AN665 Datasheet, PDF (1/17 Pages) Microchip Technology – USING KEELOQ® TO GENERATE HOPPING PASSWORDS

M
AN665
Using KEELOQ¨ to Generate Hopping Passwords
Author: Lucio Di Jasio
Arizona Microchip Technology, Italy
INTRODUCTION
The purpose of this application note is to demonstrate
how KEELOQ^Ò code hopping technology can be conve-
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
standard PC/AT ^® keyboard. Since this conversion pro-
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
range of encoders based on the proprietary KEELOQ
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
HCS300
S0 1
S1 2
S2 3
S3 4
8 VDD
7 LED
6 PWM
5 ^VSS
PIC12C508
VDD
1
GP5/OSC1/CLKIN
2
GP4/OSC2
3
GP3/MCLR/VPP
4
8
VSS
7
GP0
6
GP1
5
GP2/T0CKI
KEELOQ is a registered trademark of Microchip Technology, Inc.
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
ã 1997 Microchip Technology Inc.
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