Fintech · Yuki

Custody: from encrypted keys to MPC and smart accounts

How Yuki's wallet custody evolved: envelope-encrypted keys to ship fast, then a composable V2 built around threshold signing and on-chain smart-contract wallets.

Founder · CEO · Product lead

The decision

Every wallet product starts with one question: who holds the key? The answer decides the security model, what happens when a user loses a phone, the gas bill and the regulatory posture. Yuki answered it twice.

Version one: ship with encrypted custody

The first platform generated a wallet per customer and kept its key material encrypted at rest with a managed key service, so keys never sat in the database in the clear and decryption required a separate, audited service. It was the fastest credible way to put stablecoin wallets in partners' hands, and it is what carried Yuki's production traffic.

Version two: composable MPC

Celo described the V2 direction as "composable MPC wallet-as-a-service infrastructure". The redesign split the monolith into composable services, a gateway and a wallet service talking over a message broker with typed contracts, so custody could be swapped without touching the rest. Behind it, the team explored three custody primitives side by side:

Threshold signing (MPC). The key never exists in one place; independent parties each hold a share and sign together. No single breach can move funds.
Smart-contract wallets. The wallet is a contract with its own rules: recovery, spending limits, sponsored gas. Prototyped and deployed on test networks.
Multi-chain HD wallets. One seed deriving accounts across several EVM chains, so a customer has one identity everywhere.

The trade-off

Encrypted custody is simple and fast but concentrates trust. MPC removes the single point of compromise at the cost of coordination and latency. Smart accounts give the best user experience and recovery, but every rule costs gas. Building V2 as composable services meant Yuki could offer partners the model that fit their licence, instead of forcing one answer on everyone.

System

Wallet-as-a-service for African fintechs

One API that gives any fintech wallets, stablecoins, on/off ramps and KYC for its customers, normalising a patchwork of regional providers behind a single contract. 20,000+ requests a day.

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System

Stablecoin swaps and DEX pricing

Local-currency stablecoin swaps inside a mobile wallet, and an event-driven pricing library that quotes across dozens of exchanges from memory instead of hammering nodes.

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Custody: from encrypted keys to MPC and smart accounts

The decision

Version one: ship with encrypted custody

Version two: composable MPC

The trade-off

Related systems

Wallet-as-a-service for African fintechs

Stablecoin swaps and DEX pricing

The decision

Version one: ship with encrypted custody

Version two: composable MPC

The trade-off