How an MPC wallet signs
The shares are created by a joint protocol, so the full key is never assembled, not even at the start. To sign, a set number of share holders, for example two of three, exchange messages and each computes part of the signature.
The result is one ordinary signature. The blockchain sees a normal address and a normal transaction, and cannot tell that several parties were involved.
Shares can be refreshed, replaced with new ones for the same key, so that a share stolen earlier stops being useful. A holder can also be added or removed without changing the address.
Why custody providers use MPC
There is no single machine to steal the key from, and no single employee who can move funds. An attacker has to compromise several independent systems at the same time.
MPC works on any blockchain whose signature scheme the protocol supports, because the chain itself needs no special feature. Approval policies, such as limits, allow-lists and who must take part, are enforced by the share holders off-chain.
The same technique sits behind many embedded wallets: one share on the user's device and one with the provider. Whether that amounts to custody depends on whether the provider could ever sign alone.
MPC, multisig or HSM?
- Multisig
- Several complete keys, each producing its own signature, checked by the blockchain. Visible on-chain, and dependent on each chain's support.
- Hardware security module (HSM)
- Keeps a whole key inside tamper-resistant hardware. Often combined with MPC, each share sitting in its own module.
- Shamir's secret sharing
- Splits a key for backup, but the key has to be put back together in one place to sign. MPC never reassembles the key.
Where multi-party computation shows up in a platform
Modules that do the work
- Custody & walletsWallets on every major chain, with MPC custody from leading providers or your own keys.
- AuthenticationSign-in, passkeys and second factors for customers and staff, on your identity provider or ours.
- BackofficeOne console to run the platform: customers, money, risk, content and staff permissions.