ATSHA204 ATMEL [ATMEL Corporation], ATSHA204 Datasheet - Page 16

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

3.4
3.4.1
3.4.2
For GenDig and all other commands, KeyID values less than 0x8000 always reference keys that are stored in the data zone of
the EEPROM. In these cases, only the four least-significant bits of KeyID are used to determine the slot number, while the
entire 16-bit KeyID as input is used in any SHA-256 message calculation.
Security Features
Physical Security
The ATSHA204 incorporates a number of physical security features designed to protect the EEPROM contents from
unauthorized exposure. The security measures include:
Pre-programmed transport keys stored on the ATSHA204 are encrypted in such a way as to make retrieval of their values
using outside analysis very difficult.
Both the logic clock and logic supply voltage are internally generated, preventing any direct attack on these two signals using
the pins of the device.
Random Number Generator (RNG)
The ATSHA204 includes a high-quality random number generator that returns 32 random bytes to the system. The device
combines this generated number with a separate input number to form a nonce that is stored within the device in TempKey
and may be used by subsequent commands.
The system may use this random number generator for any purpose. One common purpose would be as the input challenge
to the MAC command on a separate CryptoAuthentication device. The device provides a special Random command for such
purposes, which does do not affect the internally stored nonce.
To simplify system testing, prior to config locking the random number generator always returns the following value:
where ff is the first byte read from the device and the first byte into the SHA message.
To prevent replay attacks on encrypted data that is passed to or from the ATSHA204, the device requires that a new, internally
generated nonce be included as part of the encryption sequence used to protect the data being written or read. To implement
this requirement, the data protection key generated by GenDig and used by the Read or Write command must use the internal
random number generator during the creation of the Nonce.
Random numbers are generated from a combination of the output of a hardware random number generator and an internal
seed value, which is not externally accessible. The internal seed is stored in the EEPROM, and is normally updated once after
every power-up or sleep/wake cycle. After the update, this seed value is retained in registers within the device that are
invalidated if the devicez enters sleep mode or the power is removed.
Because there is an EEPROM endurance specification that limits the number of times the EEPROM seed can be updated, the
host system should manage power cycles to minimize the number of required updates. In certain circumstances, the system
may choose to suppress the EEPROM seed update using the mode parameter to the Nonce and Random commands.
4.
5.
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•
•
•
•
ff ff 00 00 ff ff 00 00 …
Use that personalization key to write all the secret slots, ending with the final value of the personalization key slot
Lock the data zone
An Active Shield over the part
Internal Memory Encryption
Secure Test Modes
Glitch Protection
Voltage Tamper Detection
itself, using the following sequence repeated as necessary:
a.
b.
c.
Nonce command to generate a random nonce in TempKey
Gendig specifying the personalization key
Encrypted write to the target slot
Atmel ATSHA204 [DATASHEET]
8740D−CRYPTO−3/12
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