[jimmySixDOF]

Not sure if anyone is doing that as a service now but the old methods like post it to yourself in a sealed envelope or email are just as legally binding (ie not binding at all in any way) as the most sophisticated hashed blockchain you can throw at the problem today. There's a reason lawyers use signatures on paper over registered mail. We are talking about a part of the law where the last technical advance was to accept Fax Machines.