Challenge-response method and system

Method for performing a challenge-response session between a system (5) and a user. the system outputs challenge information, which is displayed (3) in bitmap, graphic or any other non-computer-readable format. The user sends response information (6) which is related to the content of the challenge. The system checks the response with the challenge. The challenge comprises a response instruction w.r.t. the response to be sent by the user to the system. The system checks the response with the challenge, taking into account the semantic content of the relevant instruction. Preferably, the content of the instruction is variable per session. The content of the instruction may be related to a user identifier, e.g. a user password.