ATSHA204A datasheet(53/93 Pages) MICROCHIP

53 / 93 page

If zone is Data and SlotConfig<SlotID>.CheckOnly is one, the SHA-256 message body used to create the

resulting new TempKey consists of the following bytes:

32 bytes

Slot<SlotID>

4 bytes

OtherData

1 byte

SN<8>

2 bytes

SN<0:1>

25 bytes

Zeros

32 bytes

TempKey.value

In all other cases, the message use to create TempKey is as follows:

32 bytes

Config<SlotID> or OTP<SlotID> or Data.slot<SlotID> or TransportKey<SlotID>

1 byte

Opcode

1 byte

Param1

2 bytes

Param2

1 byte

SN<8>

2 bytes

SN<0:1>

25 bytes

Zeros

32 bytes

TempKey.value

8.5.9

HMAC Command

The HMAC command computes an HMAC/SHA-256 digest of a key stored in the device, a challenge and

other information on the device. The output of this command is the output of the HMAC algorithm

computed over this key and message. If the message includes the serial number of the device, the

response is said to be “diversified”.

The normal command flow to use this command is as follows:

Run the Nonce command to load input challenge and optionally combine it with a generated

random number. The result of this operation is a nonce stored internally on the device.

Optionally run the GenDig command to combine one or more stored EEPROM locations in the

device with the nonce. The result is stored internally in the device.

Run this HMAC command to combine the output of step one (and Step 2 if desired) with an

EEPROM key to generate an output response.

Step 2 addresses multiple use models. If the data in the EEPROM is a key, GenDig has the effect of

authenticating the challenge with multiple secret keys. Alternatively, if the contents of the slot are data

(which do not have to necessarily even be secret), GenDig has the effect of authenticating the value

stored in that location.

Table 8-17. Input Parameters

Name Size Notes

Opcode HMAC

0x11.

Param1 Mode

Controls which fields within the device are used in the message.

Param2 SlotID

Which key is to be used to generate the response.

ATSHA204A

Security Commands

DS40002025A-page 53

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