ATSHA204A datasheet(56/93 Pages) MICROCHIP

56 / 93 page

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 within tempkey. 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 1 (and Step 2 if desired) with an EEPROM

key to generate an output response (or digest).

Table 8-22. Input Parameters

Name

Size

Notes

Opcode

MAC

0x08.

Param1

Mode

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

Param2

SlotID

Which internal key is to be used to generate the response.

Bits 3-0 only are used to select a slot but 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 8-23. Output Parameter

Name

Size

Notes

Response

SHA-256 digest.

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

32 bytes

key<SlotID> or TempKey (See table below)

32 bytes

Challenge or TempKey (See table below)

1 byte

Opcode (Always 0x08.)

1 byte

Mode

2 bytes

Param2

8 bytes

OTP<0:7> or zeros (See table below)

3 bytes

OTP<8:10> or zeros (See table below)

1 byte

SN<8>

4 bytes

SN<4:7> or zeros (See table below)

2 bytes

SN<0:1>

2 bytes

SN<2:3> or zeros (See table below)

Table 8-24. Mode Encoding

Bits Meaning

Must be zero.

0 =

Set the bits corresponding to SN<2:3> and SN<4:7> to 0.

Include the 48 bits SN<2:3> and SN<4:7> in the message.

If Mode<4> is set, the value of this mode bit is ignored.

0 =

Set the Corresponding 64 OTP bits to 0.

ATSHA204A

Security Commands

DS40002025A-page 56

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