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ATSHA204
(Rev.:2012)

Atmel CryptoAuthentication

Atmel Corporation 

2.2 Static RAM (SRAM)

The device includes an SRAM array that is used to store the input command or output result, intermediate computation values,

and/or an ephemeral key. The entire contents of this memory are always invalidated whenever the device goes into sleep

mode or the power is removed. The ephemeral key is named TempKey, and can be used as an input to the MAC, HMAC,

CheckMac, GenDig, and DeriveKey commands. It is also used as the data protection (encryption or decryption) key by the

Read and Write commands. See below for more details on TempKey.

2.2.1

TempKey

TempKey is a storage register in the SRAM array that can be used to store an ephemeral result value from the Nonce or

GenDig commands. The contents of this register can never be read from the device (although the device itself can read and

use the contents internally).

This register contains the elements shown in Table 2-8.

Table 2-8. TempKey Storage Register

Name

TempKey

KeyID

SourceFlag

GenData

CheckFlag

Valid

Length

256 bits

(32 bytes)

4 bits

1 bit

1 bit

1 bit

1 bit

Description

Nonce (from nonce command) or Digest (from GenDig command)

If TempKey was generated by GenDig (see the GenData and CheckFlag bits), these bits

indicate which key was used in its computation. The four bits represent one of the slots of

the data zone.

The source of the randomness in TempKey:

0 = Internally generated random number (Rand).

1 = Input seed only, no internal random generation (Input).

0 = TempKey.KeyID is not meaningful, and is ignored.

1 = The contents of TempKey were generated by GenDig using one of the slots in the data

zone (and TempKey.KeyID will be meaningful).

If 1, the contents of TempKey were generated by the GenDig command and at least one of

the keys used in that generation is restricted to the CheckMac command

(SlotConfig.CheckOnly is 1). Otherwise, this bit will be 0.

0 = The information in TempKey is invalid.

1 = The information in TempKey is valid.

In this specification, the name “TempKey” refers to the contents of the 256-bit data register. The remaining bit fields are

referred to as TempKey.SourceFlag, TempKey.GenData, and so on.

The TempKey.Valid bit is cleared to zero under any of the following circumstances:

• Power up, sleep, brown out, watchdog expiration, or tamper detection. The contents of TempKey, however, are

retained when the device enters idle mode.

• After the execution of any command other than Nonce or GenDig, regardless of whether or not the command

execution succeeds. It may be cleared by the CheckMac command unless a successful copy takes place. It is not

cleared if there is a communications problem, as evidenced by a cyclic redundancy check (CRC) error.

• An error during the parsing or execution of GenDig and/or Nonce.

• Execution of GenDig replaces any previous output of the Nonce command with the output of the GenDig command.

Execution of the Nonce command likewise replaces any previous output of the GenDig command.

Atmel ATSHA204 [DATASHEET] 11

8740D−CRYPTO−3/12

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