ATECC508A datasheet(11/109 Pages) MICROCHIP

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store an ECC public key or ECDSA certificate/signature. When a slot is used for a private or secret key,

the excess memory not required by the particular algorithm is generally unusable. The following table lists

the typical uses for each group of slots, along with any special characteristics of slots within that group.

Table 2-3. Data Zone

Slot Blocks(N

ote)

Bytes Bits Typical Use

Notes

0-7

288 Private or Secret

Key

Can also be used for data.

416

3328 Data

Reads and Writes can be configured to be restricted in the

same manner as all other slots. If this slot is used as a key,

then the remaining bytes not required for the secret or private

key storage will be ignored.

9-14

576 Public Key,

Signature or

Certificate

For curves supported by this device, these slots are large

enough to contain both the X and Y components of an

ECDSA public key or the R and S components of an ECDSA

signature.

576 Private Data,

Secret Key,

Signature, or

Certificate

This is the only slot that supports the 128 count limited use

feature (Section Limited Use Key (Slot 15 only)), If this feature

is not required, then it can otherwise be used for the same

purposes as slots 9 through 14.

Note: The last block in some data slots contains fewer than 32 bytes.

Data slots which contain ECC public or private keys should be formatted according to Section ECC Key

Formatting. The device uses the KeyType and PubInfo fields of KeyConfig to determine what is stored in

a slot. Private keys can never be read from the device under any circumstances. ECC key slot contents

may not be usable by the ECC commands unless they are validated as follows:

•

ECC Private Keys:

Prior to the first PrivWrite or GenKey(Create) command execution on a slot, private keys are

invalid. The key may also be invalid if the PrivWrite command is started, but power is interrupted

prior to its completion.

•

ECC Public Keys:

The key must be validated using an input signature and the ECC Verify command if the PubInfo

bit of KeyConfig is one. If that bit is zero, then ECC usage does not depend on the key Verify

operation. These keys may be stored in slots 8 through 15 only. This feature is optional.

2.1.1

Certificate Storage

The amount of storage required for a full X.509 Certificate within the device can rapidly use up multiple

EEPROM memory slots. Depending on the actual application it may or may not be desirable to use these

slots for certificate storage. Due to these memory limitations, Microchip has defined an encoding that

allows for a full X.509 Certificate to be reconstructed from a minimal amount of information.

The host system would actually be responsible for reconstructing the full X.509 Certificate, but how to do

this will be determined by the data stored in the encoded certificate. Data that is common to all devices

for a given system can be readily stored in the host system. Other data can be readily calculated or

extracted from data that is already stored in the device. Table 2-4 indicates the type of data that is stored

in an X.509 Certificate and how it can be encoded to fit into a single 72 byte slot.

ATECC508A

Device Organization

Datasheet Complete

DS20005927A-page 11

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