Keyfob: Split Your Phrase

A recovery phrase on one piece of paper has two ways to end badly: the paper is lost, or you are, and nobody left knows what the paper was for. Keyfob is for both, and it does nothing else. WHAT IT DOES You already hold a recovery phrase. Keyfob turns it into a set of cards and spreads them across people and places, so losing any one of them is a shrug rather than a catastrophe. Choose how many cards exist and how many are needed to rebuild the phrase: three of five is the usual answer. Any two cards, held apart, tell an attacker nothing at all. WHAT IT IS NOT It is not a wallet. It never sees an address, a balance or a transaction, and it cannot move anything. It cannot even reach the network to try: the app declares no internet permission, which you can check on this listing before you install it. THE MATHEMATICS, PLAINLY Your phrase is a number. Keyfob picks a random curve through that number and hands each guardian one point on it. Fewer points than the threshold and every possible phrase stays exactly as likely as it was before, which is a stronger promise than encryption can make. Reach the threshold and the curve, and the phrase, come back. This is Shamir secret sharing over the field of 256 elements, the same arithmetic behind SLIP-39 hardware wallet backups. CARDS A HUMAN CAN HANDLE Each card is written in the same 2048 BIP-39 words a recovery phrase uses, so a guardian is copying out one familiar kind of thing. The card carries its own number, the threshold and a checksum, so a mistyped or swapped word is caught before anything is built on it. Print them, write them out, seal them. THE PLAN, WRITTEN DOWN Who holds which card, where it lives, and what the person who has to do this without you should do first. A letter you write once, kept with the plan. A rehearsal drill: ring the people holding the cards, type them back in, and watch the app rebuild the phrase and check it against its fingerprint without ever showing it. A recovery plan nobody has practised is a wish. There is also a short quiz that checks you understood what you just set up, and a threat model in plain words about what this defends against and what it does not. WHAT IT LOOKS LIKE One drawing carries the whole app: a key, broken cleanly into as many shards as you have cards. A shard is solid when its holder has confirmed the card, hatched when nobody has checked lately, and drifted out of line when it is gone. During a drill the shards light up one by one as the cards come in, and when the threshold is reached the key is whole again for a moment. YOUR PHRASE IS NEVER STORED It is held in memory only while the cards are being built and is gone the moment you leave that screen. It is never written to storage, to a log, or to any file the app makes. The plan and the guardians' names stay on your device. There is no account, no cloud, no analytics and no advertising ID. Free and complete. A tool for the day something goes wrong should not have a subscription.