ã 1997 Microchip Technology Inc.

DS00665A-page 1

M

INTRODUCTION

The purpose of this application note is to demonstrate

niently employed to implement an automatic code hop-

ping password generator/keypad. Using a PIC12C508,

the hopping code produced by an HCS300 is converted

to a string of 16 hex digits. This string is then trans-

ferred to the PC via the keyboard line, thereby emulat-

ing the actual pressure of a sequence of keys on a

cess is transparent to any application, it appears as if

the user is simply typing on a PC/AT-type keyboard.

An ideal situation for implementing this application

would be in creating a “super password” for general,

access-control secure logins when transmitting infor-

mation onto the internet (i.e., through a browser) or a

Java applet.

THE “HOPPING” ADVANTAGE

Password-based access control systems are very pop-

ular today, but the level of security they provide are

often overestimated. Being basically a unidirectional

transmission, a password-based system has two very

important shortcomings which can lead to unautho-

rized access: the code is fixed, and the number of pos-

sible combinations is relatively low.

The growing speed of communication lines and the

computing power of available systems increases the

chance of a brute force attack or “code scanning.” The

use of unsecure means of transmission, where code

“grabbing” is possible (i.e., a typical modem connection

over phone lines), can make the use of a fixed code

highly undesirable. Note that these are the same situa-

tion that led to the introduction of the “code hopping”

concept in the remote control market.

The basic idea is to have the access code change each

time it is used through a sequence where the new

codes cannot be predicted even knowing a very large

number of previously used ones. Producing such a

sequence requires the use of a solid encryption engine.

Microchip Technology is currently offering a broad

code hopping technology. These encoders make pro-

ducing a code hopping remote control easy, but as we

will see, can also be conveniently used to add the hop-

ping advantage to old password based access control

systems in a transparent way.

FIGURE 1: HCS300 AND PIC12C508 PINOUT DIAGRAMS

Author:

Lucio Di Jasio

Arizona Microchip Technology, Italy

PIC12C508

8

7

6

5

1

2

3

4

GP0

GP1

GP2/T0CKI

GP5/OSC1/CLKIN

GP4/OSC2

1

2

3

4

8

7

6

5

S0

S1

S2

S3

LED

PWM

HCS300

AN665

Microchip’s Secure Data Products are covered by some or all of the following patents:

Code hopping encoder patents issued in Europe, U.S.A., and R.S.A. — U.S.A.: 5,517,187; Europe: 0459781; R.S.A.: ZA93/4726

Secure learning patents issued in the U.S.A. and R.S.A. — U.S.A.: 5,686,904; R.S.A.: 95/5429

IBM PC-AT, IBM and AT are registered trademarks of International Business Machines Corporation