Beginning with SGOS 5.3, Blue coat offers the option of using signed system images when upgrading. This option can be selected in the Maintenance > Upgrade tab.
A signed system image is one that is cryptographically signed with a key known only to Blue Coat, and the signature is verified when the image is downloaded to the system.
Blue Coat is currently providing signed and unsigned images for SGOS releases 5.3 and later. At some point, only signed images will be offered.
When upgrading from a pre-5.3 version, you will need to upgrade to an unsigned version of 5.3 before upgrading to a signed version of a later release.