Depending on the personalization profile, the establish key command needs to be MACed and its answer is

encrypted to avoid eavesdropping on the shared secret. The scenario assumes that the local host has set up a

host C-MAC and cipher keys as described in Section 3: Pairing with local host. It is also assumed that the local

host knows the host C-MAC sequence counter; if not, it can send a query command to the STSAFE-A120.

Command flow

1. The remote host server sends its certificate to the local host. The local host extracts the public key and can

optionally verify the validity of the certificate. In its response, the local host sends the STSAFE-A120

certificate.

2. The remote server verifies the STSAFE-A120 X.509 public key certificate with the CA public key (the host is

responsible for getting this key). When the verification succeeds, the remote server has an authentic copy of

the STSAFE-A120 public key.

3. The remote server then computes a shared secret (Z) by doing a scalar multiplication of the host’s private key

with the STSAFE-A120 public key.

4. The remote server requests the local host to establish a secure connection.

5. The local host computes the host’s C-MAC for the establish key command

6. The local host sends the STSAFE-A120 an establish key command providing the remote host’s public key

appended with the previously computed host’s C-MAC. The STSAFE-A120 does the same operation as the

remote host server, and performs the scalar multiplication of its private key with the remote server’s public key

to compute the shared secret (Z). It then encrypts the response using the host’s cipher key.

7. The local host reads the STSAFE-A120 answer and deciphers the shared secret (Z) with the locally stored

host’s cipher key.

8. The remote host server and the local host have a shared secret Z.

Figure 7. Key establishment command flow

IoT device

Remote server

Local host

(if needed)

STSAFE-A120

Host

Extracts public key

from certificate

Computes Z = scalar

multiplication of host private key

with STSAFE-A120 public key

I2C

RF or hardwired connection

1. Read STSAFE-A120 certificate

Certificate

Sends STSAFE-A120 certificate

Sends host public key

Establishes secure channel

Computes host C-MAC for

Establish Key command

2. Establish Key (Host public key)

Ciphered Z

Deciphers Z

Done

2.5

Entity authentication

An STSAFE-A120 device can authenticate an off-chip entity by verifying a digital signature generated by this

entity with a private key. This is done over a challenge that was previously generated by the STSAFE-A120.

STSAFE-A120

Product use cases

DS14609 - Rev 1

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