Intent System

When a user wants to settle their funds onchain via a batch swap, they submit an intent to the operator. Arkade intents are valid-but-unmineable Bitcoin transactions that encode an ownership proof of the inputs a user wants to redeem - whether it’s offchain VTXOs, an onchain UTXO or recoverable coins (eg. expired coins or in form of an Arkade Note) - and define the outputs they wish to receive. Arkade intents are based on BIP322, a standardized Bitcoin message signing protocol. The system was introduced with the v0.7.0 arkd release.

The usage of BIP322 enables users to choose between renewing their expiring offchain funds themselves or delegating the renewal of such VTXOs. The delegated intent workflow can be found under Intent Delegation

Intent Structure

This section defines the intent schema that expresses what is being spent and where it goes. It covers the top-level fields, the Receiver format (onchain address vs. offchain pubkey), and the IntentMessage that captures execution parameters and the intent’s validity window.

Intent structure

The Intent domain is a user-submitted presigned Bitcoin transaction that:

Contains an IntentId(UUID)
Specifies the Inputs(VTXOs, UTXOs, or expired coins)
Specifies the Outputs(via Receivers). Either:
- onchain (via OnchainAddress)
- offchain (via PubKeyto create new VTXOs)
Contains a Proofof ownership of those funds via a bip322signature
Contains a Message (bip322 message) with intent details

type Intent struct {
	Id        string
	Inputs    []Vtxo
	Receivers []Receiver
	Proof     string
	Message   string
}

Receiver Structure

Receivers is defined via an Amount, an OnchainAddressand a PubKey(of which at least one must be present during Intent submission. Those are used to construct Arkade transaction outputs or direct onchain payments.

type Receiver struct {
	Amount         uint64
	OnchainAddress string // onchain
	PubKey         string // offchain
}

IntentMessage Format

IntentMessage contains intent details in JSON structure:

The Typeindicates if the intent is for renewal or ownership proof to delete another
InputTapTreesis the revealed Taproot tree of all inputs of the intent. Revelation occurs like witness data in Bitcoin
OnchainOutputIndexes: Indicates which outputs become UTXOs; others are VTXOs
ValidAt: Time (seconds) when the intent becomes valid; 0 = valid right away
ExpireAt: Time (seconds) when the intent expires; 0 = no expiry
CosignersPublicKeys: Public keys signing the VTXO tree; typically one, but can be more to support flexible user needs

type IntentMessage struct {
	BaseIntentMessage
	// InputTapTrees is the list of taproot trees associated with the spent inputs
	// the index of the taproot tree in the list corresponds to the index of the input + 1
	// (we ignore the first bip322 input, as it is duplicate of the second one)
	InputTapTrees []string `json:"input_tap_trees"`
	// OnchainOutputIndexes specifies what are the outputs in the proof tx
	// that should be considered as onchain by the Ark operator
	OnchainOutputIndexes []int `json:"onchain_output_indexes"`
	// ValidAt is the timestamp (in seconds) at which the proof should be considered valid
	// if set to 0, the proof will be considered valid indefinitely or until ExpireAt is reached
	ValidAt int64 `json:"valid_at"`
	// ExpireAt is the timestamp (in seconds) at which the proof should be considered invalid
	// if set to 0, the proof will be considered valid indefinitely
	ExpireAt int64 `json:"expire_at"`
	// CosignersPublicKeys contains the public keys of the cosigners
	// if the outputs are not registered in the proof or all the outputs are onchain, this field is not required
	// it is required only if one of the outputs is offchain
	CosignersPublicKeys []string `json:"cosigners_public_keys"`
}

Intent Lifecycle

The Arkade event stream (GetEventStream) is a server-side streaming RPC method in the ArkService) that provides real-time batch processing coordination events to clients including batch start, finalization, and failure notifications. The server uses this stream to indicate the next required action and corresponding API call. The full Intent lifecycle is as follows:

Create a BIP322 Signature

Intents use BIP322 message signing protocol for proving ownership of coins.

Code example: BIP322 signature implementation from the TS-SDK

 export function create(
  message: string,
  inputs: TransactionInput[],
  outputs: TransactionOutput[] = []
): FullProof {
  if (inputs.length == 0) throw ErrMissingInputs;
  if (!validateInputs(inputs)) throw ErrMissingData;
  if (!validateOutputs(outputs)) throw ErrMissingData;

  // create the initial transaction to spend
  const toSpend = craftToSpendTx(message, inputs[0].witnessUtxo.script);

  // create the transaction to sign
  return craftToSignTx(toSpend, inputs, outputs);
}

Source: TS-SDK

The create function takes a string message corresponding to the associated action described below (“Register” or “Delete”).

Register an Intent

The core of intent registration is the RegisterIntentRequest which contains a Bip322Signaturefield and an intent message:

Code example: RegisterIntentRequest

message RegisterIntentRequest {
// BIP322 signature embeds the outpoints to be spent and new ones to be created, as well as the
// the proof of funds.
Bip322Signature intent = 1;
}
message RegisterIntentResponse {
string intent_id = 1;
}

Intent message format

Example: intent message

   const message = {
       type: "register",
       input_tap_trees: inputTapTrees,
       onchain_output_indexes: onchainOutputsIndexes,
       valid_at: nowSeconds,
       expire_at: nowSeconds + 2 * 60, // valid for 2 minutes
       cosigners_public_keys: cosignerPubKeys,
   };

Source: Typescript SDK

The server then responds with a RegisterIntentResponsecontaining an intent_idstring for tracking.

Confirm Registration

After receiving a BatchStartedEventcontaining their intent ID hash, clients must call ConfirmRegistrationwith a ConfirmRegistrationRequestcontaining the intent_idto confirm participation. The server responds with an empty ConfirmRegistrationResponse.

Delete Intent

The DeleteIntentmethod accepts a DeleteIntentRequestwith a Bip322Signatureproof that demonstrates ownership of any input VTXOs from the original intent. The server responds with an empty DeleteIntentResponseupon successful deletion.

Code example: delete from Typescript SDK

Recovery Mechanisms

The intent system provides additional options for edge cases like recoverable VTXOs:

Expired VTXOs: Recover unspent and swept VTXOs (recoverVtxos)
Note Redemption: Redeem Arkade notes (redeemNotes)
Sub-dust VTXOs: Amounts below the Bitcoin dust threshold (`SubDustScript)

Arkade Workflows

API Layer

Components

Server Security

Intent Structure

Intent Lifecycle

Create a BIP322 Signature

Register an Intent

Confirm Registration

Delete Intent

Recovery Mechanisms

Arkade Workflows

API Layer

Components

Server Security

​Intent Structure

​Intent Lifecycle

Create a BIP322 Signature

Register an Intent

Confirm Registration

Delete Intent

​Recovery Mechanisms

Intent Structure

Intent Lifecycle

Recovery Mechanisms