Method and apparatus for a provider to verify a client's secret identifier includes structure and steps for the client to scramble his/her predetermined secret identifier in a random way with random data. The scrambled data is transmitted to the provider, and the provider determines whether the client's secret identifier is present in the received scrambled data. Preferably, the provider rejects a transaction if the random data in the received scrambled data is substantially the same as random data received in a previous transaction corresponding to said client.