A portable device enables access to a host computer via a guest computer. The portable device is connected to the guest computer, and a program stored in a memory on the portable device is activated, the program including instructions for establishing communication with the host computer such that input to the guest computer serves as input to the host computer, and output displays from the host computer are displayed on the guest computer. The portable device includes a cryptographic processor for performing cryptographic processing for communicating with the host computer. The portable device also includes a protected memory for storing a private key accessible to the cryptographic processor, the private key being used during cryptographic processing. The protected memory can be internal or external to the cryptographic processor.