ATECC508A datasheet(85/109 Pages) MICROCHIP

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•

External Message Generation (Mode<7> is 1)

–

The system should externally compile the information to be signed and compute the digest of

that information using an external hash algorithm. This digest should then be loaded into

TempKey using the Nonce command. The ATECC508A cannot compute the SHA-256 digest

of a random external message.

–

External signatures must be enabled using SlotConfig.ReadKey<0> or else this command will

return an error.

•

Internal Message Generation (Mode<7> is 0)

–

The message to be signed is internally generated. A typical use for this mode is to sign an

internally generated random key.

–

The message is comprised of the output of the GenDig or GenKey commands (stored in

TempKey), plus various other state information according to the description below. If TempKey

is invalid or if it was not generated using GenDig or GenKey, then this command will return

an error.

–

Internal signatures must be enabled using SlotConfig.ReadKey<1> or this command will

return an error.

–

If the data zone has not been locked, then internal signatures will always generate an error

code.

The slot indicated by this command must be configured via KeyConfig.Private to contain an ECC private

key, and SlotConfig.IsSecret must be set to one or else this command will fail.

There is a small statistical probability that the device will fail to properly generate the ephemeral key, in

which case this command will return a single byte containing the ECC fault code (see Table 9-3). The

command will generally succeed if resubmitted.

Table 9-49. Input Parameters

Name

Size

Notes

Opcode

Sign

0x41

Param1

Mode

See Table 9-51.

Param2

KeyID

The internally-stored private key to be used to generate the signature.

Table 9-50. Output Parameter

Name

Size

Notes

Response

The signature composed of R and S, or an error code.

Table 9-51. Mode Encoding

Bits

Meaning

0 =

The message to be signed is internally generated as below.

1 =

The message to be signed is in TempKey and sign extended as above.

0 =

48 Message bits corresponding to SN<2:3> and SN<4:7> are set to zero.

1 =

Include the 48 bits SN<2:3> and SN<4:7> in the message for internal signatures.

This bit is ignored if mode<7> is one.

ATECC508A

Security Commands

Datasheet Complete

DS20005927A-page 85

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