Cross Peg-Out Endpoints

• Location: ./packages/contracts/bridge-stacks/contracts/

• Deployed contracts:cross-peg-out-endpoint-v2-01, cross-peg-out-v2-01-agg

This technical document provides a detailed overview of the contracts responsible for managing the peg-out process, enabling the transfer of SIP-010 bridged tokens from the Stacks network to EVM-compatible blockchain networks as EVM-based assets. The contracts manage token transfers by validating amounts and applying fees. During the process, tokens are either burned or transferred (depending on the token's configuration) to a designated address on the EVM chain.

The core functionalities of the module are implemented through public and governance functions in the following contracts:

• cross-peg-out-endpoint-v2-01: handles bridging of aBTC tokens from Stacks to EVM-compatible blockchains.

• cross-peg-out-v2-01-agg: enables the bridging of tokens from Stacks to an EVM-compatible blockchain to perform a swap on a non-ALEX liquidity aggregator.

Storage

All contracts include the following variables unless otherwise specified.

is-paused

Data	Type
Variable	bool

A flag that indicates whether the peg-out process is active. If set to true, all peg-out operations are paused, preventing any new transactions. The contract is deployed in a paused state by default.

use-whitelist

Only present in cross-peg-out-endpoint-v2-01.

Data	Type
Variable	bool

A flag that determines whether a whitelist is enforced for peg-out operations. If set to true, only addresses explicitly whitelisted can execute peg-out transactions. By default, this feature is disabled.

whitelisted-users

Only present in cross-peg-out-endpoint-v2-01.

Data	Type
Map	principal → bool

This map stores the whitelist status of users. Each entry associates a principal with a boolean value indicating whether they are authorized to perform peg-out transactions. When use-whitelist is set to true, users with a value of true in this map are whitelisted and permitted to participate in the process; otherwise, the whitelist is not enforced.

Features

Peg-out feature

transfer-to-unwrap

Only present in cross-peg-out-endpoint-v2-01.

This function manages the peg-out process, enabling users to transfer SIP-010 bridged tokens from the Stacks network to EVM-compatible blockchains. It begins by validating the transfer through the validate-transfer-to-unwrap function, performing checks such as token and chain approval, whitelist status, amount thresholds, and sufficient reserves for the operation. Once validated, the function calculates the required fee and determines the net amount to be transferred. Depending on the token's properties, the function either burns the net amount directly from the user's balance or transfers the entire amount (including the fee) to the cross-bridge-registry-v2-01. At the end of the process, the function logs key destination details, including the settle-address, which represents the recipient's address on the EVM-compatible blockchain.

Parameters

(token-trait <ft-trait>)
(amount-in-fixed uint)
(dest-chain-id uint)
(settle-address (buff 256))

transfer-to-swap

Only present in cross-peg-out-v2-01-agg.

This function handles the peg-out process from Stacks to an EVM-compatible blockchain. It prepares the transaction to be sent to an external liquidity aggregator in the BridgeEndpointWithSwap.sol contract. Its core functionality consists of validating amounts, deducting fees, transferring tokens and emitting an event with the transaction details. The function begins by validating the transfer through the validate-transfer-to-swap function, performing checks such as token and chain approval, amount thresholds, and sufficient reserves for the operation. Once validated, the function calculates the required fee and determines the net amount to be transferred. Depending on the token's properties, the function either burns the tokens or transfers them to the cross-bridge-registry-v2-01 contract. This action records the transaction and prepares it for further processing. At this point, an event is emitted, signaling relayers to detect and forward the transaction to BridgeEndpointWithSwap, which completes the operation through an external liquidity aggregator.

(amount-in-fixed uint)
(token-in-trait <ft-trait>)
(token-out principal)
(min-amount-out (optional uint))
(dest-chain-id uint)
(settle-details { address: (buff 256), chain-id: (optional uint) })

Governance features

is-dao-or-extension

This standard protocol function checks whether a caller (tx-sender) is the DAO executor or an authorized extension, delegating the extensions check to the executor-dao contract.

is-whitelisted

Only present in cross-peg-out-endpoint-v2-01.

A read-only function that checks whether a specific user is included in the whitelisted-users map. It returns true if the user is whitelisted; otherwise, it returns false.

Parameters

(user principal)

get-paused

A read-only function that checks the operational status of the contract.

set-paused

A public function, governed through the is-dao-or-extension, that can change the contract's operational status.

Parameters

(paused bool)

apply-whitelist

Only present in cross-peg-out-endpoint-v2-01.

A public function, governed through the is-dao-or-extension, that enables or disables the whitelist mechanism for peg-out transactions. When enabled, only users listed in the whitelisted-users map can perform peg-out operations.

Parameters

(new-use-whitelist bool)

whitelist

Only present in cross-peg-out-endpoint-v2-01.

A public function, governed through the is-dao-or-extension, that allows the DAO to manage user access to the peg-out feature. It updates the whitelisted-users map to either include or remove a specific user. If the whitelisted parameter is set to true, the user is added to the whitelist; if set to false, the user is removed.

Parameters

(user principal)
(whitelisted bool)

whitelist-many

Only present in cross-peg-out-endpoint-v2-01.

A public function, governed through the is-dao-or-extension, that allows the DAO to update the whitelist for multiple users in a single call. It works as a batch operation for the whitelist function, applying the specified whitelist status (true to add or false to remove) for each user in the provided list. The whitelisted-users map is updated.

Parameters

(users (list 2000 principal))
(whitelisted (list 2000 bool))

Getters

get-use-whitelist

Only present in cross-peg-out-endpoint-v2-01.

get-approved-chain-or-fail

Parameters

(dest-chain-id uint)

get-token-reserve-or-default

Parameters

(pair { token: principal, chain-id: uint })

get-min-fee-or-default

Parameters

(pair { token: principal, chain-id: uint })

get-approved-pair-or-fail

Parameters

(pair { token: principal, chain-id: uint })

Contract calls (interactions)

• executor-dao: Calls are made to verify whether a certain contract-caller is designated as an extension.

• cross-bridge-registry-v2-01: This contract is called to verify key components of the peg-out process. It validates approved tokens and chain pairings, manages the deduction and queries of token reserves, and records accrued fees. It also handles updates to transaction statuses.

• token-trait: In the cross-peg-out process, this trait is employed to manage token operations such as burning tokens when required or transferring them to the cross-bridge-registry-v2-01 contract. It is a customized version of Stacks' standard definition for Fungible Tokens (sip-010), with support for 8-digit fixed notation.

Errors

Error Name	Value
ERR-NOT-AUTHORIZED	(err u1000)
ERR-PAUSED	(err u1015)
ERR-USER-NOT-WHITELISTED	(err u1016)
ERR-AMOUNT-LESS-THAN-MIN-FEE	(err u1017)
ERR-INVALID-AMOUNT	(err u1019)