Once the entity has been authenticated, the STSAFE-A120 grants access to the commands and to the data

partition zones that require the authentic entity status to be true.

This functionality requires that the STSAFE-A120 contains an authentic copy of the public key that corresponds to

the private key used by the signature generation process of the off-chip entity.

Figure 8. Example of entity authentication

Remote host

Peripheral

STSAFE-A120

Wire or wireless

Local host

Challenge

Verifies signature with the stored

public key, and grants access to

commands and data zones that

require authentic entity

Computes

challenge’s

signature with

private key

I²C

1. Generate challenge

2. Verify entity sIgnature (public key slot, signature)

2.6

Public key signature verification

STSAFE-A120 offers signature verification services for host devices that does not support ECC. These services

can be used to allow an IoT device to verify the authenticity of a remote server or of the local host firmware that is

used at secure boot or loaded for a firmware update.

The verify signature command uses ECDSA or EdDSA with curves defined below:

•

Curves for usage with ECDSA, EdDSA, and ECDH:

–

NIST P-256 P-384, P-521

–

Brainpool P-256 P-384, P-521

–

Edwards 25519

–

Curve25519

Signature verification with EdDSA is implemented as defined in RFC8032. The host must indicate in the

command data whether the verification must be done with the pure Ed25519 variant in which case the host must

give the message in the command data or with the prehashed Ed25519 variant in which case the host must itself

compute the hash of the message and give the hash in the command data.

The host must always give the public key and the hash in the command data except for the pure Ed25519

signature verification where the message itself (not the hash) must be given in the command data.

One use case of the verify signature command is illustrated in the command flow and in the figure below where a

local host authenticates a remote host using STSAFE-A120.

Command flow

1. The local host asks for the remote host public key certificate and the remote host responds.

2. The local host extracts the public key, generates a challenge (random number), and asks the remote host to

sign this challenge. The remote host responds with the signature.

3. STSAFE-A120 verifies the challenge's signature using the extracted public key, and replies with "valid" or

"invalid".

STSAFE-A120

Product use cases

DS14609 - Rev 1

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