ATSGA204A View Datasheet(PDF) - Microchip Technology
Part Name
Description
MFG CO.
'ATSGA204A' PDF : 93 Pages View PDF
Figure 8-1. Data Flow for CheckMac Command
Slot<SlotID> TempKey
Mode
Mode
SHA
(AUTH)
Response
Match
0X01
ATSHA204A
Security Commands
8.5.6
0x00
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.
© 2018 Microchip Technology Inc.
DS40002025A-page 49
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