Here’s an app note from Microchip describing how to implement data encryption routines on PIC24 and dsPIC microcontrollers and digital signal processors.
This application note discusses the implementation of two of these for PIC24 and dsPIC devices: Triple Data Encryption Standard (TDES) and Advanced Encryption Standard (AES). Source code for this application note is available through the “Data Encryption Libraries” in the download section below.