ATECC508A datasheet(29/109 Pages) MICROCHIP

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3.2

Key Uses and Restrictions

Any slot in the EEPROM data zone can be used to store a secret or private key. There are a number of

ways in which the keys stored within the device can be used and/or their access restricted. See the

following sections Diversified Keys to Authorized Keys for some of these concepts.

The device should be properly configured to prevent any unwanted read and write access to all key slots,

including the setting of the IsSecret bit. Private keys can never be read from the device regardless of the

values in the configuration zone.

With the exception of transport keys documented in section Transport Keys, the most significant 12 bits of

all KeyID parameters should be zero.

3.2.1

Diversified Keys

If the host or validating entity has a place to securely store secrets, or contains an ATECC508A device,

the secret key values stored in the EEPROM slot(s) of the clients can be diversified by using the serial

number embedded in the device (SN<0:8>). In this manner, every client device can have a unique key,

which can provide extra protection against known plaintext attacks and permit compromised serial

numbers to be identified and blacklisted.

To implement this operation, a root secret is externally combined with the device’s serial number during

personalization by using some cryptographic algorithm, and the result is written to the ATECC508A key

slot.

The ATECC508A GenDig and CheckMac commands provide a mechanism to securely generate and

compare diversified keys, thereby eliminating this requirement from the host system.

Consult the following application note for more details:

http://ww1.microchip.com/downloads/en/appnotes/doc8666.pdf

3.2.2

Rolled Keys

In order to prevent repeated uses of the same secret key value, the ATECC508A supports key rolling.

Normally, after a certain number of uses (perhaps as few as one), the current key value is replaced with

the SHA-256 digest of its current value combined with some offset, which may either be a constant,

something related to the current system (for example, a serial number or model number), or a random

number.

This capability is implemented using the DeriveKey command. Prior to execution of the DeriveKey

command, the Nonce command must be run to load the offset into TempKey.

One use for this capability is to permanently remove the original key from the device, and replace it with a

key that is only useful in a particular environment. After the key is rolled, there is no possible way to

retrieve the old key’s value, which improves the security of the system.

Note: Any power interruption during the execution of the DeriveKey command in Roll mode may cause

the key to have an unknown value. If writing to a slot is enabled using bit 14 of SlotConfig, such keys can

be written in encrypted and authenticated form using the Write command. Alternatively, multiple copies

of the key can be stored in multiple slots so that failure of a single slot does not incapacitate the system.

3.2.3

Created ECC Keys

For the highest security, private ECC keys may be created within the ATECC508A using the internal high

quality RNG. These keys are guaranteed to be unique to this device since there is no mechanism for

reading the value of an ECC private key from the ATECC508A.

ATECC508A

Security Information

Datasheet Complete

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