I am currently working on creating a digital signature pad that customers can sign instead of a traditional paper receipt for credit card transactions. Currently I have the ability for them to sign a mobile device via a webpage. I am using this signature pad to do so: https://github.com/szimek/signature_pad. I was wondering if anyone would be able to provide me with advice on how I can prove that the signature is valid and was intended for a specific transaction in the event of a charge back from the merchant. Also our employees will watch all customers as they complete the signature and the signature will never be made on their own device so this will limit the chance of tampering by a malicious payer.
I am currently thinking there may be a way for me to create a text based image listing the total and date for said transaction. Digitally sign that image in someway. Then have the customer physically sign on top of the secure image and then digitally sign and timestamp the signature to prove that they signed on a secure image between two time stamps. The issue is I am not completely sure how I would go about doing this. Also I'm not terribly familiar with the evidence required by the credit card companies in the event of a charge-back so it may be a situation where even if I can prove the signature electronically they may not want to spend their time verifying my method on a single transaction.