<P> To get the next password in the series from the previous passwords, one needs to find a way of calculating the inverse function f . Since f was chosen to be one - way, this is extremely difficult to do . If f is a cryptographic hash function, which is generally the case, it is (so far as is known) a computationally infeasible task . An intruder who happens to see a one - time password may have access for one time period or login, but it becomes useless once that period expires . The S / KEY one - time password system and its derivative OTP are based on Lamport's scheme . </P> <P> In some mathematical algorithm schemes, it is possible for the user to provide the server with a static key for use as an encryption key, by only sending a one - time password . </P> <P> The use of challenge - response one - time passwords requires a user to provide a response to a challenge . For example, this can be done by inputting the value that the token has generated into the token itself . To avoid duplicates, an additional counter is usually involved, so if one happens to get the same challenge twice, this still results in different one - time passwords . However, the computation does not usually involve the previous one - time password; that is, usually this or another algorithm is used, rather than using both algorithms . </P> <P> The methods of delivering the OTP which are token - based may use either of these types of algorithm instead of time - synchronization . </P>

One time password (otp) authentication failed