DA-04 · LESSON 05

Coordinator trust boundary

KEY DEFINITIONS

Terms used in this lesson

Guardian: A Ducat policy-checking signer that can authorize or reject defined protocol transactions.
FROST: Flexible Round-Optimized Schnorr Threshold signing: a protocol in which a required subset produces one group signature.
Schnorr signature: Bitcoin’s Taproot signature scheme, whose algebra supports compact verification and advanced multi-party signing.

Signature aggregation is public; safety depends on each honest Guardian independently rejecting invalid signing requests.

35 min Difficulty 4/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · ducat vaultA Ducat action moves one Bitcoin-anchored vault state to a successorSelect any state to inspect its exact role.

RELATIONSHIP MAPInputs, rule, and outputs

01
PREDECESSORCurrent vault outpoint
The current Bitcoin output anchors collateral and identifies the state being spent.
02
REQUESTCoordinator trust boundary
The Coordinator trust boundary workflow proposes one exact action and successor state.
03
POLICYProfile, oracle, signatures
Applicable policy, price evidence, authorization, and transaction shape are checked.
04
BITCOIN SPENDScript-valid transaction
Bitcoin miners and nodes can include only a transaction that satisfies Bitcoin rules.
05
SUCCESSORNew outpoint and state
Ducat validators reconstruct debt, collateral, token, and account changes from the confirmed transaction.

Rule shown: Bitcoin checks the spend; Ducat components independently reconstruct and validate the additional protocol state.

STAGE 1 OF 5PREDECESSOR: Current vault outpoint

The current Bitcoin output anchors collateral and identifies the state being spent.

Governing relationship: Bitcoin checks the spend; Ducat components independently reconstruct and validate the additional protocol state. Complete path: 1. PREDECESSOR, Current vault outpoint. The current Bitcoin output anchors collateral and identifies the state being spent. 2. REQUEST, Coordinator trust boundary. The Coordinator trust boundary workflow proposes one exact action and successor state. 3. POLICY, Profile, oracle, signatures. Applicable policy, price evidence, authorization, and transaction shape are checked. 4. BITCOIN SPEND, Script-valid transaction. Bitcoin miners and nodes can include only a transaction that satisfies Bitcoin rules. 5. SUCCESSOR, New outpoint and state. Ducat validators reconstruct debt, collateral, token, and account changes from the confirmed transaction.

1. State transitionWhat exists, what changes, and what results

BEFORE

Carol and Dana approved request repay-17 for transaction hash 11aa with their own mapped commitments. Required practice uses another package.

MECHANISM

Each Guardian independently validates the Ducat request before entering threshold signing.

AFTER

The coordinator can abort or withhold, but cannot forge without the threshold; aborted nonce state is discarded. Required practice uses a separate server-executed vector.

Text equivalent: Starting from Carol and Dana approved request repay-17 for transaction hash 11aa with their own mapped commitments. Required practice uses another package. The mechanism begins when Each Guardian independently validates the Ducat request before entering threshold signing. The resulting lesson is The coordinator can abort or withhold, but cannot forge without the threshold; aborted nonce state is discarded. Required practice uses a separate server-executed vector.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 4

Receive fixed approvals

Each Guardian independently validates the Ducat request before entering threshold signing.

Complete path: 1. Receive fixed approvals: Each Guardian independently validates the Ducat request before entering threshold signing. 2. Build one signing package: The coordinator names the message, exact ordered participant set, and which fresh nonce commitment belongs to each participant. 3. Verify before sharing: Participants reject substituted messages, participant sets, reused commitments, or policy mismatches. 4. Aggregate verifiable shares: The coordinator checks shares and combines only valid ones into the final Schnorr signature.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

The worked case executes the named inputs and checks in the displayed order. The coordinator checks shares and combines only valid ones into the final Schnorr signature.

CHANGED CONDITION

A missing, malformed, stale, unauthorized, or inconsistent required input stops at its first named boundary. It remains a liveness, privacy, ordering, and denial-of-service boundary even when cryptography prevents forgery.

Important distinction: Because the coordinator cannot forge a signature, it is not a trust boundary. It remains a liveness, privacy, ordering, and denial-of-service boundary even when cryptography prevents forgery.

Text equivalent: The accepted path is enforced by The worked case executes the named inputs and checks in the displayed order. The coordinator checks shares and combines only valid ones into the final Schnorr signature. If the required condition changes, A missing, malformed, stale, unauthorized, or inconsistent required input stops at its first named boundary. It remains a liveness, privacy, ordering, and denial-of-service boundary even when cryptography prevents forgery.

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Learn the idea

A FROST coordinator collects public nonce commitments and signature shares and aggregates them. A signing package binds the approved message, exact ordered participant subset, and a participant-to-commitment map. Naming commitments without their owners is ambiguous. The coordinator can disrupt or reorder a round, but honest participants verify every mapping before producing shares.

GUIDED EXPLANATION1/4 ideas inspected

Receive fixed approvals

Each Guardian independently validates the Ducat request before entering threshold signing.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Because the coordinator cannot forge a signature, it is not a trust boundary.”

It remains a liveness, privacy, ordering, and denial-of-service boundary even when cryptography prevents forgery.

FINISH LEARNING FIRST

The questions unlock after every required learning activity

Inspect every guided idea, open the worked example, rebuild its mechanism, and complete the deterministic lesson tool. Your progress is saved automatically.