In various embodiments, a server may be provided. The server may respond to a request for a service, from a processing device, with a challenge. The challenge may include a partial key for a memory-intensive operation, a number of iterations of the memory-intensive operation to perform, and a result of performing the number of iterations of the memory-intensive operation. Upon receiving the challenge, the processing device may choose a complete key consistent with the partial key and may produce a proposed result by performing the memory-intensive operation for the number of iterations. When the proposed result matches the result included in the challenge, the processing device may send a challenge answer, including the chosen complete key, to the server. Upon receiving a correct challenge answer from the processing device, the server may access the requested service and may return a result of the access to the processing device.