Let’s explore the differences between BIP-39 and the new Shamir backup offered on the Trezor Safe 3 & 5 models.
Trezor now defaults to the Shamir Backup Scheme in its "Trezor Safe" product line.
Background
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Shamir Secret Sharing (SLIP-0039) is a cryptographic method that allows you to split your Bitcoin wallet backup into multiple shards (shares). You can configure it to require x of n shares to recover your wallet.
On the Trezor Safe models, this means users are prompted to create a 20-word seed backup instead of the traditional 12- or 24-word BIP-39 seed.
If you need a multi-share wallet, Shamir can be a great choice. If you need a multi-signature wallet, Electrum’s multisig wallet scheme, which works with all Trezor wallets. (Note: Multi-share is recovery of a single key only, while multi-signature enables multiple keys to device to signs transaction.) While Electrum supports restoring wallets with Shamir backups, it does not support signing transactions for a Shamir wallet on a Trezor.
Pros and Cons of Using a 20-Word Shamir Seed for a Single-Share Backup vs. BIP-39
Cons of Shamir Seeds:
- No Additional Security for Single Share:
- A single-share Shamir backup offers no extra security compared to a BIP-39 backup. Both use 256 bits of entropy, so you’re managing more words (20 vs. 12/24) without gaining additional cryptographic strength.
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**Shamir Secret Sharing is a recovery solution only: **you cannot use Shamir to allow multiple people to co-sign transactions (aka multisig).
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Limited Software Support:
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While Electrum can restore Shamir wallets, it cannot use them for signing transactions on a Trezor.
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To my knowledge, no wallet clients currently support signing transactions for Shamir wallets on a Trezor.
- Locked into the Trezor Ecosystem:
- Neither Ledger nor Coldcard supports Shamir, and there seems to be resistance to adopting Shamir in other ecosystems. Coldcard prefers SSKR (a different splitting scheme).
Pros of Shamir Seeds:
- Future Multi-Share Flexibility:
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If you decide to move to a multi-share Shamir backup, check Trezor's current Shamir backup documentation before assuming you can expand an existing seed into multiple shares without regenerating it and moving your coins.
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For instance, you could keep your personal wallet as a single share and add shares for a shared business wallet with partners.
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Caveat: To secure existing coins with multi-share Shamir, you must still transfer your funds to new addresses derived from the multi-share configuration.
- Native Support in Trezor Suite:
- Shamir is the only multi-signature scheme fully supported by Trezor Suite, making it a convenient option for users already committed to the Trezor ecosystem.
- Multi-share metal seed backups are now available from Trezor. These backups make it easier to create backups of up to 16 shares.
Conclusion:
A single-share Shamir backup may not provide additional security over BIP-39, and its limited compatibility with third-party wallets is a significant drawback. However, it offers flexibility if you plan to adopt a multi-share setup in the future, particularly within the Trezor ecosystem.
For users who value simplicity and broad compatibility, **BIP-39 remains the gold standard for a single-share wallet. **However, if you prefer a multi-signature solution, don’t mind being locked into the Trezor ecosystem, and don’t require the advanced transaction features of Electrum, Shamir Sharing is a good choice.