ATECC508A datasheet(91/109 Pages) MICROCHIP

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input data should be passed to the device in the clear. If SlotConfig<14> is set to one, then the

input data should be encrypted and an input MAC calculated. If the slot is individually locked using

SlotLocked, then this command always returns an error.

Four byte writes are only permitted in the data zone if all of the following conditions are met:

•

SlotConfig.IsSecret must be zero.

•

SlotConfig.WriteConfig must be always.

•

The input data must not be encrypted, i.e. Zone<6> must be zero.

•

The data/OTP zones must be locked.

Four byte writes are only permitted in the OTP zone if all of the following conditions are met:

•

OTPmode must be consumption.

•

The input data must not be encrypted, i.e. Zone<6> must be zero.

•

The Data/OTP zones must be locked.

The two input address bytes are formed in a manner to achieve compatibility with the ATSHA204A (see

Section Address Encoding). The least significant three bits, Address<2:0>, indicate the word within the

block, or they are ignored if an entire 32 byte block is being written. Address<6:3> contains the slot

number for writes to the data zone, or the block number for the configuration and OTP zones. For the

data zones, Address<9:8> is used to indicate the block within the slot. Address values beyond the size of

the specified zone result in the command returning an error.

For Slots 8 to 15, if KeyConfig.PubInfo indicates that the slot contains an ECC public key which can be

validated, then the key will be invalidated by writing 0xA to the most significant four bits of byte zero of

Block 0 of the slot when any block within the slot is written. If KeyConfig.PubInfo is zero, then the most

significant four bits of byte zero of Block 0 of the slot are written with the data from the input parameter.

If KeyConfig.PubInfo is one and the ECC public key has been validated, then writes will fail if WriteConfig

is set to 0001 (PubInvalid). Use Verify(Invalidate) to invalidate the public key prior to writing.

Any attempt to write the OTP and/or data zones prior to the configuration zone being locked results in the

device returning an error code. When writing to the data zone, if the corresponding SlotLocked bit is zero,

then this command returns an error regardless of whether or not the OTP/data zones have been locked.

9.21.1

Input Data Encryption

The input data may be encrypted to prevent snooping on the bus during personalization or system

operation. The system should encrypt the data by XORing the plain text with the current value in

TempKey. Upon receipt, the device will XOR the input data with TempKey to restore the plain text prior to

writing to the EEPROM.

Whenever the data is encrypted, an authorizing input MAC is always required. This MAC is computed as

follows:

SHA-256(TempKey, Opcode, Param1, Param2, SN<8>, SN<0:1>, <25 bytes of zeros>, 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 bit 14 of the

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

encrypted.

If data encryption is indicated, 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.GenDigData must both

be set to one. The last slot used by GenDig for TempKey creation and stored in TempKey.KeyID must

ATECC508A

Security Commands

Datasheet Complete

DS20005927A-page 91

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