ATECC508A datasheet(77/109 Pages) MICROCHIP

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2 bytes

SN<0:1> (never zeroed out)

2 bytes

SN<2:3> (or zeros, see Mode Encoding)

Table 9-34. Mode Encoding

Bits

Meaning

Must be zero.

0 =

48 Message bits corresponding to SN<2:3> and SN<4:7> are set to zero.

1 =

Include the 48 bits SN<2:3> and SN<4:7> in the message.

5–3 Must be zero.

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

command will return an error.

0 =

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.

9.12

Nonce Command

The Nonce command generates a nonce for use by a subsequent 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 (i.e. nonce) is always

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

nonce can then be used by a subsequent ATECC508A command. Where the actual nonce value is

required to be known by an external system, software will typically be needed to externally compute this

digest value and store it externally to complete the execution of those commands.

In order to simplify the system code for some usage models, the device provides a mechanism for a host

device to compute the nonce generated on a client device. In this calculation mode, the current value in

TempKey is combined with the input parameters using SHA and the result is written back into TempKey.

The new TempKey value is also returned to the system as the output parameter. Mode<1:0> must have a

value of zero or one to enable this feature. TempKey.SourceFlag is not modified by the device in this

mode.

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

ATECC508A

Security Commands

Datasheet Complete

DS20005927A-page 77

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