Shamir Backup

A Shamir backup splits a wallet secret into several shares of which only a subset is needed to restore it, so a three-of-five scheme survives two lost shares and still resists a single stolen one. It applies Shamir's Secret Sharing, a scheme published by Adi Shamir in 1979, and appears in wallets through the SLIP-39 standard, which encodes each share as its own word list. The advantage over one written seed phrase is that no single location is both sufficient and fatal. The costs are real too: shares must be stored in genuinely separate places, the threshold has to be recorded, and SLIP-39 shares are not interchangeable with a standard BIP-39 phrase, so recovery depends on compatible software.