ATECC508A datasheet(35/109 Pages) MICROCHIP

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ECC public keys appear as the input or output parameters to several commands, and they can also be

stored in EEPROM. They are composed of an X value first on the bus or in memory, followed by a Y

value. They are formatted differently depending upon the situation as noted below:

•

The public key is an output of GenKey command or an input to Verify command:

32 bytes of X, then 32 bytes of Y. There are no pad bytes.

•

Write command:

Public keys can be written directly to the EEPROM using Write command and are always

72 bytes long, formatted as follows: 4 pad bytes, 32 bytes of X, four pad bytes, then 32 bytes of Y.

•

GenKey command:

SHA Message: Public keys can be hashed and placed in TempKey by the GenKey command. The

SHA message contains various bytes that are independent of the size of the key. These are

followed by

25 bytes of pad, followed by 32 bytes of X, then 32 bytes of Y.

•

Verify command:

SHA Message: When used to validate a stored public key, the Verify command expects an input

signature created over a SHA-256 digest of a key stored in memory. Such an inner SHA calculation

is always performed over 72 bytes formatted as they are stored in EEPROM as 4 pad bytes, 32

bytes of X, four pad bytes, then 32 bytes of Y.

When a public key is configured to be validated by the Verify command, then the most significant four

bits of the first byte in memory are used internally by the device to save the validation state. They are

always set to the invalid state (0xA) by the Write command, and then may be set to the valid state

(0x5) by the Verify command.

4.2

Sharing the Interface

Multiple cryptoAuthentication devices may share the same interface, as follows:

The system issues a wake token to wake-up all devices.

The system issues the Pause command (Section Pause Command) to put all but one of the

devices into the idle mode. Only the remaining device will then see any of the commands that the

system sends. When the system has completed talking to the one active device, it then sends an

idle flag that puts the remaining active device into the idle mode and the idle devices will ignore.

Steps 1 and 2 are repeated for each device on the wire. If the system has completed communications

with the final device, it should wake up all the devices, and then put all the devices to sleep to reduce

total power consumption.

The device uses the Selector byte within the configuration zone to determine which device stays awake.

Only that device with a Selector value that matches the input parameter of the Pause command will stay

awake. In order to facilitate late configuration of systems which use the multi-device sharing mode, the

following three update capabilities for the selector byte are supported:

•

Unlimited Updates:

At any time, the UpdateExtra command can be executed to write the value in the Selector field of

the Configuration zone. To enable this mode, clear the SelectorMode bit in ChipMode.

•

One-time Field Update:

If the SelectorMode bit is set to one, and the Selector byte has a zero value prior to locking the

configuration zone, then at any time after the configuration zone is locked the UpdateExtra

ATECC508A

General I/O Information

Datasheet Complete

DS20005927A-page 35

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