SCIS 2022
2022 Symposium on
Cryptography and Information Security
Osaka, Japan & Online, Jan. 18 – 21, 2022
The Institute of Electronics,
Information and Communication Engineers

Copyright c 2022 The Institute of Electronics,
Information and Communication Engineers

Interhead Hydra: Two Heads are Better than One
Maxim Jourenko ∗Mario Larangeira †Keisuke Tanaka ‡Keywords: blockchain, channels, Layer-2, cryptographic protocols

Abstract

supports and utilizes multi-party channels and (2) allows for collateral to be paid by multiple intermediaries
which allows to share this burden and thus improves
practicality.

Distributed ledger are maintained through consensus protocols executed by mutually distrustful parties.
However, these consensus protocols have inherent limitations thus resulting in scalability issues of the ledger.
Layer-2 protocols operate on channels and allow parties to interact with another without going throughthe consensus protocol albeit relying on its security as
fall-back. Prominent Layer-2 protocols are payment
channels for Bitcoin that allow two parties to exchangecoins and State Channels for Ethereum that allow twoparties to execute a state machine. Channels can beconcatenated into networks using techniques such as
Hashed Timelocked Contracts to execute payments or
virtual state channels as introduced by Dziembowski
et al. [3, 4] to execute state machines. These constructions allow interaction between two parties acrossa channel network, i.e. the two endpoints of a path ofchannels. This is realized by utilizing intermediaries,
which are the parties on the channel path which are inbetween both endpoints, who have to pay collateral toensure security of the constructions. Dziembowski etal. [2] introduced multi-party state channels based on avirtual channel construction and more recently Hydra
heads [1] is a channel construction that allows multiple parties the execution of Constraint Emitting Machines (CEM). While existing protocols such as HTLCs
can be extended s.t. two parties can interact with another across multi-party channels, there are no dedicated constructions that utilize multi-party channels
and similarly allow more than two parties to interact
across a network of such channels. This presentation
presents our ongoing work to address this gap by extending Hydra and introducing the Interhead construction that allows for the iterative creation of virtual
Hydra heads. Our construction is the first that (1)References[1] Manuel MT Chakravarty, Sandro Coretti, Matthias
Fitzi, Peter Gazi, Philipp Kant, Aggelos Kiayias,
and Alexander Russell. Hydra: Fast isomorphic
state channels. In International Conference on Financial Cryptography and Data Security. Springer,
2021.
[2] Stefan Dziembowski, Lisa Eckey, Sebastian Faust,
Julia Hesse, and Kristina Hostáková. Multi-party
virtual state channels. In Annual InternationalConference on the Theory and Applications ofCryptographic Techniques, pages 625–656. Springer,
2019.
[3] Stefan Dziembowski, Lisa Eckey, Sebastian Faust,and Daniel Malinowski. Perun: Virtual payment
hubs over cryptocurrencies. In Perun: Virtual Payment Hubs over Cryptocurrencies. IEEE, 2017.
[4] Stefan Dziembowski, Sebastian Faust, and Kristina
Hostáková. General state channel networks. In
Proceedings of the 2018 ACM SIGSAC Conference
on Computer and Communications Security, pages
949–966. ACM, 2018.

∗ The University of Tokyo, 7 Chome-3-1 Hongo, Bunkyo City,Tokyo 113-8654† Tokyo Institute of Technology, 2 Chome-12-1 Ookayama, Me-guro City, Tokyo 152-8550
IOHK, IOG Singapore Pte Ltd 4 Battery Road, #25-01 Bankof China Building, Singapore (049908)‡ Tokyo Institute of Technology, 2 Chome-12-1 Ookayama, Meguro City, Tokyo 152-8550

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