ATECC508A datasheet(71/109 Pages) MICROCHIP

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Table 9-22. Mode Encoding

Bits

Meaning

7–5 Must be zero.

0 = KeyID points to a private key, and Mode<2> and Mode<3> control device operation.

1 = KeyID must point to a public key, and GenKey only creates the digest in TempKey without any public

key generation operation. Bit 2 and bit 3 of the mode byte are ignored if this bit is set.

0 = No PubKey digest is created.

1 = The device creates a PubKey digest based on the private key in KeyID and places it in TempKey.

0 = A the private key currently stored in the slot is used to generate the public key.

1 = A random private key is generated and stored in the Slot specified by KeyID. KeyType must indicate an

ECC key in the KeyConfig area for this KeyID or an error will be returned.

1–0 Must be zero.

When a PubKey digest is to be calculated by the GenKey command, the following message is used as

the input to the SHA-256 algorithm:

32 bytes

TempKey

1 byte

Opcode

1 byte

Param1

2 bytes

Param2

1 byte

SN<8>

2 bytes

SN<0:1>

25 bytes

Zeros

64 bytes

X and Y coordinates of the public key

9.8

HMAC Command

The HMAC command computes an HMAC/SHA-256 digest using a key stored in the device over a

challenge, stored in the TempKey register and/or other information stored within the device. The output of

this command is the output of the HMAC algorithm computed over message using the specified key.

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.

Run this HMAC 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).

See the SHA command, which can generate an HMAC digest over an arbitrary length message without

any special formatting.

ATECC508A

Security Commands

Datasheet Complete

DS20005927A-page 71

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