the bad coins proffered, good ones retained. Banks as financial intermediaries between ultimate savers and borrowers, and their ability to generate a medium of exchange marked higher than a fiat currency's store of value, is the basis of banking. Central banking is based on the principle that no medium requires more than the guarantee of the state that it can be redeemed for payment of debt as "legal tender" – so all money equally backed by the state is considered good money, within that state. So long as that state produces anything of value to others, the medium of exchange has some value, and the currency may also be useful as a standard of deferred payment among others. Of all functions of money, the medium of exchange function has historically been the most problematic due to counterfeiting, the systematic and deliberate creation of bad money with no authorization to do so, leading to the driving out of the good money entirely. Other functions rely not on recognition of some token or weight of metal in a marketplace, where time to detect any counterfeit is limited and benefits for successful passing-off are high, but on more stable long term social contracts: one cannot easily force a whole society to accept a different standard of deferred payment, require even small groups of people to uphold a floor price for a store of value, still less to re-price everything and rewrite all accounts to a unit of account (the most stable function). Thus it tends to be the medium of exchange function that constrains what can be used as a form of financial capital. It was once common in the United States to widely accept a check () as a medium of exchange, several parties endorsing it perhaps multiple times before it would eventually be deposited for its value in units of account, and thus redeemed. This practice became less common as it was exploited by forgers and led to a domino effect of bounced checks – a forerunner of the kind of fragility that electronic systems would eventually bring. In the age of electronic money it was, and remains, common to use very long strings of difficult-to-reproduce numbers, generated by encryption methods, to authenticate transactions and commitments as having come from trusted parties. Thus the medium of exchange function has become wholly a part of the marketplace and its signals, and is