The S/Key Protocol

The S/Key login protocol is summarized in the following picture:

The host being logged into stores the sequence number m, the seed k, and the previously used hash p_m-1 in the skeykeys file (which is world readable). In what follows, the remote host is the host being connected to and the local host is the host on which the user is working. The protocol proceeds as follows:

local host

Send login name to remote host

remote host

Send m and k to local host

local host

Hash the S/Key password and k N-m times, where N is the number of one-time passwords that have been generated. Send this value, called X, to the remote host.

remote host

Hash X once. If the result is the stored p_m-1, the user has authenticated himself or herself correctly. Store X in the skeykeys file. If the result is not the stored p_m-1, the user has not authenticated properly and access is denied.