A method of obtaining access to an Internet service using a multi-step challenge response test is presented. The method calculates a threshold probability for access to the Internet service and generates a number of challenge-response tests. Each of the challenge-response tests is rendered on a display of a client device. An input corresponding to the response to one of the challenge-response tests is received and an authentication probability is calculated after each response. The authentication probability is evaluated after each response and if the authentication probability is higher than the threshold probability access to the Internet service is provided.