PS810 datasheet(23/46 Pages) MICROCHIP

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DS21904C-page 23

PS810

7.0

KEELOQ® SECURITY

The PS810 will employ KEELOQ security coding which

can ensure only proprietary batteries can be used with

the host device. Since the batteries are tailored to the

system requirements, an incompatible battery may

cause poor performance or even non-performance. A

verification challenge will be issued to the battery from

the host. The battery will encrypt the challenge data and

return it to the host. The host will encrypt the challenge

data also and verify that the battery’s encrypted

response matches. For technical KEELOQ specifications,

see Microchip application note

AN827, “Using KEELOQ®

to Validate Subsystem Compatibility” (DS00827).

The host issues a “challenge” (i.e., a 32-bit random

number) by writing it to the KEELOQ bus function on the

PS810. The PS810 firmware contains the KEELOQ

encryption algorithm and a 64-bit key. The PS810 runs

the 32-bit challenge data along with its 64-bit key

through the KEELOQ decrypt algorithm to create a 32-bit

response. The host then reads the response from the

PS810 using the KEELOQ bus function. The host verifies

the response by also running the challenge through the

decrypt algorithm using the same key. If the results are

the same, authentication is successful.

When a challenge is written to the battery, it takes time

to generate the encrypted response. Battery status

bit 1 is set to one (‘1’) when a challenge is written and

reset to zero (‘0’) when the response is ready and may

be read from the PS810.

The security of authentication relies not on the secrecy

of the algorithm, but on secrecy and management of

the key. The key, the function that stores the key and

the KEELOQ decrypt algorithm that uses it are located in

memory that is protected from reads and writes. This

prevents the key from being read except by code in the

boot block and prevents the boot block from being

altered without being first erased. The key is stored in

an encoded form and during normal execution, never

appears in its entirety in RAM.

A battery pack’s key is stored at time of manufacture.

Each pack can have a unique key, further improving

security, which is a function of a master key and the

pack’s serial number. A host would use the master key,

the pack serial number and a key generation algorithm

to determine an individual pack’s key.

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