XCLAIM: Financially Trustless Cross-Chain Communication

XCLAIM is a framework for achieving trustless and efficient cross-chain exchanges using cryptocurrency-backed assets (CbAs). XCLAIM allows to create assets which are 1:1 backed by existing cryptocurrencies, without requiring trust in a central operator. While this approach is applicable to a wide range of cryptocurrencies, we currently focus on implementing Bitcoin-backed tokens on Ethereum, i.e. XCLAIM(BTC,ETH).

XCLAIM introduces three protocols to achieve decentralized, transparent, consistent, atomic, and censorship resistant cross-blockchain swaps:

  • Issue: Create Bitcoin-backed tokens on Ethereum.
  • Swap: Swap Bitcoin-backed tokens on Ethereum against Ether or any ERC-20 token.
  • Redeem: Burn Bitcoin-backed tokens on Ethereum and receive the 1:1 amount of Bitcoin in return,

The current XCLAIM prototype is compliant with the ERC20 standard. An overview of the protocol is presented below:


Design Principles

XCLAIM guarantees that Bitcoin-backed tokens can be redeemed for the corresponding amount of Bitcoin, or the equivalent economic value in Ethereum. Thereby, XCLAIM overcomes the limitations of centralized approaches through three primary techniques:

  • Secure audit logs: Logs are constructed to record actions of all users both on Bitcoin and Ethereum.
  • Transaction inclusion proofs: Chain relays are used to prove correct behavior on Bitcoin to the smart contract on Ethereum.
  • Proof-or-Punishment: Instead of relying on timely fraud proofs (reactive), XCLAIM requires correct behavior to be proven proactively.
  • Over-collateralization: Non-trusted intermediaries are bound by collateral, with mechanisms in place to mitigate exchange rate fluctuations.

XCLAIM vs HTLC Atomic Swaps

To test performance and costs, we implemented both XCLAIM and HTLC ACCS between Bitcoin and Ethereum. Our experiments show:
XCLAIM is cheaper and faster than HTLC Atomic Swaps.


Storage and execution costs are in USD as per exchange rates of 30 Nov. 2018: BTC/USD 3717.38 and ETH/USD 105.71. Performance is measured in minutes and includes recommended security parameters: 6 conf. a 10 min for Bitcoin; 12 conf. a 14 sec for Ethereum.

Conferences, Press, and Industry Applications

XCLAIM was presented at the following conferences:

Scaling Bitcoin 2018 Scaling Bitcoin, Tokyo
6-7 Oct. 2018
Ethereum Community Conference 2019 EthCC 2019, Paris
5-7 Mar. 2019
Community Ethereum Development Conerence 2019 EDCON, Sydney
8-14 Apr. 2019
IEEE S&P 2019 IEEE S&P, San Francisco
20-22 May. 2019

and was featured in the following articles:

Coindesk article Article on XCLAIM-based Bitcoin-Polkadot bridge
5 May. 2020

XCLAIM is being implemented as by these industry projects:

Interlay Interlay is building a bridge from Bitcoin to Polkadot