HCS500-I Datasheet, PDF (17/34 Pages) Microchip Technology – KEELOQ Code Hopping Decoder

HCS500
6.0 KEY GENERATION
The HCS500 supports three learning schemes which are selected during the initialization of the system EEPROM. The
learning schemes are:
• Normal learn using the KEELOQ decryption algorithm
• Secure learn using the KEELOQ decryption algorithm
• Secure learn using the XOR algorithm
6.1 Normal (Serial Number derived) Learn using the KEELOQ Decryption Algorithm
This learning scheme uses the KEELOQ decryption algorithm and the 28-bit serial number of the transmitter to derive
the crypt key. The 28-bit serial number is patched with predefined values as indicated below to form two 32-bit seeds.
SourceH = 60000000 00000000H + Serial Number | _{28 Bits}
SourceL = 20000000 00000000H + Serial Number | _{28 Bits}
Then, using the KEELOQ decryption algorithm and the manufacturer’s code the crypt key is derived as follows:
KeyH _{Upper 32 bits} = F _{KEELOQ Decryption} (SourceH) | _{64-Bit Manufacturer’s Code}
KeyL _{Lower 32 bits} = F _{KEELOQ Decryption} (SourceL) | _{64-Bit Manufacturer’s Code}
6.2 Secure (Seed Derived) Learn using the KEELOQ Decryption Algorithm
This scheme uses the secure seed transmitted by the encoder to derive the two input seeds. The decoder always uses
the lower 64 bits of the transmission to form a 60-bit seed. The upper 4 bits are always forced to zero.
For 32-bit seed encoders (HCS200, HCS201, HCS300, HCS301):
SourceH = Serial Number _{Lower 28 bits}
SourceL = Seed _{32 bits}
For 48-bit seed encoders (HCS360, HCS361):
SourceH = Serial Number (with upper 4 bits set to zero) _{Upper 16 bits} <<16 + Seed _{Upper 16 bits}
SourceL = Seed _{Lower 32 bits}
For 60-bit seed encoders (HCS362, HCS365, HCS370, HCS410, HCS412, HCS473):
SourceH = Seed _{Upper 32 bits} (with upper 4 bits set to zero)
SourceL = Seed _{Lower 32 bits}
The KEELOQ decryption algorithm and the manufacturer’s code is used to derive the crypt key as follows:
KeyH _{Upper 32 bits} = Decrypt (SourceH) _{64 Bit Manufacturer’s Code}
KeyL _{Lower 32 bits} = Decrypt (SourceL) _{64 Bit Manufacturer’s Code}
6.3 Secure (Seed Derived) Learn using the XOR Algorithm
This scheme uses the seed transmitted by the encoder to derive the two input seeds. The decoder always use the lower
64 bits of the transmission to form a 60-bit seed. The upper 4 bits are always forced to zero.
For 32-bit seed encoders (HCS200, HCS201, HCS300, HCS301):
SourceH = Serial Number _{Lower 28 bits}
SourceL = Seed _{32 bits}
For 48-bit seed encoders (HCS360/HCS361):
SourceH = Serial Number (with upper 4 bits set to zero) _{Upper 16 bits} <<16 + Seed _{Upper 16 bits}
SourceL = Seed _{Lower 32 bits}
For 60-bit seed encoders (HCS362, HCS365, HCS370, HCS410, HCS412, HCS473):
SourceH = Seed _{Upper 32 bits} with upper 4 bits set to zero
SourceL = Seed _{Lower 32 bits}
Then, using the manufacturer’s code the crypt key is derived as follows:
KeyH _{Upper 32 bits} = SourceH XOR 64-Bit Manufacturer’s Code _{Upper 32 bits}
KeyL _{Lower 32 bits} = SourceL XOR 64-Bit Manufacturer’s Code _{Lower 32 bits}
© 2002 Microchip Technology Inc.
DS40153C-page 17