This application note describes the design of a
microcontroller-based KEELOQ™ Hopping Encoder
using the XTEA encryption algorithm. This encoder is
implemented on the Microchip PIC16F636
microcontroller. A description of the encoding process,
the encoding hardware and description of the software
modules are included within this application note. The
software was designed to emulate an HCS365 dual
encoder. As it is, this design can be used to implement
a secure system transmitter that will have the flexibility
to be designed into various types of KEELOQ receiver/
decoders.
BACKGROUND
XTEA stands for Tiny Encryption Algorithm Version 2.
This encryption algorithm is an improvement over the
original TEA algorithm. It was developed by David
Wheeler and Roger Needham of the Cambridge
Computer Laboratory. XTEA is practical both for its
security and the small size of its algorithm.
XTEA security is achieved by the number of iterations
it goes through. The implementation in this KEELOQ
Hoppping Decoder uses 32 iterations. If a higher level
of security is needed, 64 iterations can be used.
For a more detailed description of the XTEA encryption
algorithm please refer to AN953, “Data Encryption
Routines for the PIC18”.