ATSHA204 ATMEL [ATMEL Corporation], ATSHA204 Datasheet - Page 48

• Manufacturer Part Number: ATSHA204
• Description: Atmel CryptoAuthentication
• Manufacturer: ATMEL [ATMEL Corporation]

8.9
Nonce Command
This 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 ─ 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 on, 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 per 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 random number generator produces a 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.
Table 8-25. Input Parameters
Table 8-26. Output Parameter
If Mode[0:1] is zero or one, the input NumIn parameter must be 20 bytes long, and the SHA-256 message body used to create
the nonce stored internally in TempKey consists of the following:
Opode
Param1
Param2
Data
Name
RandOut
32 bytes
20 bytes
1 byte
1 byte
1 byte
Name
Nonce
Mode
Zero
NumIn
RandOut
NumIn from input stream
Opcode (always 0x16)
Mode
LSB of Param2 (should always be 0x00)
1 or 32
20,32
Size
Size
1
1
2
Notes
0x16
Controls the mechanism of the internal random number generator and seed update.
Must be 0x0000.
Input value from system.
Notes
The output of the random number generator or a single byte with a value of zero if
Mode[0:1] is three.
Atmel ATSHA204 [DATASHEET]
8740D−CRYPTO−3/12
48