ATSGA204A View Datasheet(PDF) - Microchip Technology
Part Name
Description
MFG CO.
'ATSGA204A' PDF : 93 Pages View PDF
ATSHA204A
Security Commands
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.
4
1 = Include the first 88 OTP bits (OTP<0> through OTP<10>) in the message.
3 Must be zero.
2
If either Mode<0> or Mode<1> are set, Mode<2> must match the value in TempKey.SourceFlag or the
command returns an error.
0= The first 32 bytes of the SHA message are loaded from one of the data slots.
1
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.
0
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.
© 2018 Microchip Technology Inc.
DS40002025A-page 57
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