ATSHA204A datasheet(65/93 Pages) MICROCHIP

65 / 93 page

Whenever the input data is encrypted an authorizing input MAC is always required when writing the Data

zone. This MAC is computed as follows:

SHA-256 (TempKey, Opcode, Param1, Param2, SN<8>, SN<0:1>, <25 bytes of 0’s>, PlainTextData)

Prior to locking of the OTP/Data zones, Zone<6> is used to indicate to the device whether or not the input

data is encrypted. After locking of the OTP/Data zones, Zone<6> is ignored and only SlotConfig<14>

corresponding to the slot being written is used to determine whether or not the input data is encrypted.

If data encryption is indicated, then TempKey must be valid prior to this command being called and it

must be the result of GenDig. Specifically, this means that TempKey.Valid and TempKey.GenDig must

both be set to one. Prior to data locking, any key can be used to generate TempKey. After locking, the last

slot used by GenDig for TempKey creation and stored in TempKey.SlotID must match that in

SlotConfig.WriteKey. If the slot number being written is even, then TempKey.SourceFlag must be RAND.

If the slot number is odd, then TempKey.SourceFlag must match the value in Config.CheckMacSource

corresponding to the slot.

Table 8-40. Input Parameters

Name

Size

Notes

Opcode Write

0x12

Param1 Zone

Bit 7:

0 = 4 bytes of date are written to the specified zone.

1 = 32 bytes of data are written to the specified zone.

Bit 6:

0 = Data is written in the clear.

1 = Input data must be encrypted.

Must be zero if Data/OTP zones are locked.

Bits 5-2:

Must be zero.

Bits 1-0:

Select among Config, OTP or Data. See Section Zone Encoding.

Param2 Address

Address of first word to be written within the zone. See Section Address Encoding.

Data_1 Value

4 or 32 Information to be written to the zone; may be encrypted.

Data_2 Mac

0 or 32 Message authentication code to validate address and data.

Table 8-41. Output Parameter

Name

Size

Notes

Success

Upon successful completion, the ATSHA204A returns a value of 0x00.

ATSHA204A

Security Commands

DS40002025A-page 65

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