A challenge/response authentication procedure determines whether a response is a correct response, a unique incorrect response, or a non-unique incorrect response, the unique incorrect response and non-unique incorrect response being differentiated by comparing the response value with a store of unique incorrect response values. For the correct response, client access to protected computer system resources is allowed, and the challenge value is discarded so as not to be used again. For the unique incorrect response, (1) when a predetermined limit of unique incorrect responses has not been reached, then the response value is added to the store of unique incorrect response values and the process is repeated with reuse of the challenge value, and (2) when the predetermined limit has been reached, then the client is locked out. For the non-unique incorrect response, the process is repeated with reuse of the challenge value.