ATSHA204 datasheet(48/65 Pages) ATMEL

48 / 65 page

Atmel ATSHA204 [DATASHEET]

8740D

−CRYPTO−3/12

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

Name

Size

Notes

Opode

Nonce

0x16

Param1

Mode

Controls the mechanism of the internal random number generator and seed update.

Param2

Zero

Must be 0x0000.

Data

NumIn

20,32

Input value from system.

Table 8-26. Output Parameter

Name

Size

Notes

RandOut

1 or 32

The output of the random number generator or a single byte with a value of zero if

Mode[0:1] is three.

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:

32 bytes

RandOut

20 bytes

NumIn from input stream

1 byte

Opcode (always 0x16)

1 byte

Mode

1 byte

LSB of Param2 (should always be 0x00)

ATSHA204 Datasheet(PDF) 48 Page - ATMEL Corporation

ATSHA204 Datasheet(HTML) 48 Page - ATMEL Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Datasheet Download

Link URL