ATECC508A datasheet(18/109 Pages) MICROCHIP

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2.2.3

Write Permissions

The 4-bit WriteConfig field is interpreted by the Write, DeriveKey, GenKey and PrivWrite

commands as shown in Table 2-8, Table 2-9, Table 2-10 and Table 2-11 where “X” means don't care.

Note: The tables overlap: for example, a code of 0110 indicates a slot which can be written in encrypted

form using the Write command and can also be the target of an unauthorized DeriveKey command with

the target as the source.

KeyType in the KeyConfig field (see Table 2-12) indicates whether the GenKey or DeriveKey

commands can be used on a particular slot; with GenKey for ECC keys only, and DeriveKey for

SHA-256 keys.

See Section Writing ECC Private Keys for special information regarding the writing of ECC private keys.

ECC public keys are treated as normal data, and Write permissions for those slots are described in this

section.

Table 2-8. Write Configuration Bits: Write Command

Bit

Mode Name Description

Always

Clear text writes are always permitted on this slot. Slots set to always

should never be used as key storage. Either 4 or 32 bytes may be

written to this slot.

PubInvalid

If a validated public key is stored in the slot, writes are prohibited. Use

Verify(Invalidate) to invalidate prior to writing. Do not use this

mode unless the slot contains a public key.

Never

Writes are never permitted on this slot using the Write command.

Slots set to never can still be used as key storage.

Never

Writes are never permitted on this slot using the Write command.

Slots set to never can still be used as key storage.

Encrypt

Writes to this slot require a properly computed MAC, and the input

data must be encrypted by the system with WriteKey using the

encryption algorithm documented in the Write command description

(Section Write Command). 4 byte writes to this slot are prohibited.

Table 2-9. Write Configuration Bits: DeriveKey Command

Bit 15 Bit 14 Bit 13 Bit 12 Source

Key(Note)

Description

Target

DeriveKey command can be run without authorizing MAC.

(Roll)

Target

Authorizing MAC required for DeriveKey command. (Roll)

Parent

DeriveKey command can be run without authorizing MAC.

(Create)

Parent

Authorizing MAC required for DeriveKey command. (Create)

—

Slots with this value in the WriteConfig field may not be used as

the target of the DeriveKey command.

ATECC508A

Device Organization

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DS20005927A-page 18

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