Building trustless cross-blockchain trading protocols is challenging. Therefore, centralized liquidity providers remain the preferred route to execute transfers across chains - which fundamentally contradicts the purpose of permissionless ledgers to replace trusted intermediaries. Enabling cross-blockchain trades could not only enable currently competing blockchain projects to better collaborate, but seems of particular importance to decentralized exchanges as those are currently limited to the trade of digital assets within their respective blockchain ecosystem. In this paper we systematize the notion of cryptocurrency-backed tokens, an approach towards trustless cross-chain communication. We propose XCLAIM, a protocol for issuing, trading, and redeeming e.g. Bitcoin-backed tokens on Ethereum. We provide implementations for three possible protocol versions and evaluate their security and on-chain costs. With XCLAIM, it costs at most USD 1.17 to issue an arbitrary amount of Bitcoin-backed tokens on Ethereum, given current blockchain transaction fees. Our protocol requires no modifications to Bitcoin's and Ethereum's consensus rules and is general enough to support other cryptocurrencies.