ATECC508A datasheet(76/109 Pages) MICROCHIP

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9.11

MAC Command

The MAC command computes a 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 digest of this 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 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 GenDig command to combine one or more stored EEPROM locations in the device

with the nonce. The result is stored internally in the device. This capability permits two or more keys

to be used as part of the response generation.

Run this MAC command to combine the output of step one (and step two if desired) with an

EEPROM key to generate an output response (i.e. digest).

Alternatively, data in any slot (which does not have to necessarily even be secret) can be accumulated

into the response through the same GenDig mechanism. This has the effect of authenticating the value

stored in that location.

Table 9-32. Input Parameters

Name

Size

Notes

Opcode

MAC

0x08

Param1

Mode

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

Param2

KeyID

The internal key is to be used to generate the response. Bits 3:0 only are used to

select a slot; however, all 16 bits are used in the SHA-256 message.

Data

Challenge 0 or 32 Input portion of message to be digested, ignored if Mode<0> is one.

Table 9-33. Output Parameter

Name

Size

Notes

Response

SHA-256 digest

The message that will be hashed with the SHA-256 algorithm consists of the following information:

32 bytes

Slot<KeyID> or TempKey (see Mode Encoding )

32 bytes

Challenge or TempKey (see Mode Encoding)

1 byte

Opcode (always 0x08)

1 byte

Mode

2 bytes

KeyID

8 bytes

Zeros

3 bytes

Zeros

1 byte

SN<8> (never zeroed out)

4 bytes

SN<4:7> (or zeros, see Mode Encoding)

ATECC508A

Security Commands

Datasheet Complete

DS20005927A-page 76

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