ATSHA204 ATMEL [ATMEL Corporation], ATSHA204 Datasheet - Page 43

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

8.6
HMAC Command
Computes a HMAC/SHA-256 digest of a key stored in the device, a challenge, and other information on the device. The output
of this command is the output of the HMAC algorithm computed over this key and message. If the message includes the serial
number of the device, the response is said to be diversified.
The normal command flow to use this command is as follows:
Step two addresses multiple use models. If the data in the EEPROM is a key, GenDig has the effect of authenticating the
challenge with multiple secret keys. Alternatively, if the contents of the slot are data (which does not have to necessarily even
be secret), GenDig has the effect of authenticating the value stored in that location.
Table 8-17. Input parameters
Table 8-18. Output parameter
The HMAC digest is computed using the key at KeyID as the HMAC key over a message consisting of the following
information:
Opcode
Param1
Param2
Data
Name
Response
1.
2.
3.
32 bytes
32 bytes
1 byte
1 byte
2 bytes
8 bytes
3 bytes
1 byte
4 bytes
2 bytes
2 bytes
Run Nonce command to load input challenge and optionally combine it with a generated random number. The result
Optionally run GenDig command to combine one or more stored EEPROM locations in the device with the nonce.
Run this HMAC command to combine the output of steps one (and step two if desired) with an EEPROM key to
of this operation is a nonce stored internally on the device.
The result is stored internally in the device.
generate an output response.
Name
HMAC
Mode
KeyID
-
Zeros
TempKey
Opcode (always 0x11)
Mode
KeyID
OTP[0:7] (or zeros, see Table 8-19)
OTP[8:10] (or zeros, see Table 8-19)
SN[8] bits (never zeroed out)
SN[4:7] bits (or zeros, see Table 8-19)
SN[0:1] (never zeroed out)
SN[2:3] (or zeros, see Table 8-19)
Size
Size
32
1
1
2
0
Notes
HMAC digest
Notes
0x11
Controls which fields within the device are used in the message.
Which key is to be used to generate the response.
Bits 0:3 only are used to select a slot but all 16 bits are used in the HMAC message.
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Atmel ATSHA204 [DATASHEET]
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