there are enough solar installations out there that several communities are likely to retain significant power even in the event of a societal collapse. But current crypto's consensus mechanisms aren't built for intermittent connectivity: you'd need to build and design something that is eventually consistent, where a weeks-long fork does not result in two competing cryptocurrencies.