ATSHA204A datasheet(49/93 Pages) MICROCHIP

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Figure 8-1. Data Flow for CheckMac Command

Slot<SlotID>

TempKey

Response

0X01

SHA

(AUTH)

Match

0x00

Mode

8.5.6

DeriveKey Command

The device combines the current value of a key with the nonce stored in TempKey using SHA-256 and

places the result into the target key slot. SlotConfig<TargetKey>.Bit13 must be set or DeriveKey returns

an error.

If SlotConfig<TargetKey>.Bit12 is zero, the source key that is combined with TempKey is the target key

specified in the command line (Roll-Key operation). If SlotConfig<TargetKey>.Bit12 is one, the source key

is the parent key of the target key, which is found in SlotConfig<TargetKey>.WriteKey (Create Key

operation).

Prior to execution of the DeriveKey command, the Nonce command must have been run to create a

valid nonce in TempKey. Depending upon the state of bit two of the input mode, this nonce would have

been created with the internal RNG, or it would have been fixed.

If SlotConfig<TargetKey>.Bit15 is set, an input MAC must be present and had been computed as follows:

SHA-256(ParentKey, Opcode, Param1, Param2, SN<8>, SN<0:1>)

where the ParentKey ID is always SlotConfig<TargetKey>.WriteKey.

If SlotConfig<TargetKey>.Bit12 or SlotConfig<TargetKey>.Bit15 is set and

SlotConfig<ParentKey>.LimitedUse is also set, DeriveKey returns an error if UseFlag<ParentKey> is

0x00. DeriveKey ignores LimitedUse and UseFlag for the target key if SlotConfig<TargetKey>.Bit12

and SlotConfig<TargetKey>.Bit15 are both zero.

For slots 0 through 7 only, if input parsing and the optional MAC check succeed, UseFlag<TargetKey>

gets set to 0xFF and UpdateCount<TargetKey> is incremented. If UpdateCount currently has a value of

0xFF, then it wraps to zero. If the command fails for any reason, these bytes can not be updated. The

value of UpdateCount may be corrupted if power is interrupted during the execution of DeriveKey.

ATSHA204A

Security Commands

DS40002025A-page 49

ATSHA204A 데이터시트(PDF) 49 Page - Microchip Technology