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AN663

Using KEELOQ® to Generate Hopping Passwords

Microchip Technology 

AN665

INTRODUCTION TO KEELOQ

ENCODERS

All KEELOQ encoders use the KEELOQ code hopping

technology to make each transmission by an encoder

unique. The encoder transmissions have two parts. The

first part changes each time the encoder is activated

and is called the code hopping part and is encrypted.

The second part is the unencrypted part of the trans-

mission, principally containing the encoder serial num-

ber identifying it to a decoder.

The code hopping contains function information, a dis-

crimination value, and a synchronization counter. This

information is encrypted by an encryption algorithm

before being transmitted. A 64-bit encryption key is

used by the encryption algorithm. If one bit in the data

that is encrypted changes, the result is that an average

of half the bits in the output will change. As a result, the

code hopping changes dramatically for each transmis-

sion and can not be predicted.

The synchronization information is used at the decoder

to determine whether a transmission is valid or is a rep-

etition of a previous transmission. Previous codes are

rejected to safeguard against code grabbers.

The HSC300 and HCS301 encoders transmit two over-

flow bits which may be used to extend the range of the

synchronization counter from 65,536 to 196,608 button

operations. The HCS300 and HCS301 encoders

include provision for four bits of function information

and two status bits in the fixed code portion of its trans-

mission. The two status bits indicate whether a

repeated transmission is being sent, and whether the

battery voltage is low.

The Microchip HCSXXX encoders all have the ability to

transmit a fixed seed. The seed value is programmed

into the encoder when the encoder is first initialized

along with the counters, key, serial number, and other

information. The seed length differs from encoder to

encoder, with the HCS300 and HCS301 having a 32-bit

seed.

FIGURE 2: KEELOQ ENCODER CODE WORD TRANSMISSION FORMAT

LOGIC ‘0’

Bit

Period

LOGIC ‘1’

Preamble

TP

Header

TH

Encrypted Portion

of Transmission

THOP

Fixed Portion of

Transmission

TFIX

Guard

Time

TG

FIGURE 3: KEELOQ ENCODER CODE WORD ORGANIZATION

Fixed Code Data

Encrypted Code Data

VLOW and Button

Button Overflow Discrimination

Repeat Status Status 28-bit Serial Number Status bits

bits

(2 bits)

(4 bits)

(4 bits) (2 bits)

(10 bits)

16-bit

Sync Value

+ + 2 bits

of Status

Serial Number and Button

Status (32 bits)

Encrypted using

BLOCK CIPHER Algorithm

32 bits of Encrypted Data

Transmission Direction

DS00665A-page 2

ã 1997 Microchip Technology Inc.

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