ATECC508A datasheet(32/109 Pages) MICROCHIP

32 / 109 page

particular, do not assume that the other bits in the configuration word for a particular slot will override the

enablement of this capability specified by ReadKey = 0.

This capability is only enabled if the mode parameter to CheckMac has a value of 0x01, indicating the

following:

•

The first 32 bytes of the SHA-256 message are stored in a data slot in the EEPROM (i.e. the

password).

•

The second 32 bytes of the SHA-256 message must be a randomly generated Nonce in the

TempKey register.

If the above conditions are met and the input response matches the internally generated digest, then the

contents of the target key are copied to TempKey. The other TempKey register bits are set as follows:

•

SourceFlag is set to one (not Random).

•

GenDigData is set to zero (not generate by the GenDig(Data) command).

•

NoMacFlag is set to zero (TempKey is usable by MAC, HMAC, and Read commands).

•

Valid is set to one.

See the Microchip website for application notes with more detail on this capability.

3.2.8

Transport Keys

The ATECC508A device includes an internal hardware array of keys that are used for secure

personalization (i.e. transport keys). The values of the hardware keys are kept secret and are made

available only to qualified customers upon request to Microchip. These keys can be used with the

GenDig command only and are indicated by a KeyID value greater than or equal to 0x8000.

For GenDig and all other commands, KeyID values of less than 0x8000 always reference keys that are

stored in the Data zone of the EEPROM. In these cases, only the four least-significant bits of KeyID are

used to determine the slot number, while the entire 16-bit KeyID as input is used in any SHA-256

message calculation.

3.2.9

Authorized Keys

The ATECC508A device provides an optional mechanism for restricting the use of any key to those users

with knowledge of the appropriate authorization information.

Key authorization is a standard cryptographic requirement in many systems and can be used to prevent

fraudulent use of a key if the device containing the key is stolen or lost. For instance, if a key is used as

identification for a person, the authorizing value could be a password known only to that person. If the

device with the ID is stolen, then the thief cannot use the device to sign fraudulent messages since he or

she does not know the password.

The device can use either the CheckMac or Verify commands to implement this capability. If the

validation succeeds, then an internal AuthValid flag is set and the authorizing slot number is internally

retained in AuthKeyID. The AuthValid flag is cleared whenever the device wakes from sleep or is powered

on. It is also cleared when any operation is performed on a key which requires authorization. Prior to the

authorization check, the Nonce command must be run to load TempKey with a nonce.

•

CheckMac

The authorization value is stored in any slot configured to contain a secret, and it is validated with a

MAC calculated using that secret and the nonce stored in TempKey.

•

Verify

The authorizing slot must contain a valid ECC public key. The authorization value should be a

ATECC508A

Security Information

Datasheet Complete

DS20005927A-page 32

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