ATSHA204A datasheet(83/93 Pages) MICROCHIP

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13.

Reference and Application Notes

The ATSHA204A implements a challenge-response protocol using either SHA-256 or HMAC/SHA-256,

details are noted below. The response is always a 256-bit digest.

The Nonce command (see Section Nonce Command) accepts an input challenge from the system and

optionally combines it with an internally generated random number to generate a nonce (for example

number used once) for the calculation. The combination is the seed and it is then combined with a secret

key as part of the authentication calculation for any of the crypto commands (for instance MAC, HMAC,

Read, Write, or GenDig). The input challenge can also be passed directly to the MAC command.

The device can guarantee the uniqueness of the nonce only if the device has included the output of its

RNG in the calculation; that is because the system input may or may not be unique. Every random nonce

is in fact guaranteed to be unique when compared to all previous nonces, ensuring that each transaction

is unique over all time.

13.1

SHA-256

The ATSHA204A MAC command calculates the digest of a secret key concatenated with the challenge or

nonce. It optionally includes various other pieces of information stored on the device within the digested

message.

The ATSHA204A computes the SHA-256 digest based on the algorithm documented at the following site:

http://csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf

The complete SHA-256 message processed by the ATSHA204A is listed in the command description for

each command that use the algorithm. Most standard software implementations of the algorithm

automatically add the appropriate number of pad and length bits to this message to match that the

operation the device performs internally.

The ATSHA204A can also calculate a SHA-256 digest using the SHA command. The caller is responsible

for providing the pad and length bytes to the message. Message size must be a multiple of 64 bytes

including the pad bytes.

The SHA-256 algorithm is used for encryption by taking the output digest of the hash algorithm and

XORing it with the plain text data to produce the ciphertext. Decryption is the reverse operation, namely

the ciphertext is XORed with the digest with the result being the plain text.

13.2

HMAC/SHA-256

The response to the challenge can also be computed using the HMAC algorithm based on SHA-256

documented at the following site:

http://csrc.nist.gov/publications/fips/fips198/fips-198a.pdf

Because of the increased computation complexity, the HMAC command is not as flexible as the MAC

command and the computation time for HMAC is extended. While the HMAC sequence is not necessary

to ensure the security of the digest, it is included for compatibility with various software packages.

ATSHA204A

Reference and Application Notes

DS40002025A-page 83

ATSHA204A датащи(PDF) 83 Page - Microchip Technology