In the wake of a significant entropy bug affecting Coldcard wallets, self-custody advocates and security experts are reassessing standard custody practices. The vulnerability, which went undiscovered since at least 2021, has led many within the Bitcoin community to question long-held assumptions regarding single-signature wallets.
The Threat Model and Single-Sig Vulnerabilities
Self-custody is widely recommended to protect funds from exchange failures, but recent events have driven users to reevaluate their security setups. According to Casa CEO Nick Neuman, approximately 233,000 bitcoins were moved to safety following the Coldcard security issue.
Security practices often begin with developing a personal threat model, which involves assessing potential risks, evaluating their likelihood, and determining the catastrophic impact they might have on funds. Historically, common causes of fund loss include user error, poor backups, lost passwords, and targeted theft. Bad entropy attacks rank among the most successful exploits against self-custody, with several wallet providers having suffered similar vulnerabilities.
Rise of Multi-Vendor Multisignature Solutions
In response to hardware manufacturer errors, multi-vendor multisig has emerged as a widely recommended standard for long-term Bitcoin holders. Multisig wallets protect users by requiring valid signatures from multiple private keys and distinct devices before a transaction can be processed on the Bitcoin network.
Multi-vendor multisig theory dictates that every keypair used to construct a Bitcoin multisig address should be generated by a different wallet vendor. For example, a setup might use a Trezor device for one key, a Ledger device for a second key, and a recovery key from a specialized multisig provider, operating under a threshold requirement such as two signatures out of three.
By utilizing multiple hardware providers, users minimize trust in any single manufacturer. Prominent multisig wallet providers and interfaces include Casa, Nunchuck, Sparrow desktop wallet, and Unchained Capital.
Benefits and Drawbacks of Multisig
Beyond mitigating hardware vulnerabilities, advanced multisig configurations offer potential resistance against physical extortion or "wrench attacks." By incorporating multi-jurisdictional key placement, geographical distribution, or time-locked recovery keys, users can introduce spending delays that protect against fast-paced coercion and phishing schemes. Additionally, structured multisig setups have enabled novel forms of Bitcoin-denominated insurance, such as offerings from AnchorWatch.
However, multisig setups introduce unique complexities. Users must securely store a copy of the multisig script or template alongside their key material. This template is necessary to recreate the smart contract and spending conditions independently should a third-party multisig provider go offline.
