HTLC Deep-dive

At the heart of Kaleido’s Token Swap service is the Hashed Timelock Contract (HTLC). It can be viewed as a safety deposit box that temporarily holds the tokens offered for swap. The lock box allows the designated swap counterparty to claim the tokens after presenting the required credentials. It also supports refunding in case the swap deal falls apart.

Anatomy of a Hashed Time Lock Contract (HTLC)

Let’s take a look at how things work under the covers.

To support the secure and atomic trades between two parties without having to assume that the parties will behave honestly and fulfill their duties, the HTLC contract must have the following properties:

  • As with all Ethereum smart contracts, the HTLC contract can hold token balances. A swap is created by first depositing the offered tokens into the contract’s lock box. The lock box is told about the intended beneficiary of the tokens, so that only that particular account can claim the tokens.
  • The contract safeguards the tokens with a hash lock. In order to get the contract to release the deposited funds, not only must the designated beneficiary be the signer of the claim() transaction, but the transaction must also present the original secret that produces the hash value used by the hash lock.
  • The contract also supports refunds with a time lock. It is a timeout period to signal to the beneficiary when to make the claim. If the timeout expires, the swap initiator will be able to call refund() to get the tokens back. So the beneficiary is incentivized to make the claim before the timeout expires.
  • A single deployed HTLC contract can keep track of multiple concurrent swaps. In fact, in a Kaleido environment all ERC20 token swaps are supported by the same HTLC contract instance, and likewise for all ERC721 swaps.
  • It is possible to swap both types of ERC20 and ERC721 tokens, including cross-type swaps:
    • ERC20 tokens <=> ERC20 tokens
    • ERC721 token <=> ERC721 token
    • ERC20 tokens <=> ERC721 token