I started to ask this question in the Writing SE, but realized that I want both a method and a mechanism, so decided that for the latter, I'd come over here.
I'm building a novel where the MCs are transported, unprepared, from the 24th Century to the 14th. They decide (the details of why aren't important here) to establish a business that will grow over the centuries. This has been taken care of. Afterward, they attempt to return home and find themselves 200 years closer to their starting point, but clearly not all the way there. They need to access their resources, particularly finances.
What mechanism could they have established in the late 14th C. that would allow them to clearly identify themselves as approved to have access to the funds that have been growing that entire time?
I've thought of a seal, or perhaps a seal and a passphrase. Both of those are simple enough, but not necessarily secure, since a signet ring or stamp could be reverse-engineered from the result. Is there a way to document, in a way that a 14th or 16th Century accountant could understand, something like a rudimentary public/private keypair system?