Building on Secure Intents and Transaction Validation principles, we show how Multi-Party Computation (MPC) hardens both custody and governance while remaining universal and chain-agnostic. Previously, we discussed how to make sure that something you sign equals something you intended; in the following, we explore how to ensure that no single entity can force this signature.

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We remain within the same model. A user submits an intent (e.g., transfer 100 ETH to an external address). The system asks the user to confirm the human-readable intent on the mobile device. Then a policy engine evaluates the request; amounts above certain limits, external addresses, or high-risk operations may require additional approvals from other users. Only after the required quorum approval can we craft the raw, unsigned transaction. Just before signing, the transaction is decoded and validated to confirm that it still matches the approved intent - including asset values, fees, addresses, and other parameters.

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To avoid a single point of trust, we distribute it across several MPC Policy Authority (MPA) nodes. These independent nodes have three responsibilities:

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• Enforce policy on the intent.
  
  
• Check that an unsigned transaction matches an approved intent.
  
  
• Produce a signature via a threshold signature scheme (TSS).

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For every operation, each MPA node independently evaluates the policy and verifies whether the intent matches the prepared unsigned transaction. If there is any disagreement between MPA nodes, signing is aborted. Only a transaction that matches the initial intent across all MPA nodes will be signed.

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With MPC, trust is scaled out and distributed. In prior approaches, much depended on the validation and security of the signing entity. With MPC, trust rests on two independent “rails” that must both succeed for assets to move:

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• Quorum approval in accordance with policy.
  
  
• Threshold cryptography across multiple signing entities.

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Changing the policy, adding approvals, and modifying the overall state of the system are subject to the same workflow.

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This MPC-based design is universal and chain-agnostic. The policy engine, the approval quorum, transaction validation, and MPC signing happen off-chain. Onchain, the transaction looks like it was produced by a standard externally owned account (EOA). This allows broad protocol coverage and predictable gas costs.

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Furthermore, it allows custody distribution. No single party assumes custodianship of a key and, therefore, of the funds it protects. From the signing perspective, the key shares can be distributed between an end user and a service provider. From a quorum perspective, the approval flow can be shared between an exchange and a custodian for settlement-based transactions.

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