STSAFE-A110 datasheet(14/36 Pages) STMICROELECTRONICS

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Asymmetric cryptography use cases

STSAFE-A110

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DS13039 Rev 1

The TLS server sends the Server Key Exchange message including the Diffie-Hellman

public key of the TLS server and a signature over the server key exchange parameters.

Upon reception of this message, the local host of the TLS client may use the STSAFE-

A110 for verifying the signature with the Verify Signature command (3) and the same

mechanisms that were applied in step 2.

When the signature is valid, the local host may use the Generate Key command 4 of

the STSAFE-A110 for generating an ephemeral key pair in the STSAFE-A110. The

command data take a reference to the curve that must be used, and the response data

includes the public key of the freshly generated key pair. The Generate Key command

(4) requires a Host C-MAC in the command but this is not illustrated here (see

Section 3.1).

The local host can now use the Establish Key command (5) of the STSAFE-A110 and

give the public key of the TLS server in the command data. The response data contains

the shared secret that is computed with ECDHE using the ephemeral private key in the

STSAFE-A110 and the public key of the TLS server. The shared secret in the response

data is encrypted with the Host’s Cipher key and the Establish key command also

requires a Host C-MAC. The local host must compute the Host C-MAC and decrypt the

response data to obtain the plain-text shared secret that can be used as the pre-master

secret of the TLS handshake protocol. The mechanisms linked to the Host C-MAC and

Host’s Cipher key are not illustrated here (see Section 3.1). The local host can now

derive the pre-master secret to the master secret and apply the expansion algorithm to

obtain the key block; these functions, however, cannot be executed by the STSAFE-

A110.

The TLS server sends the Certificate Request message including the signature and

hash algorithms that are supported by the TLS server. The TLS server also sends the

Server Hello Done message. The TLS client sends the Certificate message including

the X509 certificate chain of the TLS client. This chain may include the X509 certificate

of the static private key of the STSAFE-A110, which can be read from it with a Read

command (6). This command can typically be executed upon setup of the IoT device

and can then be cached by the IoT device so that there is no need any longer to read it

from the STSAFE-A110. In case of long certificates, the Read command may be sent

multiple times but this is not illustrated in Figure 7.

The TLS client sends the Client Key Exchange message including the ephemeral

Diffie-Hellman public key that was obtained in step 5 in the Establish Key response

data from the STSAFE-A110. The TLS client sends the Certificate Verify message

including a signature over all handshake messages that have been exchanged so far.

The local host may use the STSAFE-A110 for generating this signature. The local host

must therefore hash the To Be Signed message and send it to the STSAFE-A110 in the

command data of the Generate Signature command (7). The STSAFE-A110 uses its

static private key for generating the signature that is returned in the response data.

The handshake protocol continues without any further interaction with the STSAFE-A110.

•

The TLS client sends the Change Cipher Spec command.

•

The TLS client sends the Finished command including the verify data computed with

the Pseudo Random Function, the master secret and all handshake messages

exchanged so far.

•

The TLS server sends the Change Cipher Spec command.

•

The TLS server sends its own Finished command that must be verified by the TLS

client.

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