Secure Document Validator

Hi Guys

I have created a custom doctype that essentially acts as a confirmation letter. Upon completing the letter a user emails it or prints it out.

My question is how can I prevent fraudulent duplication of the said letter? Can I have a unique random PIN code generated with every letter and have a facility inside ERPNext wherein the person receiving that letter can punch in the said PIN code and have a Boolean query done that validates that letter.

This is important also for Purchase Orders, we have a problem with people faking Purchase Orders and RFQs, if such a solution doesn’t already exist within the community can someone please redirect me to where I can find a solution.