The N870 has default from the factory Client certificates installed on the device.

The Factory installed Client certificates can be found here:

/config/ssl/gigaset_factory_cert.pem
/config/ssl/gigaset_factory_key.pem

The Client certificates can be uploaded from the web-interface, go to: SETTINGS - Telephony - VoIP.

<?xml version="1.0" encoding="UTF-8"?>
<provisioning version="1.1" productID="e2">

<firmware>

</firmware>

<nvm>

</nvm>

<custom>

<step type="certificate" url="<URL to certificate>" key="<URL to Private key>" flags="CLIENT_CERT" />

</custom>

</provisioning>

You can generate your own Client certificates on the N870 CLI if you have no access to a Linux machine.

To create self-signed client certificate you have to follow the instructions below. If you have own CA you can skip the first step.
1. Generate CA certificate and key:

openssl genrsa -des3 -out ca.key 4096
openssl req -new -x509 -days 365 -key ca.key -out ca.crt

and follow the on-screen instructions.

2. Generate Client Key, Certificate Signing Request, and Signed Client Certificate:

openssl genrsa -des3 -out N870.key 4096
openssl req -new -key N870.key -out N870.csr
openssl x509 -req -days 365 -in N870.csr -CA ca.crt -CAkey ca.key -set_serial 01 -out N870.crt
openssl rsa -in N870.key -out N870_key.pem

As Common Name you can put the MAC address of the device (with capital letters).

You can download the N870.crt and N870.pem file using WinSCP.

In the WebUI you have to upload N870.crt and N870.pem files.