ATECC508A datasheet(22/109 Pages) MICROCHIP

22 / 109 page

•

KeyType:

The four ECC commands that use ECC keys (i.e. GenKey, Sign, ECDH, and Verify) will operate

only on data slots in which this field is set to one of the legal ECC key types. Any attempt to use

any SHA-256 computation commands (i.e. CheckMac, DeriveKey, MAC, or HMAC) on a slot

configured to be an ECC private key will result in an error.

Keys that will be the source or destination of the SHA-256 computation commands (i.e. CheckMac,

DeriveKey, MAC, or HMAC) should be set to a KeyType of 0b111. Proper operation of the device is

not guaranteed if these commands are attempted with any other KeyType. The GenDig command

may operate on any slot type other than for ECC private keys.

•

ReqRandom:

This field is useful in preventing replays of authorization and/or other cryptographic operations.

Keys that control encrypted reads and/or writes should have this field set to one under normal

circumstances in order to provide data security.

If this field is set to one, then prior to the execution of the CheckMac, GenDig, DeriveKey,

Verify, MAC, and HMAC commands, the Random Number Generator (RNG) must have been used

by the Nonce command to generate the contents of TempKey.

If GenKey is used to generate a public key digest of either a public or private key stored in a Data

zone slot, then the ReqRandom field is used to ensure that the nonce in TempKey included the

RNG.

•

ReqAuth:

If this bit is set, then prior authorization of the key at KeyConfig.AuthKey must have been

completed prior to execution of any cryptographic command (i.e. CheckMac, DeriveKey, GenDig,

GenKey, MAC, HMAC, Sign, or Verify) that uses this key. The DeriveKey command checks for

usage authorization only for the parent key, and never the target key, unless it is the same as the

parent key.

The GenKey command checks for usage authorization even when generating a new key to prevent

denial of service attacks.

The authorization state is stored in two internal volatile registers:

–

AuthValid

–

AuthKeyID

These registers are retained as long as power is applied, and the device does not enter the

Sleep mode.

These registers are set by means of the execution of a successful CheckMac or Verify

command with the key to be authorized as the target key of the command. CheckMac must

be run with Mode<1> set to one, or Verify must be run in Stored mode to set AuthKeyID to

the value in the KeyID parameter to these commands. The CheckMac and Verify

commands do not clear these bits on an unsuccessful authorization attempt unless the keys

also happen to be used as the source key.

AuthValid is cleared under the following situations:

–

The device enters Sleep mode or power is removed.

–

Any command is executed that uses a key requiring prior authorization, regardless of which

slot has been authorized and/or which slot was required to be authorized for this key. If there

are multiple state or configuration errors preventing the proper execution of the command,

then AuthValid may or may not be cleared depending upon the specific error conditions

encountered.

ATECC508A

Device Organization

Datasheet Complete

DS20005927A-page 22

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