I have some little background on signatures validation. Signature validation is a different kind of a scenario that require special image recognition techniques to compare the signatures. There are some proven techniques and software applications that are specialized on signature validation.
The best option for you to take in this scenario would be, do some research on what signature validations software are available. In those techniques, check whether those have a different API kind of a method which you can use to connect UiPath with the software so that you can use UiPath to send signature data and get the validation result back. This would be the best and the most effective way of getting it done. If you are to so it through RPA only, it will not be possible. The best would be to integrate RPA mechanisms with those software systems to get the job done