1. A mobile PDA computer system, comprising: a secure user processor, comprising: a trusted microprocessor; a trusted operating system executing on said trusted microprocessor; a trusted application software executing on said trusted microprocessor; a non-secure user processor, comprising: a non-trusted microprocessor; a non-trusted operating system executing on said non-trusted microprocessor; a non-trusted application software executing on said non-trusted microprocessor; a cryptographic engine configured for encrypting and decrypting data comprising: a trusted cryptographic processor; a trusted operating system executing on said trusted cryptographic processor; a first data communication link for communicating data between said secure user processor and said cryptographic engine; a second data communication link for communicating data between said non-secure user processor and said cryptographic engine; and a shared human/machine interface comprising a digital multiplexer that is exclusively responsive to said cryptographic engine for selectively enabling exclusive bi-directional communication of information between a user and one of said secure user processor, said non-secure user processor and said cryptographic engine; and a third data communication link for communicating control information between said cryptographic engine to said shared human/machine interface; wherein said non-secure user processor is operatively connected to a communications transceiver, said secure processor is configured to utilize said non-secure processor to exchange information with said communications transceiver, and said cryptographic engine is further configured to command said shared human/machine interface to delete sensitive information contained therein and to command said digital multiplexer to connect to said non-secure user processor only after said sensitive information has been deleted from said shared human/machine interface.