STSAFE-A110 datasheet(11/36 Pages) STMICROELECTRONICS

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STSAFE-A110

Asymmetric cryptography use cases

2.2

Key establishment

This use case shows how to generate the same shared secret in the local host and in the

remote host without having to exchange it. The principle consists in generating two ECC

ephemeral key pairs on both sides. Then, after exchanging the public keys of these two key

pairs, the local host and the remote host run an ECDH scheme to calculate the shared

secret.

The goal of this use case is to share a secret between the local host and the remote server

using the elliptic curve Diffie-Hellman (ECDH) scheme with a static key in the STSAFE-

A110. The STSAFE-A110 also supports ECDHE that utilizes an ephemeral key, but this is

not illustrated.

The shared secret should further be derived to one or more session keys, but this is not

illustrated here. The session keys can then be used in communication protocols like TLS for

example for protecting the confidentiality, integrity and authenticity of the data that are

exchanged between the local host and the remote server. Below are some examples of

data:

•

From the local host to the remote server: power consumption of a smart meter, alarm of

a fire sensor or blood pressure data of a health sensor.

•

From the remote server to the local host: activating the recharge of the battery of an

electric car, activating home appliances like air conditioning or water heaters, or

pushing firmware upgrades to IoT devices.

Because 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 3.1: Host secure channel setup

use case. 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-A110.

Command flow (see Figure 6)

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-A110’s certificate.

The remote server verifies the STSAFE-A110’s 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-A110’s public key.

The remote server then computes a shared secret (Z) by doing a scalar multiplication

of the Host’s Private key with the STSAFE-A110’s public key.

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

The local host computes the Host’s C-MAC for the Establish Key command

The local host sends the STSAFE-A110 an Establish Key command providing the

remote host’s public key appended with the previously computed host’s C-MAC. The

STSAFE-A110 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.

The local host reads the STSAFE-A110’s answer and deciphers the shared secret (Z)

with the locally stored host’s cipher key.

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

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