ATSHA204A datasheet(57/93 Pages) MICROCHIP

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Bits Meaning

1 =

Include the first 64 OTP bits (OTP<0> through OTP<7>) in the message.

0 =

Set the Corresponding 88 OTP bits to 0.

1 =

Include the first 88 OTP bits (OTP<0> through OTP<10>) in the message.

Must be zero.

If either Mode<0> or Mode<1> are set, Mode<2> must match the value in TempKey.SourceFlag or the

command returns an error.

The first 32 bytes of the SHA message are loaded from one of the data slots.

1 =

The first 32 bytes are filled with TempKey.

0 = The second 32 bytes of the SHA message are taken from the input Challenge parameter.

1 = The second 32 bytes are filled with the value in TempKey. This mode is recommended for all use.

8.5.12

Nonce Command

The Nonce command generates a nonce for use by a subsequent GenDig, MAC, HMAC, Read, or Write

command by combining an internally generated random number with an input value from the system. The

resulting nonce is stored internally in TempKey and the generated random number is returned to the

system.

The input value is designed to prevent replay attacks against the Host and it must be externally

generated by the system and passed into the device using this command. It may be any value that

changes consistently, such as a nonvolatile counter, current real time of day and so forth; or it can be an

externally generated random number.

To provide a nonce value for subsequent crypto commands, the input number and output random number

are hashed together according to the information listed below. The resulting digest (nonce) is always

stored in the TempKey register, TempKey.Valid is set and TempKey.SourceFlag is set to “Rand”. The

nonce can be used by a subsequent GenDig, Read, Write, HMAC, or MAC command, thus the system

must externally compute this digest value and store it externally to complete the execution of those

commands.

Alternatively, this command can also be run in a pass-through mode if a fixed nonce is required for

subsequent commands. In this case, the input value must be 32 bytes long and it is passed directly to

TempKey without modification. No SHA-256 calculation is performed and TempKey.SourceFlag is set to

“Input.” The nonce value in TempKey may not be used with Read or Write commands. If operated in this

mode and with a repeated input number value, the device provides no protection against replay attacks.

Prior to the configuration section being locked, the RNG produces a 32 byte value of 0xFF FF 00 00

FF FF 00 00... to facilitate testing. This test value is combined with the input value in the manner

described above.

ATSHA204A

Security Commands

DS40002025A-page 57

ATSHA204A Datasheet(PDF) 57 Page - Microchip Technology

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