Shamir secret sharing

Shamir secret sharing, published by Adi Shamir in 1979, splits a secret into n shares with a threshold k. Any k shares rebuild the secret; any k − 1 reveal nothing about it, not even a hint.

For example, with a 3-of-5 split, five people each hold a share. Any three can restore the key together. Two can't, and losing two shares doesn't lose the key.

Good practice

• Give shares to different people, in different places.
• Choose a threshold that survives losing shares but needs real cooperation to use.
• Rehearse recovery, so you know it works before you need it.

Shamir sharing protects a secret at rest. To use the key it must be rebuilt in one place, which is why it's used for backups. Signing without ever rebuilding the key is what MPC does.

On Valtu

The backup of Valtu's production keys, created in the key ceremony, is split with Shamir secret sharing and held offline. Day-to-day signing happens inside hardware enclaves, so the key is never rebuilt outside one.