ATSHA204A datasheet(14/93 Pages) MICROCHIP

14 / 93 page

Table 2-7. Write Configuration Bits — Write Command

Bit 15 Bit 14 Bit 13

Mode

Name

Description

Always

Clear text writes are always permitted on this slot. Slots set to “always” should

never be used as key storage. Either 4 or 32 bytes may be written to this slot.

Never

Writes are never permitted on this slot using the Write command

Slots set to “never” can still be used as key storage.

Never

Writes are never permitted on this slot using the Write command

Slots set to “never” can still be used as key storage.

Encrypt

Writes to this slot require a properly computed MAC and the input data must

be encrypted by the system with WriteKey using the encryption algorithm

documented in the Write command description Section (8.5.18 Write

Command). 4-byte writes to this slot are prohibited.

The 4-bit WriteConfig field is interpreted by the Write command as shown in Table Write Configuration

Bits —Write Command, where X means don’t care.

The tables overlap. For example, a code of 0b0110 indicates that a slot can be written in encrypted form

by using the Write command and it can also be the target of an unauthorized DeriveKey command

with the target as the source.

The IsSecret bit controls internal circuitry necessary for proper security for slots in which reads and/or

writes must be encrypted or are prohibited altogether. It must also be set for all slots that are to be used

as keys, including those created or modified with DeriveKey. Specifically, to enable proper device

operation, this bit must be set unless WriteConfig is “Always”. 4-byte accesses are prohibited to/from slots

in which this bit is set.

Slots used to store key values should always have IsSecret set to one and EncryptRead set to zero

(reads prohibited) for maximum security. For fixed key values, WriteConfig should be set to “Never”.

When configured in this way, there is no way to read or write the key after the Data zone is locked. It may

only be used for crypto operations.

Some security policies require secrets to be updated from time to time. The ATSHA204A supports this

capability in the following way:

•

WriteConfig for the particular slot should be set to “Encrypt” and SlotConfig.WriteKey should point

back to the same slot by setting WriteKey to the slot ID. A standard Write command can be then

used to write a new value to this slot provided that the authentication MAC is computed using the

old (current) key value.

2.1.2.15 Special Memory Values in the Configuration Zone (Bytes 0 – 12)

Various fixed information is included in the ATSHA204A that can never be written under any

circumstances and can always be read, regardless of the state of the lock bits.

•

SerialNum

Nine bytes (SN<0:8>) which together form a unique value that is never repeated for any device in

the CryptoAuthentication family. The serial number is divided into two groups:

1.1.

SN<0:1> and SN<8>

The values of these bits are fixed at manufacturing time in most versions of the

ATSHA204A. Their default value is (0x01 0x23 0xEE). These 24 bits are always included

in the SHA-256 computations made by the ATSHA204A.

ATSHA204A

Device Organization

DS40002025A-page 14

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