MCS3122-ISLASHST datasheet(10/26 Pages) MICROCHIP

10 / 26 page

MCS3122

DS40001762A-page 10

 2014 Microchip Technology Inc.

4.5

Code Word Format

The Advanced KEELOQ code word is either 160 or

192-bit long. It comprises three sections (see Figure 4-1):

• 32 Bits of the Encoder’s Serial Number

• 128 Bits of the Encrypted Hopping Code

• 32 Bits of the Authorization Code (optional)

These segments are described in detail in the following

sections.

FIGURE 4-1:

ADVANCED KEELOQ® CODE WORD FORMAT

4.5.1

FIXED CODE PORTION

The fixed code portion consists of 32 bits of the serial

number.

4.5.2

HOPPING CODE PORTION

The hopping code portion is calculated by encrypting

the synchronization counter and function code with the

encoder key. The hopping code is calculated when a

button press is registered.

4.5.3

AUTHORIZATION CODE PORTION

The authorization code is a cryptographically-strong

industry standard representation of the code word

suitable for authentication and integrity verification. It is

generated by using the on-board AES encryption

algorithm in CBC-MAC mode. The calculation takes

place

over the entire code word, including the

encrypted and unencrypted portions, using the

authorization key as input. Figure 4-2 shows a

representation of how this calculation is performed.

This calculation is truncated to its Least Significant 32

bits for transmission.

The authorization code requires a shared secret called

the authorization key. This key is set in the

Authorization Key Configuration register, listed in

Table 3-6.

FIGURE 4-2:

AUTHORIZATION CODE

CALCULATION

The authorization code portion consists of the 32-bit Least

Significant bits of the authorization code.

Serial Number

0x55AA55

32 bits

Fixed Portion

128 bits Encrypted Hopping Code

Authorization

Code

32 bits

Auth Portion

Sync.

Counter

0x55

Function

Code

0xAA55AA55

0xAA55

32-bits

24-bits

8-bits

32-bits

16-bits

32-bits

Note:

The data is sent LSB first (in this figure from right to left).

Serial

Encrypted

Authorization

Key

Authorization

Code

Number

Code Word

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