Ask HN: Could secure offline transactions be possible?
On the firsts case (machine distributor), the amount/credit is saved inside the card without a global synchronization mechanism; so an attacker could read/change the card, generate new money and put them in the network. (http://esec-pentest.sogeti.com/post/playing-with-nfc-for-fun-and-coffee)
On the second one (cardlocal transport card), Tthe balance isn't stored centrally on oyster cards - rather changes are uploaded to the central system IF you have registered it, at the end of the day. (https://www.reddit.com/r/london/comments/2dmasr/has_there_ever_been_a_case_of_someone_hacking/)
On the last case (Visa/Mastercard), the operation should be saved on the card and on the reader POS devices (POS). Most of the Credit Card Processors (like Square) accept transactions while in Offline Mode will be processed when you regain internet connectivity within 72 hours. ( https://squareup.com/help/us/en/article/5095-process-payments-with-offline-mode )
Is it possible build some extra features for offline system to increase the security? (ok.. cryptology is a suggestion but not a solution) PS: on the other hand, bitcoin network is completely distributed and totally synchronized by nature.