MCS3142-ISLASHST datasheet(19/37 Pages) MICROCHIP

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 2014 Microchip Technology Inc.

DS40001747A-page 19

MCS3142

5.8

Authorization Code

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 5-1 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, described in

Table 3-13.

FIGURE 5-1:

AUTHORIZATION CODE

CALCULATION

5.9

Serial Number

Each Ultimate KEELOQ encoder transmits its 32-bit

serial number with each transmission. It is intended

that this serial number be unique to a system. It is set

in the Serial Number Configuration registers, described

in Table 3-12.

Serial

Encrypted

Authorization

Key

Authorization

Code

Number

Code Word

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