Methods for（ ）people differ significantly from those for authenticating machinesand programs,and this is because of the major differences in the capabilitiesof people versuscomputers.Computers are great at doing（ ）calculations quickly and correctly,and theyhave largememories into which they can store and later retrieve Gigabytes of informationHumans don't.So we need to use different methods to authenticate people.In particular,

the(作答此空)protocols we've already discussed are not well suited if the principal beingauthenticated is a person(with all the associated limitations)

All approaches for human authentication rely on at least one ofthe following:

·Something you know (eg.a password).This is the most common kind of authentication usedfor humans.We use passwords every day to access our systems.Unfortunately,somethingthat you know can become something you just forgot.And if you write it down,then otherpeople might find it.

Something you（ ）(eg.a smart card).This form of human authentication removes theproblem of forgetting something you know,but some object now must be with you any timeyou want to be authenticated.And such an object might be stolenand thenbecomes somethingthe attacker has

Something you are (eg.a fingerprint).Base authentication on something （ ）to theprincipal being authenticated.It's much harder to lose a fingerprint than a wallet.Unfortunately,biometric sensors are fairly expensive and(at present)not very accurate.