ATSHA204A datasheet(7/93 Pages) MICROCHIP

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any other key ever generated on any device. By “activating” a Host-Client pair in the field in this manner,

a clone of a single Client can not work on any other Host.

In a Host-Client configuration where the Host (for example a mobile phone) needs to verify a Client (for

instance an OEM battery), there is a need to store the secret in the Host in order to validate the response

from the Client. The CheckMac command (Section CheckMac Command) allows the Host device to

securely store the Client’s secret and hide the correct response value from the pins, returning only a

yes/no answer to the system.

Where a user-entered password is required, the CheckMac command also provides a way to both verify

the password without exposing it on the communications bus and map the password to a stored value

that can have much higher entropy. See Section Password Checking for details.

Finally, the hash combination (for example digest) of a challenge and secret key can be kept on the

device and XORed with the contents of a slot to implement an encrypted read (Section Read Command),

or it can be XORed with encrypted input data to implement an encrypted write (Section Write

Command).

Each of these operations can be protected against replay attacks by including a random nonce (Section

Nonce Command) in the calculation.

All security functions are implemented using the industry-standard SHA-256 secure hash algorithm,

which is part of the latest set of high-security cryptographic algorithms recommended by various

government agencies and cryptographic experts. Section SHA-256 includes a reference to the algorithm

details. If desired, the SHA-256 algorithm can also be included in an HMAC sequence (See Section HMAC

Command). The ATSHA204A employs full-sized, 256-bit secret keys to prevent any kind of exhaustive

attack.

ATSHA204A

Introduction

DS40002025A-page 7

ATSHA204A датащи(PDF) 7 Page - Microchip Technology