A method is provided for constructing a secret code in a processing unit when in communication with a portable security unit. Mutual authentication messages are exchanged between a linked portable security unit and processing unit. A first portion of the secret code is communicated to the processing unit. The processing unit combines the first portion and a second portion of the secret code stored in the non-volatile memory of the processing unit. The secret code is stored in a volatile memory of the processing unit. A secure operation is performed using the secret code. The portable security unit is de-linked from the processing unit. At least a portion of the secret code is deleted from the volatile memory of the processing unit.