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You store your crypto in a brainwallet. After the war internet will reboot ( yes there are bunkers fot servers) and all Blockchains will be online.
Children born after the war will not understand central banking and fiat curency.

But will bitcoin's still be considered worth something at that time? All currency is a general agreement that it is worth something. In the extreme, bartering of goods will be the first "currency", then precious metals, then whatever is agreed upon. That's my opinion anyway.