# Threshold Signature Scheme

## Decentralized TSS Vaults

Helios secures Bitcoin deposits with a **Threshold-Signature Scheme (TSS)** implemented on the MIDL layer. Vault keys are split among an on-chain **quorum of validators**; no single party ever controls a complete key. This design removes the single-custodian risk common in traditional solutions while retaining Bitcoin’s native settlement guarantees.

## Withdrawal Process - Three Mandatory Checks

* **User Intent**: The depositor signs a **BIP-322** message that proves ownership of the UTXO.
* **Validator Co-Signature**: A super-majority of validators jointly generate the TSS signature (t-of-N).
* **Protocol Verification**: Helios smart contracts confirm the action aligns with protocol rules (e.g., loan repayment, liquidation) before broadcasting the transaction.\
  Only when **all three conditions** are met does the vault release funds, ensuring continuous **self-custody** for the depositor.

## Additional Security Safeguards - Slashing & Watchtowers

* **Economic Penalties**: Validators stake BTC that is **slashed** for invalid signatures or censorship.
* **Fraud-Proof Window**: Each outbound spend is subject to a short challenge period, during which **independent watchtowers** can submit fraud proofs.\
  A malicious validator majority cannot move assets without the depositor’s signature, and off-path signatures are rejected because the TSS key functions **only** through the custody module.

## Resilience & Recovery

If validator liveness drops below safe thresholds, the protocol supports **on-chain validator rotation** and **time-locked recovery scripts**, enabling depositors to migrate funds without disruption.

## Institutional Alignment - Seamless, Revenue-Positive Integration

**• Plug-and-Play Stack:** Helios relies on the **TSS primitives** already deployed by many enterprise custody platforms.

**• Policy-Engine Continuity:** Existing **role-based approvals, velocity limits, and whitelists** map one-for-one to Helios contract calls, allowing operations teams to integrate the product without rewriting internal control matrices.

**• New Yield Product:** A simple **“Bitcoin-native earn”** toggle can be added to current dashboards, giving clients access to DeFi returns while their BTC never leaves the base layer—enabling the provider to charge management or performance fees.

**• Compliance by Design:** Real-time **Merkle solvency proofs**, Travel-Rule metadata feeds, and automated audit logs plug into existing reporting pipelines and preserve SOC 2 / ISO accreditation.

**• Risk Ring-Fenced:** Slashable validator stakes, mandatory user co-signatures, and immutable contract guardrails eliminate rehypothecation risk and satisfy fiduciary capital-protection mandates.

**• Competitive Differentiation:** Offering native BTC yield—without bridges or wrapped assets— that allows **1-block withdrawal** instead of long staking period broadens the custody provider’s product suite and positions it ahead of peers still limited to wrapped-token DeFi.


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