EECS 373 Final Project

Fall 2004

Audio Encryption Unit

Victor Chan - Jonathan Dobbie

Project Overview:

Our project is an audio encryption and decryption device. A signal is inputted into the board from the analog to digital decoder. The data is then encrypted with TEA (Tiny Encryption Algorithm), then decrypted and sent out through the digital to analog converter.

Algorithm Choices and Details

The TEA was selected for three main reasons:
1.   Relative Strength
2.   Fast execution time
3.   No need for floating point hardware.
“TEA has a few weaknesses. Most notably, it suffers from equivalent keys — each key is equivalent to three others, and this means that the effective key size is only 126 bits (Kelsey et. al., 1996). This weakness led to a method for hacking Microsoft's Xbox game console, where the cipher was used as a hash function. TEA is also susceptible to a related-key attack which requires 223 chosen plaintexts under a related-key pair, with 232 time complexity” (Kelsey et. al., 1997). [Lifted from the wikipedia entry on TEA] And while this may be so, and we could have used the more secure XTEA, it was decided that our project did not warrant this amount of concern.
The source code for the algorithm is available on http://www.simonshepherd.supanet.com/tea.htm

Choices and Pitfalls:

We initially intended on integrating a 16-character keypad and a 16x4 character LCD into our design. We were never able to clean the keypad’s input enough to use reliably and we reverse biased our LCD screen in final testing. We also intended on using the serial connection to transmit the data, but we were never able to work out the timing issues involved. The original design also called for a public key exchange, but doing RSA without floating point hardware along with the issues with the serial connection made this unfeasible.

Hardware Limitations:

The DAC0830 as used on the lab board is not correctly biased, as so, the conversion is very inaccurate. As seen below, this would not be a suitable set up for real-world use.

The Triangle wave as coming out of the function generator	The square wave as coming out of the function generator
The Triangle wave as coming out of the DAC	The Square Wave as coming out of the DAC

Project Duty Breakdown

Jonathan
·   Xilinx schematic
·   Failed hardware construction
·   Interrupt service routines
·   General ASM and C debugging and reworking
·   Algorithmic research

Victor
·   RSA research
·   C coding
·   ASM timer set up

Practical Outcomes

This project did prove the ability for a system of very limited processing power to use a fairly secure modern encryption algorithm. The knowledge and research from this project will hopefully be used to build an underground encryption system to allow persons with a spare external Hayes compatible modem and a computer to establish a secure telephone line with another person who has these same pieces of equipment. The extra processing power and inclusion of a floating point unit in almost any machine that one would see in 2004 would also allow for RSA key exchanges and other features that would increase the usability and security of the system. It is hoped that if this system is developed, it would provide a low cost way for persons to securely communicate when they feel that a third party may be trying to listen in.

Web Sites used

http://www.simonshepherd.supanet.com/tea.htm
This is a site linked form the wikipedia entry that has code for the TEA both in C and in assembly for various assembly languages.

http://en.wikipedia.org/wiki/TEA

Works Cited (via wikipedia)

John Kelsey, Bruce Schneier, and David Wagner. Related-key cryptanalysis of 3-WAY, Biham-DES, CAST, DES-X NewDES, RC2, and TEA. Lecture Notes in Computer Science, 1334: pp233–246, 1997.