HCS512

© 1997 Microchip Technology Inc.

DS40151C-page 11

7.0

KEY GENERATION SCHEMES

The HCS512 decoder has two key generation schemes. Normal learning uses the transmitter’s serial number to derive

two input seeds which a

µre used as inputs to the key generation algorithm. Secure learning uses the seed transmission

to derive the two input seeds. Two key generation algorithms are available to convert the inputs seeds to secret keys.

The appropriate scheme is selected in the configuration word.

FIGURE 7-1:

7.1

Normal Learning (Serial Number Derived)

The two input seeds are composed from the serial number in two ways, depending on the encoder type. The encoder

type is determined from the number of bits in the incoming transmission. SourceH is used to calculate the upper 32 bits

of the encoder key, and SourceL, for the lower 32 bits.

For 24-bit serial number encoders (56-bit transmissions):

SourceH = 65H + 24 bit Serial Number

SourceL = 2BH + 24 bit Serial Number

For 28-bit serial number encoders (66 / 67-bit transmissions):

SourceH = 6H + 28 bit Serial Number

SourceL = 2H + 28 bit Serial Number

7.2

Secure Learning (Seed Derived)

The two input seeds are composed from the seed value that is transmitted during secure learning. The lower 32 bits of

the seed transmission is used to compose the lower seed, and the upper 32 bits, for the upper seed. The upper 4 bits

(function code) are set to zero.

For 32-bit seed encoders:

For 48-bit seed encoders:

For 64-bit seed encoders:

Note:

64-bit seeds are handled as 48-bit seeds

Seed

Patched

Serial

Number

Key Generation

Algorithms

-------------------

Decrypt

XOR

Encoder

Key

Manufacturer’s

Key