ATECC508A datasheet(66/109 Pages) MICROCHIP

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Else the success code of 0x00

If any error has occurred, this parameter will contain the error code.

9.6

GenDig Command

The GenDig command uses SHA-256 to combine a stored or input value with the contents of TempKey,

which must have been valid prior to the execution of this command. The stored value can come from one

of the data slots, the Configuration zone, either of the OTP pages, the monotonic counters, or be

retrieved from the hardware transport key array. The resulting digest is retained in TempKey, and can be

used in one of four ways:

It can be included as part of the message used by the MAC, Sign, CheckMac, or HMAC commands.

Because the MAC response output incorporates both the data used in the GenDig calculation and

the secret key from the MAC command, it serves to authenticate the data stored in the data and/or

OTP zones.

A subsequent Read or Write command can use the digest to provide authentication and/or

confidentiality for the data, in which case it is known as a data protection digest.

The command can be used for secure personalization by using a value from the transport key

array. The resulting data protection digest would then be used by Write.

The input value, typically a nonce from a remote device, is combined with the current TempKey

value to create a shared nonce in which both devices can attest to the inclusion of the RNG.

If zone is 0x02 (i.e. Data), and KeyID is less than 0x8000, then the GenDig command sets

TempKey.GenDigData to one, and TempKey.KeyID to the input KeyID; otherwise, TempKey.GenDigData

is set to zero. If KeyConfig.ReqRandom is set for KeyID, and the Data zone is locked, then the value in

the TempKey register must have been originally computed using a random number via the Nonce

command; otherwise, GenDig will fail. Regardless of how the resulting digest is computed, it can never

be read from the device.

If TempKey.Valid is invalid, this command returns an error. Upon command completion, the

TempKey.Valid bit is set indicating that a digest has been loaded and is ready for use. The TempKey.Valid

bit is cleared when the next command is executed. See Section Static RAM (SRAM) Memory for more

details.

For all KeyID values less than 0x8000, the device uses the least-significant four bits of KeyID to

determine the slot number from which to retrieve the key value from the Data zone of the EEPROM.

KeyID values above 0x8000 reference keys stored in the masks of the design. These keys can only be

used if the nonce value stored in TempKey has been generated using the on-board RNG. In any event, all

16 bits of the KeyID as input to the device are used as Param2 in the SHA-256 calculation.

When the key specified on input to GenDig has the NoMac bit set, GenDig can be used to generate

ephemeral keys matching those generated on client CryptoAuthentication devices using the DeriveKey

command. Keys which have the NoMac bit set represent situations in which the device is acting as a

host. In this case, the opcode and parameter bytes that would normally be included in the SHA

calculation are replaced with bytes from the input stream.

ATECC508A

Security Commands

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