How Seedcase works.
From a simple idea to mathematical certainty. Here's the full picture.
Curve of degree 2, as a threshold of 3 requires. Any three shares redraw it and reveal the secret at f(255); any two leave it undetermined.
Shamir's Secret Sharing.
In 1979, cryptographer Adi Shamir solved a fundamental problem: how do you split a secret so that no single piece reveals anything, but enough pieces together reveal everything? His answer was a mathematical scheme based on polynomial interpolation. Any M fragments reconstruct the original. Any fewer and the secret remains completely out of reach, not by luck, but by mathematics.
Seedcase uses SLIP-39, a standard created by SatoshiLabs, also implemented by hardware wallets like Trezor. Each fragment becomes a sequence of words, easy to write down or store on any medium.
SLIP-39 has been audited and reviewed by the security community for years.
Read the SLIP-39 specificationYou decide the threshold.
Choose how many fragments to create and how many are needed to reconstruct. Every threshold is mathematically valid.
Example of a 3-of-5 scheme. Five fragments are created, only three are needed to reconstruct the seed phrase.
Personal use
Three fragments, any two reconstruct. Simple, resilient, ideal for everyday peace of mind.
Family redundancy
Five fragments distributed across trusted locations and people. Survives multiple losses without compromising security.
Institutional
Seven fragments, five required. Maximum redundancy for high-value or shared-control scenarios.
A second lock.
When you create your fragments, you can protect the whole set with a single password, the same one for every fragment. Even someone who gathers enough fragments to reconstruct your seed phrase gets nothing without it.
We never store it. It doesn't exist anywhere except where you put it. The threshold scheme and the password protection operate independently: breaking one doesn't compromise the other.
Organize fragments into groups.
Real-world trust isn't flat. Your spouse, fully? Your siblings, a little less? A notary, within their professional role? Each circle should play a different role in recovering your seed.
Seedcase lets you organize fragments into groups. Give each circle its own group, with its own threshold. For example: in the family, any two of three are enough. Among the professionals, one of two is enough. You need both groups to recover the seed. The family cannot do it without a professional, and a professional cannot do it without the family.
Store each fragment where it makes sense.
Paper, metal, computer: picking a medium is the easy part. What really matters is deciding where each fragment goes, and who you trust with it. That allocation is what decides whether your scheme survives a fire, a burglary or a remote attack. Four rules help you get it right, at any threshold.
One location per fragment.
If two fragments share a roof, your threshold is lower than it looks: a single disaster destroys both. Your home, a bank vault, the house of someone close: locations that won't disappear on the same day.
Mix the kinds of trust.
Spread your fragments across yourself, the people close to you, and an institution. Each one can fail, but never for the same reason: an accident leaves you unable to act, a falling-out distances a sibling, a firm closes its doors.
The right medium for the role.
A fragment you store away for years, untouched, is worth engraving on metal: it survives a fire as well as a flood. The one you reach for regularly can live on the computer you already use. Not every fragment has to face the same risks.
Prefer offline storage.
Fragments kept on a device connected to the internet are the most exposed ones. Prefer keeping them offline: on paper, on metal, or on a device that never goes online. Your threshold then stays out of the network's reach.
Custodians, not just objects.
A fragment doesn't have to sit on an object you own. You can hand it to a custodian, such as a notary: a sealed fragment deposited with a legal professional is institutionally recognized and independent from any digital infrastructure. It is also useless on its own. That fragment becomes necessary to reconstruct the seed phrase, without ever giving the notary access to your funds.