Unlike asynchronous, in synchronous the authenticator produces a password as a function of its internal clock at the time the user demands it. This authenticator does not involve the host challenges. All the schemes mentioned above require that both the authenticator and the host know a common algorithm for computing the one-time passwords. The algorithm is either industry standard or proprietary, depending on the vendor's implementation. One aspect of their operation is analogous to handheld authenticators in that they calculate one-time passwords in response to a host challenge.
Smart cards communicate directly with the challenging entity through a smart card reader.
After the PIN is entered, the reader processes the challenge, enabling authentication to occur without further human involvement. This automatic function coupled with onboard key storage makes the use of lengthy keys possible with no added inconvenience to the user. In trusted third party technique, the host does not have to rely solely on credentials supplied by the user or a device in his possession. Nor does the user have to entrust the host with secret information such as a password. A KDC does not distinguish between users and hosts, or more correctly, server programs on hosts.