Introducing Erk: Ark without liveness requirements

Last weekend at OP_NEXT, we unveiled Erk—a novel variation of the Ark protocol that completely removes the need for user interactivity in rounds. This addresses one of Ark's key limitations: the requirement for users to come online before their VTXOs expire.

The innovation behind Erk

Erk (named after our CTO Erik who conceived the protocol!) introduces "refund transactions" with rebindable signatures, allowing users to provide signatures in advance that can be applied to future outputs. The server can then safely refresh VTXOs without requiring users to be online for each round.

refund tx:
| inputs             |        outputs | 
+====================+================+
| old exit tx for A  | exit policy A' | 
+--------------------+----------------+
| new exit tx for A' |              S | 
+--------------------+----------------+

Another powerful feature of Erk is "perpetual offline refresh"—users can bulk pre-sign future refreshes in sequence. With watchtowers monitoring the protocol, a user could remain offline indefinitely while their funds stay secure.

Beyond Erk: hArk

We've also developed "hArk"—which works alongside Erk. While Erk excels at single-input refreshes, hArk is optimized for consolidating multiple inputs into a single output through a clever secret-revealing mechanism.

Like Erk, hArk also requires CTV (but doesn't rely on CSFS).

What this means for Ark

These innovations build on recent improvements to our Ark implementation, including making arkoor the standard for payments and benefiting from Bitcoin Core's package relay update.

While these improvements require CTV, we continue to work toward launching Ark on bitcoin as it is now—there's still a lot of great scaling benefits users can enjoy without these enhancements!

For the full technical breakdown of how Erk and hArk leverage CTV and CSFS, check out our detailed post on the Delving Bitcoin forum.