ATECC508A datasheet(64/109 Pages) MICROCHIP

64 / 109 page

DeriveKey. If performing a Create Key operation and KeyConfig<ParentKey>.ReqAuth is one, then the

appropriate authorization must have been performed using KeyConfig<ParentKey>.AuthKey prior to the

execution of DeriveKey.

If an input MAC is required and KeyConfig<ParentKey>.ReqAuth is one, then the appropriate

authorization must have been performed using KeyConfig<ParentKey>.AuthKey prior to the execution of

DeriveKey.

If a parent key is involved in the operation (either SlotConfig<TargetKey>.Bit12 or

SlotConfig<TargetKey>.Bit15 are set) and SlotConfig<ParentKey>.LimitedUse is also set, DeriveKey

returns an error if Counter<0> has reached its limit. DeriveKey always ignores LimitedUse for the target

key.

Note: If the source and target key are the same, then there is a risk of permanent loss of the key value if

power is interrupted during the write operation. If the configuration bits permit it, then the key value may

be recovered using an authenticated and encrypted Write based on the parent key.

Table 9-14. Input Parameters

Name

Size

Notes

Opcode DeriveKey

0x1C

Param1 Mode

Bits 7-3: Must be zero.

Bit 2:

The value of this bit must match the value in TempKey.SourceFlag or the

command will return an error.

Bits 1-0: Must be zero.

Param2 TargetKey

Key slot to be written.

Data

MAC

0 or 32 Optional MAC used to validate operation.

Table 9-15. Output Parameter

Name

Size

Notes

Success

Upon successful completion, the ATECC508A returns a value of zero.

The key written to the target slot is the result of SHA-256 of the following message:

32 bytes

Target or Parent key (depending upon SlotConfig<12>)

1 byte

Opcode

1 byte

Param1

2 bytes

Param2

1 byte

SN<8>

2 bytes

SN<0:1>

25 bytes

Zeros

32 bytes

TempKey.value

The data flow for this command is illustrated below.

ATECC508A

Security Commands

Datasheet Complete

DS20005927A-page 64

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