A method and apparatus are provided for verifying the identity of a system user. Each user has a token which processes an inputted seed to generate a stored value which is either a current seed value or a function thereof. This value is then changed to generate a new current seed in response to each of a plurality of selectively generated trigger signals. The stored current seed or a selected function thereof is outputted, for example by being displayed on the token, and is received and inputted into a host verification unit. This unit either has or generates the current seed value (and the outputted function thereof where required) for the user, the user being indicated by an input also provided to the host with the token output, and compares the received and generated seed function values to verify the user.