What is a Shamir backup (SLIP-39)?
A backup split into several word lists, so any threshold of them — say 2 of 3 — restores the wallet, while one alone reveals nothing.
Instead of one list of recovery wordsRecovery words (seed phrase)A list of 12, 15, 18, 21 or 24 words that can restore wallet keys. Anyone who has them can take your bitcoin. to lose or have stolen, a Shamir backup makes several shares. The wallet restores from any threshold of them; a single share on its own gives an attacker nothing. It's based on Shamir's secret sharing, a 1979 cryptographic scheme.
SLIP-39 is the standard many Trezor wallets use for this. It is not the same as multisigMultisig (multi-signature)A wallet whose coins need signatures from several keys, such as 2 of 3, so no single key can spend alone.: the shares rebuild one key, rather than several keys signing separately. Each share needs the same careful, offline storage as recovery words.
References
- SatoshiLabs, “SLIP-0039: Shamir's Secret-Sharing for Mnemonic Codes”.