=Paper=
{{Paper
|id=Vol-3407/paper4
|storemode=property
|title=Blockchain Interoperability
|pdfUrl=https://ceur-ws.org/Vol-3407/paper4.pdf
|volume=Vol-3407
|authors=Guzmán Llambías
|dblpUrl=https://dblp.org/rec/conf/caise/Llambias23
}}
==Blockchain Interoperability==
https://ceur-ws.org/Vol-3407/paper4.pdf
Blockchain Interoperability
Guzmán Llambías1,2
1
Facultad de ingeniería, Universidad de la República, Montevideo, Uruguay
2
Pyxis, Montevideo, Uruguay
Abstract
Blockchain is one of the most disrupting technologies in recent years and has been used in several
domains, such as health, supply chain and finance. In addition, organisations started integrating their
blockchains with other blockchains or other external software systems. However, this is challenging for
blockchain systems as interoperability is not a native design feature. Several academic and industrial
efforts were performed to enable blockchain interoperability in the last years, but they were proposed for
specific domain scenarios. To the best of our knowledge, there is not a general-purpose interoperability
solution that enables blockchain interoperability. This research project aims to define a general-purpose
blockchain interoperability framework to enable blockchain interoperability. This framework will
comprise methods, guidelines, specifications and software pieces that ease interoperability between
blockchains. This paper presents this research project, describing its research questions, the adopted and
projected research methodologies, preliminary results and its current status.
Keywords
Distributed Ledger Technologies (DLT), blockchain, interoperability, cross-chain transactions,
1. Introduction
Blockchain is a technology that enables decentralised payments across participants without the
need of a centralised trusted third-party. Furthermore, blockchain became a suitable technology
for other domains besides finance, such as health, Internet of Things (IOT) and supply chain,
among others. Indeed, blockchains are used on open and permissionless scenarios where
security, transparency and anonymity are key requirements, but also on closed and permissioned
scenarios, where user identity, authorisation and privacy are needed. In addition, organisations
started to increasingly require that permissionless and permissioned blockchains interoperate
with other blockchains and external software systems. This requirement represents a challenge,
as blockchains are isolated information systems, and interoperability was not part of their
design. In particular, organisations are required to implement data transfer, asset transfer and
asset exchange scenarios. A data transfer scenario involves copying information from a source
blockchain to a target blockchain. An asset transfer scenario requires the transfer of an asset
(e.g. cryptocurrency) from a source blockchain to a target blockchain, while an asset exchange
scenario involves the atomic exchange of assets that belong to different blockchains.
Proceedings of the Doctoral Consortium Papers Presented at the 35th International Conference on Advanced Information
Systems Engineering (CAiSE 2023), June 12–16, 2023, Zaragoza, Spain
$ gllambi@fing.edu.uy,guzman.llambias@pyxis.com.uy (G. Llambías)
0000-0002-0877-7063 (G. Llambías)
© 2023 Copyright for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).
CEUR
Workshop
Proceedings
http://ceur-ws.org
ISSN 1613-0073
CEUR Workshop Proceedings (CEUR-WS.org)
25
Blockchain interoperability differs from traditional software interoperability. Blockchain
systems maintain a state following rules defined by their consensus protocol, and only data that
satisfy these rules may be registered on the blockchain. Therefore, enabling interoperability
between blockchains implies that there must be a consensus between blockchains about the
validity of the exchanged information, which is a complex challenge. Furthermore, being
blockchain a decentralised system where participants need to reach finality about their results,
it is challenging for this technology to access external data that can be dynamic by nature (e.g.
invoking an API of an external system). Secure integration mechanisms need to be used to
ensure the blockchain’s consistency.
In the last years, efforts to enable blockchain interoperability were proposed with quite a
success for domain specific scenarios - such as asset transfer - (e.g. Polygon PoS Bridge [1])
and between specific blockchains (e.g. Ethereum [2] and Polygon). However, to the best of
our knowledge, a general-purpose interoperability solution for blockchain systems is not yet
available.
This paper provides an overview of the PhD thesis’s research goals, which are focused on
blockchain interoperability. The paper is structured as follows: Section 2 introduces related
work on blockchain interoperability. Section 3 presents the objective of this thesis and the
proposed research questions. Section 4 presents the research methodologies to be used in this
thesis. Section 5 describes preliminary results and contributions, while Section 6 presents the
conclusions.
2. Related work
Blockchain interoperability has gained special attention in the research community in recent
years, where several literature reviews analysed the state-of-the-art [3, 4]. In particular, Belchior
et al. presented the most exhaustive survey [3], where they proposed categorising blockchain
interoperability solutions into three categories: Public Connectors, Hyrbrid Connectors and
Blockchain of Blockchains. These categories also have the following subcategories: Notary
Scheme, Hash Time Lock Contracts (HTLC), Sidechains, Trusted Relays, Agnostic Protocols
and Blockchain Migrators. Scheid et al. [5] and Li et al. [6] presented two examples of Notary
Scheme solutions, while there are several proposals for HTLC [4]. Zendoo is an example of a
Sidechain solution using Zero Knowledge Proof for cryptocurrency transfers [7]. Abebe et al. [8]
presented a solution based on Trusted Relays between two blockchains, based on Hyperledger
Fabric on a data exchange scenario. Liu et al. [9] presented Hyperservice, belonging to the
Agnostic Protocols category, where they proposed a platform for the modelling and specifying
smart contracts to enable interoperability between blockchains in an asset exchange scenario.
Scheid et al. [10] proposed a Blockchain Migrator solution, defining a framework based on
policies for blockchain selection. On the other hand, Blockchain of Blockchains is the most
novel and least studied category [4]. Other authors provided other classifications but were less
exhaustive. For example, Koens et al. [11] proposed a classification based on twelve properties,
but only included Notary Scheme, Sidechains and HTLC categories.
Regarding software architecture, Jin et al. proposed a reference architecture for blockchain
interoperability based on the OSI network layers [12]. In addition, Hardjono et al. proposed an
26
interoperability architecture based on Gateways, taking the internet model as an inspiration
[13].
Some formalisation of blockchain interoperability has been proposed. Kiayias et al. [14] and
then Gaži et al. [15] formalized the Sidechain solution for two consensus protocols: Proof-of-
Work and Proof-of-Stake. Herlihy [16] formalised the HTLC solution through game theory.
Zamyatin et al. [17] proved that achieving cross-chain communication without a trusted third
party is impossible. At the same time, Lafourcade and Lombard-Platet [18] showed that it is
impossible to achieve interoperability between two blockchains under the classical definition
of blockchain. However, if this definition is relaxed, it is possible to achieve interoperability,
creating a two-in-one blockchain (i.e. a blockchain with two ledgers).
Most of the academic community had focused on enabling interoperability between non-
permissioned blockchains, while a small-scale effort was put into permissioned blockchain
interoperability. This effort is even less for enabling interoperability between permissioned and
permissionless blockchains [4]. Abebe et al. [8] and Bradach et al. [19] provide two solutions
for permissioned blockchain interoperability. Franzoni [20] and Falazi et al. [21] propose two
solutions to interoperate permissioned and permissionless blockchains but do not provide
cross-chain transaction validations.
On the other hand, the industry has been active in proposing solutions for blockchain inter-
operability. Blocknet enables blockchain interoperability based on two components: XBridge
and XRouter [22]. By design, Blocknet may enable interoperability between permissioned and
permissionless blockchains, but to the best of our knowledge, only permissionless blockchains
are supported. ARK is a permissionless blockchain that enables blockchain interoperability by
using two protocols: Specific Smart Bridge and Protocol-Agnostic Smart Bridge [23]. ARK en-
ables interoperability between Bitcoin and Ethereum, as well as other Bitcoin-based Blockchains.
Hyperledger Cactus [24], Weaver [25], and YIU [26] are three incubated projects developed by
Hyperledger to achieve blockchain interoperability. Hyperledger Cactus provides a framework
that business applications may use to interoperate with heterogeneous blockchains and cur-
rently supports Hyperledger Fabric [27], Hyperledger Besu [28], Corda [29] and Quorum [30].
Weaver is a Trusted Relay based solution, where each blockchain must have an IOP module
and a Trusted Relay. The IOP module is responsible for cross-chain transaction verification,
while the Relay is responsible for cross-blockchain communication. YIU implements the IBC
(Inter-Blockchain Communication) protocol proposed by Cosmos [31] [32]. With this approach,
every blockchain must have an IBC module that implements the protocol and is responsible
of the connectivity and verification of cross-chain transactions. YIU currently supports Hy-
perledger Besu, Hyperledger Fabric and Corda. Finally, Cosmos and Polkadot [33] are two
blockchain of blockchains solutions with similar behaviour. Cosmos relies on the IBC protocol
to enable interoperability between blockchains and requires the interoperating blockchains to
modify their source code to support the protocol. On the other hand, Polkadot uses bridges
to enable interoperability between blockchains and also requires changes in the blockchain
source code. Both solutions use a main blockchain that provides a consensus protocol to val-
idate cross-blockchain transactions. Finally, Optimism [34] and Polygon PoS Bridge [1] are
two examples of Sidechain solution that allows asset transfer between Ethereum and other
Ethereum Virtual Machine based blockchains. In particular, Optimism enables asset transfer be-
tween Ethereum and Optimism, and vice versa, while Polygon PoS Bridge enables asset transfer
27
between Ethereum and Polygon. Both solutions provide cross-chain transaction verification
but are specific solutions for asset transfer between two specific permissionless blockchains.
Considering the existing work, blockchain interoperability has had significant advances since
the first proposals, but there are still challenges to be solved. Despite the current work, there is
no consensus in the scientific community about a blockchain interoperability definition nor
a categorisation of the existing solutions [4]. Existing approaches are platform specific (e.g.
Polygon) and do not yet provide a general-purpose interoperability solution.
3. Objective and research questions
The main objective of this PhD thesis is to provide an approach to enable interoperability
between blockchain platforms for general-purpose scenarios. To this end, this PhD thesis
proposes the following research questions.
RQ1: What challenges exist to enable blockchain interoperability?
RQ2: What factors need to be considered to enable blockchain interoperability?
RQ3: What approaches exist to enable blockchain interoperability?
RQ4: What challenges are not covered by the existing approaches, and how can they be
addressed?
RQ5: How can a reference framework be developed to support blockchain interoperability?
RQ6: How can this framework be applied to existing blockchains?
4. Research methodology
This PhD thesis follows the Design Science research (DSR) as it constitutes a suitable methodol-
ogy to reach the research goals and provide solutions to the research questions.
The DSR is an incremental process that designs and evaluates artefacts intended to solve
identified organisational problems [35]. DSR uses existing knowledge (e.g. theories, frameworks,
methods, methodologies) applied to existing needs of organisations and people to build new
artefacts or improve existing ones in an iterative process.
Figure 1: Proposed methodology based on Design Science Research
The followed methodology is described in Fig. 1 and is composed of four stages. At stage one,
a literature review is performed to understand the motivation of blockchain interoperability,
identify challenges and define the requirements of the artefact (i.e. the reference framework).
Stage two takes these requirements to design the first iteration of the artefact. In stage three,
the artefact is evaluated through design evaluation methods (e.g. observational, analytical,
28
experimental). At this stage and after the evaluation of the obtained results, the process may
return to stage two for the improvement of the artefact. The results are later communicated at
conferences, workshops and journals at stage four, where new feedback is gathered from the
research community. After this last stage is finished, the process starts again. It continues on
stage two unless a defined number of iterations is reached and enables the finalisation of this
PhD thesis.
Stage one is partially completed and was developed by adopting some features of the PRISMA
statement [36].
We plan to execute stage two through an iterative process, following a bottom-up approach.
The first iterations of the methodology will focus on the foundations of the artefact, and the
following iterations will focus on building new features and its evolution.
We plan to evaluate the artefact through several evaluation methods at stage three [37].
In particular, we plan to use prototypes and illustrative scenarios to demonstrate its utility.
Technical experiments, such as performance testing, will be performed to evaluate the technical
performance of the artefact. An assessment through expert evaluation is planned to show
the suitability of the artefact. Subject-based experiments are planned to test the artefact with
different blockchains.
Finally, at stage four we plan to communicate our research results through scholarly research
and professional publications. The first may include academic seminars, conferences, workshops
and journals whose audience are students and researchers. The professional publications include
technical reports and industry conference talks, and aims to a practitioners audience (industry
and government).
5. Preliminary ideas, results and contributions
Fig. 2 depicts the expected blockchain interoperability framework as the artefact to be built.
The artefact is composed of a set of elements. The theoretical concepts element describes the
background concepts regarding blockchain interoperability and related concepts to understand
the framework. The decision guidelines element provides decision guidelines to select the most
suitable blockchain interoperability solution according to the business requirements. The design
patterns element describes blockchain interoperability patterns that can be used to build a
blockchain interoperability solution. The solution specification element describes a specification
of a blockchain interoperability solution that enables interoperability between two or more
blockchains. A reference implementation of the specification is another element that provides
software following the specification. This software can be used to enable interoperability
between two specific blockchains (e.g. Ethereum and Hyperledger Fabric). Finally, a set of case
scenarios that shows the usage of the framework constitutes its last element.
This PhD started with stage one to answer RQ1, RQ2 and RQ3. At this stage, a literature review
was performed, which analysed sixty-two papers and proposed a feature-based classification
framework to improve the classification of blockchain interoperability solutions [4]. This
literature review enabled the identification of challenges and requirements for developing the
reference framework. Furthermore, this work constitutes the basis of the theoretical element of
the framework.
29
Figure 2: Blockchain interoperability reference framework
The first iteration of the methodology was finished and provided the first preliminary results,
enabling interoperability between two permissioned blockchains on a specific social security
scenario [19]. These results included a first version of the specification element, a prototype
that served as a preliminary version of the reference implementation and an illustrative scenario
that will be part of the case scenario element. The prototype and the illustrative scenario served
as the evaluation methods of these results. The experiment had some limitations as it did not
consider cross-chain transaction authentication, cross-chain transaction authorisation, or data
privacy properties of permissioned blockchains. An extended version of this work was recently
submitted to a journal where further assessment was performed based on two blockchain
interoperability frameworks [38].
The ongoing work of this PhD thesis consists of starting the second iteration of the method-
ology and the redesign of the artefact based on the obtained results and research community
feedback. The first iteration provided a tailored made interoperability solution for two specific
permissioned blockchains on a specific social security scenario. The second iteration pursued
the evolution of the previous elements to build a general-purpose interoperability solution.
In this case, the evolution of the specification, reference implementation and case scenarios
elements. Furthermore, this iteration introduced permissionless blockchains and aimed to
enable interoperability between permissionless and permissioned blockchains. In particular,
between Ethereum and Hyperledger Fabric. The iteration is on stage four (communication
stage), where the obtained results are planned to be submitted to a workshop or conference.
On a third iteration, it is planned to start the development of the design patterns and the
decision framework elements. The works of Pillai et al. will inspire the latter [39] and Belchior
et al. [40] with further development.
At the end of this PhD thesis, we envision a reference blockchain interoperability framework
that practitioners and researchers may use to enable interoperability between two or more
blockchains. In addition, students and researchers may use the framework to get introduced to
this topic and extend it with new features.
30
6. Conclusions
Blockchain is a disrupting technology that was applied in multiple domains (e.g. health, IOT,
supply chain, art), but its usage is limited because of its design principles. Blockchains are
isolated systems that work as silos of information, and interoperability is not part of their
native design. It is a challenge for a blockchain to interoperate with other blockchains or
other external systems. In the last years, some industrial and academic approaches enabled
blockchain interoperability but they were limited to specific domain scenarios. To the best of
our knowledge, there is not a general-purpose interoperability solution that enables blockchain
interoperability.
This paper presents the approach of my PhD thesis, where its main objective is the develop-
ment of a general-purpose interoperability framework that enables blockchain interoperability.
This PhD thesis follows a Design Science Research methodology with different stages. The
first stage provided a survey on blockchain interoperability state of the art, identifying challenges
and defining of the proposed research questions. Further stages enabled the development of the
first elements of the artefact. The first result was a gateway-based middleware that enabled
interoperability between two permissioned blockchains on a specific based scenario. These
elements are being evolved in the next iterations of the methodology. In particular, the evolution
to include permissionless and permissioned blockchain interoperability, cross-chain transaction
authentication and new elements of the framework, like the design guidelines and design
patterns.
The main contribution of this PhD thesis is the definition of a blockchain interoperability
framework that provides solutions for general-purpose interoperability between blockchain
platforms. Future work will include the evolution of the framework by enhancing its elements
and building blocks. We envision that this framework may ease the development of blockchain
interoperability solutions and positively impact their quality. In addition, this PhD thesis will
provide an organised knowledge base that may be the baseline for future academic research.
Acknowledgments
I want to thank Dra. Laura González and Dr. Raúl Ruggia from Universidad de la República
for the supervision of this PhD thesis. Guzmán Llambías is supported by Pyxis. The research
that gives rise to the results presented in this publication received funding from the Agencia
Nacional de Investigación e Innovación under the code POS_NAC_2022_4_174476. Blockchain
icons created by Freepik - Flaticon.
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ledger technology interoperability solution?, Distrib. Ledger Technol. (2022). URL: https:
//doi.org/10.1145/3564532. doi:10.1145/3564532, just Accepted.
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