ATSHA204 datasheet(55/65 Pages) ATMEL

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Atmel ATSHA204 [DATASHEET]

8740D

−CRYPTO−3/12

8.14

Write Command

Writes either a one 4-byte word or an 8-word block of 32 bytes to one of the EEPROM zones on the device. Depending on the

value of the WriteConfig byte for this slot the data may be required to be encrypted by the system prior to being sent to the

device.

The following restrictions apply to writes within zones using this command:

Config

If the Config zone is locked or Zone:6 is set, this command returns an error. Otherwise the bytes are written

as requested. Any attempt to write any byte for which Writes are permanently prohibited (per Section 2.1.1)

results in a command error with no modifications to the EEPROM.

OTP

If the OTP zone is unlocked, all bytes can be written with this command. If the OTP zone is locked and the

OTPmode byte is read-only or legacy, then this command returns an error. Otherwise, OTP mode should be

consumption and this command sets to zero those bits in the OTP zone that correspond to the zero bits in

the input parameter value. When the OTP zone is locked, encrypted writes to it are never permitted

regardless of OTPmode.

Data

If the data zone is unlocked, all bytes in all zones can be written with either plain text or encrypted data.

After the data zone is locked, the values within the WriteConfig bytes control access to the data slots. If the

WriteConfig bits for this slot are set to “always”, the input data should be passed to the device in the clear. If

Bit:14 of SlotConfig is set to one, the input data should be encrypted and an input MAC calculated.

Four byte writes are only permitted in the data and OTP zones if all four of the following conditions are met:

• SlotConfig.IsSecret must be zero.

• SlotConfig.WriteConfig must be “always.”

• The input data must not encrypted.

• The data/OTP zones must be locked.

Four byte writes will return an error under all other circumstances.

The least significant three bits of Param2, Address[0:2], indicate the word within the block, or are ignored if an entire 32 byte

block is being written. Address[3:6] contains the slot number for writes to the data zone, or the block number for the Config

and OTP zones. Address values beyond the size of the specified zone result in the command returning an error.

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

an error code.

8.14.1 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 XOR’ing 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 input data is encrypted an authorizing input MAC is always required when writing the data zone. This MAC is

computed as:

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 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, 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.KeyID must match that in SlotConfig.WriteKey and the random number generator must have been used to originally

generate TempKey prior to GenDig.

ATSHA204 Datasheet(PDF) 55 Page - ATMEL Corporation

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