Ask HN: How to do a secure file transfer with contractually binding receipt

I'm trying to find/figure out if there is legislation (obviously regionally different) and protocols, maybe even implementations of a system that guarantees the delivery of files containing a specific filetype. The healthcare institutions we deal with are asking for SFTP and the in house lawyers flat out denied. Obviously they could agree on some some of process, but they have no interest in thinking of any sort of process, nor look for any sort of existing legislation. One app I saw is basically a broker that lets you send signed receipts that you attach to an email and then has a file sharing platform tied to it, is HIPAA certified, potentially on premise etc. I'm wondering if it's possible to cut out the broker. I have some ideas of how I would do it, but I'm trying to get a standard written that our standardization agency could then pass and force feed back to our lawyers (as opposed to what they would do, not read up on any technology, come up with a shitty solution and force feed them to the IT Dpts). To summarize the questions: 1. Is there legislation used in departments or countries for the definition of a legally binding send receipt? 2. Is there a need for a new or encapsulation of old file transfer system and if so is there an RFC or sample somewhere? 3. Is there open source software that does this?