Image Integrity Checking

This feature adds digital signature verification in ExtremeXOS image and XMOD modules. Image integrity is checked against the digital signature before actual installation.

Digital Signature

Prior to ExtremeXOS 16.1, the ExtremeXOS image or XMOD modules downloaded to the switch were only protected by CRC check. This is sufficient for checking corrupted image. However, it cannot prevent malicious attacks. For example, an attacker can de-package the image, replace certain fields, recompute CRC and then repackage the image.

Digital signature is commonly used to demonstrate of the authenticity of a digital message, in this case, the image downloaded to the switch. Only images with digital signature validated on the switch can be installed. Otherwise, the installation should be aborted.

This features uses the Public Key Infrastructure (PKI) approach. Specifically, with the RSA algorithm, two keys, the private key and the public key are generated using openssl utility. The ExtremeXOS image or XMOD module is digitally signed with the private key. The public key is installed on the swtich in the format of a X.509 certificate, which is verified before being used.

When building the image, the signature is computed for the ExtremeXOS image or XMOD module, then included in the final image provided to the customer. On the switch, during downloading process, the signature is verified against the image using the public key previously installed.

In order to deliver the public key to the customer securely, it is also digitally signed and it is distributed in the format of a X.509 certificate. In order to do so, another set of keys are generated to sign this certificate. A self-signed root certificate is also installed on the switch to verify the certifcate containing the image signing public key.

All these keys and certificates are generated offline and the private keys should be stored safely.

Transition from an image without signature to one with needs two steps, the first step is to download the EXOS image and install the public key certificates. At this time, the signature cannot be verified because there is no key to validate the image. But after the first installation, all subsequent downloaded images can be validated using the installed key.

The certificates are only included in the ExtremeXOS image; XMOD modules do not need to include certificates.

Downgrading from an ExtremeXOS version supporting digital signature to one that does not is allowed. No special handling is needed. A warning message is printed on the console to remind the user that the image is not digitally singed. The user does have the choice to either proceed with downgrading or not.