The options for comparing user entities doesn't include the signature. Here is a patch that adds the function.
Context: In updating the User Revision module I have added diff support, which makes use of the user entity comparison functions in Diff. The signature is the only embedded value that wasn't supported, hence my reason for adding it.
Closed: outdated
3.0
Code
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.
Given the proximity of EOL for Drupal 7, marking this as won't fix. The likely final release of diff has landed in https://www.drupal.org/project/diff/releases/7.x-3.5 → . If this is something still applicable to a more modern version of the module, feel free to re-open and post additional notes.