[pmarreck]

Arrange a secret phrase in advance- ideally generated randomly. Stick it up on the wall of the aging parent or grandparent- maybe in the bedroom, where guests are unlikely to go. Make it innocuous-looking (hidden in plain sight). Require that phrase to be said to prove identity. Reset it if it ever gets used on a call legitimately.

[ButlerianJihad]

Personally, I require all my aging grandparents to carry a Yubikey, with an identical one always stored in a safe-deposit box. Then, on demand, they simply mate their Yubikey with a specially-prepared GrapheneOS device, open their Firefox app, and connect to the dedicated mesh network, run by and for aging grandparents. Then they run their right ring finger over the fingerprint sensor, but it must be done in a Morse-code pattern that matches their unique tattoo (I am unable to divulge the location or encoding of this tattoo). Once these conditions are met, their physical presence is confirmed by the Yubikeys of at least 2 other aging grandparents of equal or higher reputation.

It's really simple and straightforward, and there is no need to really document the workflow here, because all the aging grandparents are extensively trained and drilled every two weeks, by the aging great-grandparents who've been using this same exact system for the past 50 years.

[ButlerianJihad]

Seriously though, I'd like to make two points:

1: Your family members already have shared "secrets" if they communicate regularly. It could be pet names, terms of endearment, shared experiences, unique monikers for things. It's language that is already familiar and you already use quite often. You should leverage that, and rely on that familiarity in a crisis, rather than trying to contrive something special for crisis-only ID. The attackers' greatest weapon is your own confusion, your own willingness to believe, and creating a sense of urgency. Your attackers' 3 greatest weapons. Don't panic.

2: My maternal grandmother was widowed and lived alone for decades. She had certain ways of knowing things. For example, every time we'd come home, she would test the doorknob. If some stranger had come and tried to jiggle the doorknob, we could tell by its feel. Just a simple mechanical giveaway. When Mom and/or Dad came by, they rang the doorbell by a special pattern. It wasn't complex, but it was distinct and recognizable from inside. It wasn't a securely encrypted ID, just a "secondary ring" that was unlike a stranger's touch on the doorbell button. And, of course, my parents can always interpret the antics of their indoor cat, in regards to who is approaching the house by car or on foot...

[pmarreck]

LOL.