HCS512 datasheet(12/20 Pages) MICROCHIP

12 / 20 page

HCS512

DS40151C-page 12

7.3

Key Generation Algorithms

There are two key generation algorithms implemented in the HCS512 decoder. The KEELOQ decryption algorithm pro-

vides a higher level of security than the XOR algorithm. Section 6.1 describes the selection of the algorithms in the con-

ﬁguration byte.

7.3.1

KEELOQ DECRYPT ALGORITHM

This algorithm uses the KEELOQ decryption algorithm and the manufacturer’s code to derive the encoder key as follows:

Key Upper 32 bits = F KEELOQ Decrypt (SourceH) | 64 Bit Manufacturers Code

Key Lower 32 bits = F KEELOQ Decrypt (SourceL) | 64 Bit Manufacturers Code

7.3.2

XOR WITH THE MANUFACTURER’S CODE

The two 32-bits seeds are XOR with the manufacturer’s code to form the 64 bit encoder key.

Key Upper 32 bits = SourceH ⊗ Manufacturers Code | Upper 32 bits

Key Lower 32 bits = SourceL ⊗ Manufacturers Code | Lower 32 bits

After programming the manufacturer’s code, the HCS512 decoder will automatically activate an Erase All function,

removing all transmitters from the system.

If LRNIN is taken low momentarily during the learn status indication, the indication will be terminated. Once a successful

learning sequence is detected, the indication can be terminated, allowing quick learning in a manufacturing set up.

FIGURE 7-2:

HCS512 KEY GENERATION

KEELOQ

Decryption

Algorithm

Padding

6/65

28/24-bit Serial Number

Padding

2/2B

28/24-bit Serial Number

MS 32 bits of Encoder Key

LS 32 bits of Encoder Key

SC_LRN = 0

LRN0 = 0

KEELOQ

Decryption

Algorithm

Padding

0000b

MS 28 bits of Seed Transmission

LS 32 bits of Seed Transmission

MS 32 bits of Encoder Key

LS 32 bits of Encoder Key

SC_LRN = 1

XOR

Padding

0000b

MS 28 bits of Seed Transmission

LS 32 bits of Seed Transmission

MS 32 bits of Encoder Key

LS 32 bits of Encoder Key

LRN0 = 1

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