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MetaOpera: A Cross-Metaverse Interoperability Protocol

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Abstract and Figures

With the rapid evolution of metaverse technologies, numerous metaverse applications have arisen for various purposes and scenarios. This makes interoperability across meta-verses becomes one of the fundamental technology enablers in the metaverse space. The aim of interoperability is to provide a seamless experience for users to interact with metaverses. However , the development of cross-metaverse interoperability is still in its initial stage in both industry and academia. In this paper, we review the state-of-the-art cross-metaverse interoperability schemes. These schemes are designed for specific interoperating scenarios and do not generalize for all types of metaverses. To this end, we propose MetaOpera, a generalized cross-metaverse interoperability protocol. By connecting to the MetaOpera, users and objects in metaverses that rely on centralized servers or decentralized blockchains are able to interoperate with each other. We also develop a proof-of-concept implementation for MetaOpera, evaluate its performance, and compare it with a state-of-the-art cross-metaverse scheme based on Sidechains. Simulation results demonstrate that the size of cross-metaverse proof and the average time of cross-metaverse transactions using the proposed solution are respectively about eight times and three times smaller than the Sidechains scheme. This paper also suggests a number of open issues and challenges faced by cross-metaverse interoperability that may inspire future research.
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MetaOpera: A Cross-Metaverse Interoperability
Protocol
Taotao Li, Changlin Yang, Member, IEEE, Qinglin Yang, Member, IEEE, Siqi Zhou, Huawei Huang, Senior
Member, IEEE, Zibin Zheng, Fellow, IEEE
Abstract—With the rapid evolution of metaverse technologies,
numerous metaverse applications have arisen for various pur-
poses and scenarios. This makes interoperability across meta-
verses becomes one of the fundamental technology enablers in
the metaverse space. The aim of interoperability is to provide a
seamless experience for users to interact with metaverses. How-
ever, the development of cross-metaverse interoperability is still
in its initial stage in both industry and academia. In this paper,
we review the state-of-the-art cross-metaverse interoperability
schemes. These schemes are designed for specific interoperating
scenarios and do not generalize for all types of metaverses. To
this end, we propose MetaOpera, a generalized cross-metaverse
interoperability protocol. By connecting to the MetaOpera, users
and objects in metaverses that rely on centralized servers or
decentralized blockchains are able to interoperate with each
other. We also develop a proof-of-concept implementation for
MetaOpera, evaluate its performance, and compare it with
a state-of-the-art cross-metaverse scheme based on Sidechains.
Simulation results demonstrate that the size of cross-metaverse
proof and the average time of cross-metaverse transactions using
the proposed solution are respectively about eight times and
three times smaller than the Sidechains scheme. This paper also
suggests a number of open issues and challenges faced by cross-
metaverse interoperability that may inspire future research.
Index Terms—Cross-Metaverse, Interoperability, Blockchain,
Oracle
I. INTRODUCTION
The metaverse attracts significant attention in recent years.
It is a computer-generated virtual world and can be intertwined
with the physical world. Users in the metaverse gain immer-
sion during interacting with virtual objects or other users,
which makes the metaverse a new paradigm of the Internet. To
achieve this, the metaverse requires the integration of various
technologies. These include but not limited to augmented
reality (AR), digital twin (DT), blockchain, interactivity, game,
artificial intelligence (AI), networking, and Internet of Things
(IoTs) [1]. For example, AR provides an immersive experi-
ence; DT generates a mirror image of physical world entities;
blockchains build metaverse economic systems.
In metaverse, an avatar is the digital representation of a
user within a virtual space. The avatar can be integrated
with blockchain, to become the digital DNA of this user.
In addition, a user may have digital assets in the metaverse,
which can be cryptocurrencies, skins, or objects that feed from
the physical world. Such assets become valuable and tradable
Copyright (c) 2015 IEEE. Personal use of this material is permitted.
However, permission to use this material for any other purposes must be
obtained from the IEEE by sending a request to pubs-permissions@ieee.org.
Corresponding Author: Huawei Huang, huanghw28@mail.sysu.edu.cn
by using blockchain. Further, blockchain, such as Ethereum,
forms the fundamental economic system in the metaverse. On
the other hand, non-fungible tokens (NFT) ensure that non-
monetary digital assets owned by a user can’t be copied or
fabricated.
To date, there are numerous metaverses have been built [2].
Examples include Roblox [3] for productivity, Sandbox [4]
for virtual real estate, Axie Infinity [5] for gaming. Moreover,
a number of top companies propose their own metaverse,
such as Meta, Nvidia, and Google, to name a few [2]. These
metaverses usually have independent economic systems, i.e.,
they use different tokens or currencies, and rules, i.e., repre-
senting avatars in 2D or 3D form. Hence, it is inconvenient
for users to interchange between metaverses. This slackens the
development of the metaverse society. Therefore, an efficient
cross-metaverse strategy is desired to enable users to switch
between metaverses with negligible effort. We call this cross-
metaverse interoperability.
In a nutshell, cross-metaverse interoperability provides a
seamless experience for users to interact with metaverses. We
now use an example in the commercial metaverses to illustrate
the key features of cross-metaverse interoperability. Consider
a user purchased a digital asset, say an ‘axe’, from metaverse
Alpha, the cross-metaverse interoperability ensures that this
asset has the same value and functions when the user using
it in another metaverse Beta. Note that, the metaverse Alpha
and Beta may have different economic systems based on Bit-
coin, Ethereum, or some type of centralized currency derived
from offline central bank. Moreover, these metaverses may
have different virtual environments, but the cross-metaverse
interoperability ensures that the digital ‘axe’ is used to ‘cut
trees’ in all of them.
In order to achieve cross-metaverse interoperability, the
following two basic components need to be interoperable
between metaverses:
Identity. The identity defines the uniqueness of users and
digital assets across metaverses. In addition, identities and
their relationship are essential to connecting users with
their actions and assets. For cross-metaverse interoper-
ability, identity is fundamental for building trustworthi-
ness. This requires the standardization of identities among
various types, such as users, assets, currencies, objects,
and motions, in all metaverses. It should be noted that
one natural person may create multiple user identities in
one or many metaverses. However, each of these identities
needs to be treated independently, the same with multiple
social accounts owned by the same person.
arXiv:2302.01600v1 [cs.CY] 3 Feb 2023
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Object. This includes avatars, digital assets, and in-
teractable entities in metaverses. Each object has its
own properties, such as gender, material, rendering, and
functionality. These properties can be fed from physical
words, e.g., via DT and 3D scanning, or created by users
out of thin air. For cross-metaverse interoperability, the
objects in different metaverse with the same identity must
have the same properties.
In this article, we first present a comprehensive review
of existing technologies that enable cross-metaverse interop-
erability. We show that these technologies lack generality,
making them hard to implement for interoperability between
arbitrary centralized or decentralized metaverses. We then
propose a metaverse interoperability protocol named Meta-
Opera. It enables interoperability between any two metaverses
regardless of their centralized or decentralized system models.
By employing NFT technologies, all metaverse components
are monetized as unique assets. Thus, they are exchangeable
and satisfy the requirement of crossing the metaverses. As an
exemplary concrete instantiation for MetaOpera, we describe
two MetaOpera workflows in detail, which demonstrate how
MetaOpera support metaverses interoperate with each other.
To evaluate the validity of MetaOpera, we develop a proof-
of-concept (PoC) implementation for MetaOpera metaverse
and Sandbox metaverse. Without loss of generality, our Meta-
Opera metaverse is also applicable to any other metaverses.
We evaluate the performance of MetaOpera and compare
it with a cross-metaverse scheme based on Sidechains [6].
Experiment results show that the cross-metaverse proof of
MetaOpera is roughly 1.7 KB, which is 8 times smaller
than the Sidechains approach; and that the average time
of cross-metaverse transactions is about 1.8 hours, which
is 3 times smaller than the Sidechains approach. Lastly,
we summarize open issues and challenges faced by cross-
metaverse interoperability, which include economic systems,
applications, physical/virtual interoperability, security, off-line
trustworthiness, universal data structure, and policy.
The rest of this paper is organized as follows. Section II in-
troduces the preliminaries and state-of-the-art cross-metaverse
technologies. Section III demonstrates the proposed Meta-
Opera protocol. The performance of MetaOpera is evaluated
in Section IV. Section V suggests open issues and challenges
for cross-metaverse interoperability. Section VI concludes this
paper.
II. PRELIMINARIES AND STATE-O F-T HE -ART
CROS S- ME TAVERSE TECHNOLOGIES
According to the system model of metaverses, cross-
metaverse interoperability is divided into two categories: (1)
the interoperability between decentralized metaverses built at
the top of blockchain technology, and (2) the interoperability
between decentralized and centralized metaverses, where the
centralized metaverses are built at the top of a centralized
server. For the former, the underlying technology of interoper-
ability is cross-chain. For the latter, the underlying technology
of interoperability is on-chain and off-chain exchange. These
two types of underlying technologies are described in detail
below.
A. Cross-Chain Technologies
To further understand cross-chain technologies, we review
state-of-the-art cross-chain works. As described in table I,
there are four kinds of cross-chain technologies: Notary,
Hash Time-Lock, Sidechains/Relay, and Relay chain. Notary
scheme is the most widely used due to its high efficiency
and convenient deployment. A typical example of a notary
scheme is centralized exchange. All cross-chain assets in the
exchange are maintained and managed by a trusted third party
called notary. It is easy to see that if the notary is fail, cross-
chain interoperability will be suspended. To avoid the single-
point-failures, the notary consists of multiple parties based on
their weights (i.e., coin) or reputation. However, the notary
still suffers from security challenges such as external trust
assumption. Indeed, Ronin bridge [43], a well-known notary
scheme, lost 624 million USD due to malware attacks. The
main reason behind this attack is that an attacker illicitly
obtains the majority keys of the notary members through the
malware.
To overcome the external trust assumption, the Hash time-
lock scheme is proposed, which utilizes a hash function and
time-lock features to achieve cross-chain interoperability. the
security of the Hash time-lock scheme is based on crypto-
graphic hardness assumptions. Consider two users: Alice with
xcoins on chain C1and Bob with ycoins on chain C2.
They want to exchange their assets at the exchange rate of
1. First, Alice generates a hash value p=H(p) via a hash
function H(·)and hereby uses it to create a transaction tx1
that transfers xcoins to Bob’s address on C1. That is, Bob can
receive these xcoins from Alice if Bob knows the preimage
pcorresponding to the h. Once the tx1is included on C1,
Bob also uses the hash value pto generate a transaction tx2
that transfers ycoins to Alice’s address on C2, and broadcasts
it into C2network. To gain these ycoins from Bob, Alice
reveals the preimage pin tx2. Meanwhile, Bob also knows the
pvia observing the tx2on C2. Subsequently, Bob publishes
the obtained preimage pin tx1and hereby receives these x
coins from Alice, completing the asset exchange. However,
this scheme only supports monetary exchange and thus has
low scalability.
The sidechains/relay scheme supports the interoperability
of multiple objects such as assets and other data, thus having
high scalability. Sidechain is a blockchain that communicates
with other blockchains via a two-way peg. In particular, the
two-way peg is a mechanism that allows bidirectional commu-
nication between blockchains. An example of a two-way peg is
simplified payment verification (SPV) in Bitcoin. Specifically,
to perceive events on a mainchain, all block headers of the
mainchain are relayed into the sidechain pegged with the
mainchain. The sidechain can determine whether an event
has occurred on the mainchain by verifying the stored block
headers of the mainchain and the Merkle tree verification path
regarding the event. However, the sidechains/relay scheme can
only be applied to specific blockchains, and thus has a low
generality.
A relay chain is a third-party blockchain that other
blockchains can connect to, which enables these blockchains
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TABLE I
CROS S-CHAIN TECHNOLOGIES
Technical
Scheme Typical Project Description Security Efficiency Generality Scalability
Notary
Interledger [7], Tokrex [8],
Croda [9], BTCB [10],
HBTC [11], tBTC [12],
ren [13], DeCus [14], Hop
Exchange [15], Hyphen
[16], Degate Bridge [17],
Wanchain [18], Fusion [19]
A mutually trusted third party is used be-
tween different blockchains to act as a
notary for cross-chain message verification
and forwarding.
Low
[20–22] High High High
Hashed
Time-Lock
WBTC [23], Bridge [24],
Lightning Network[25],
Zcash XCAT [26]
The asset receiver is forced to determine the
collection and produce proof of collection
to the payer within the cut-off time, or
the asset will be returned via hash locks
and blockchain ”time” locks. The proof of
receipt can be used by the payer to ac-
quire assets of equal value on the recipient’s
blockchain or trigger other events.
Medium
[27, 28] High Medium
[22] Low
Sidechains/Relay
BTCRelay [29], RootStock
[30], Plasma [31], Ronin
[32], Elements [33], Lay-
erZero [34], Waterloo [35],
Polygon [36],
Interoperation (transfer, communication, op-
eration) of on-chain objects (assets, data,
functions) through two-way anchoring and
relay mechanisms.
High
[37]
Medium
[37, 38]
Medium
[38, 39]
High
[37, 38]
Relay chain Polkadot [40], Cosmos [41] To achieve cross-chain object interoperation
by the relay chain. Medium Low Medium
[42] High
to communicate with each other. The relay chain scheme has
high scalability, and supports multiple objects interoperability
between blockchains connected to the relay chain. However,
as compared with other cross-chain schemes, the relay chain
scheme has relatively low efficiency, especially in terms of
cross-chain time.
B. On-Chain and Off-Chain Technologies
On-chain and off-chain technologies, often called oracle
technologies, connect on-chain blockchain systems to off-
chain systems, which further enables the interoperability of de-
centralized systems and centralized systems. Currently, oracles
can be generally classified into two types: voting-based oracles
and reputation-based oracles. The voting-based oracles are
usually designed for single centralized systems, which employ
participants’ weight to finalize interoperability outcomes. The
widely deployed strategy of achieving these oracles includes
stake-based oracles, multisignature-based oracles, schelling
point-based oracles, and conventional oracles. However, these
oracles are hard to verify the outcome integrity. On the
other hand, reputation-based oracles are suitable for multiple
centralized systems. Their aims are to authenticate the integrity
of interoperability outcomes. These oracles utilize software
(i.e., TLS protocol) or hardware (i.e., Intel Software Guard
Extension) to generate a proof. The performance comparison
of oracles is shown in Table II.
C. Cross-Metaverse Technologies
In terms of cross-metaverse interoperability, industrials have
proposed prototypes. For example, STYLE protocol [77],
which is a virtual asset infrastructure across-metaverse, has
been proposed in 2022 and will be implemented this year.
The goal of STYLE is to enable assets to exchange across
the metaverses. To achieve this, two key features: usability
and visualization for assets, are introduced into this protocol.
Cross-metaverse avatar scheme [78] aims to enable avatar
interoperability across-metaverse. Same with in the real world
to interact and communicate, avatars, a digital representation
in metaverse, also allow people to feel and travel in multiple
metaverses. Similarly, the scheme will be launched in 2023. In
academics, Huang et al. [2] presented a cross-chain ecosystem
for metaverse. As an example, this work depicts assets inter-
operability between Sandbox’s and Axie Infinity’s metaverses.
However, this study focuses only on decentralized metaverses.
Chen et al. [79] proposed a cross-platform metaverse data
management system. The system designs plug-ins for different
metaverse/games, enabling the profile and space share cross-
platform metaverse.
III. METAOPE RA PROTO COL
In this section, we present a novel cross-metaverse inter-
operability protocol: MetaOpera. We first give an overview
of MetaOpera, and then present its core components and
workflow.
A. Overview
At a high level, MetaOpera is a cross-metaverse interoper-
ability infrastructure, which enables interoperability between
any two metaverses. MetaOpera implements NFT technolo-
gies to monetize all metaverse components as unique assets.
Therefore, arbitrary metaverse users and objects are able to
interoperate with each other. Its key idea is shown in Figure
1. Briefly, the user of the metaverse connected to MetaOpera
first locks its asset, and then mints the corresponding asset,
in the form of NFT, in the MetaOpera. Next, the user of the
MetaOpera stakes its NFT and then mints the corresponding
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TABLE II
ORACLE TECHNOLOGIES
Technical Scheme Specific Type Description Security Integrity Confidentiality
Voting-based Oracles
Stake-based [44–51]
This scheme employs the stake held by partic-
ipants for finalizing the outcome. Participants
are rewarded if the outcome is matched and
penalized otherwise.
High Low Low
Multi-signature based
[52–56]
Finalizing the outcome is determined col-
lectively by multi-participants via making a
signature on the outcome.
Medium Medium Low
Schelling point based
[57–59]
According to the median value, the outcome
is derivated from a group of data providers’
answers.
Low Low Low
Conventional [60–63] The outcome is directly from the raw answer
of the data provider. Low Low Low
Reputation-based Oracles
Software-based Proof
[64–68]
Utilizing the TLS/SSL protocols, the scheme
generates a data authenticate proof which is
used to verify the integrity of outcomes.
Medium Medium Medium
Hardware-based Proof
[69–72]
The scheme provides the integrity and
conidentiality for the outcome based on the
Software Guard Extension (SGX) technology
in the Inter CPUs.
Medium High High
Proofless [73–76]
The scheme retrieves directly from data
sources without providing authenticated proof
of the outcome.
Low Low Low
NFT in another metaverse it wishes, completing metaverse
object interoperability.
B. MetaOpera Core Components
MaoetaOpera is essentially a relay metaverse built at the
top of MetaOpera blockchain, which is used to connect
another metaverse. Upon connecting to the MetaOpera, any
metaverse can communicate with each other. To achieve a
variety of metaverse interoperability, MetaOpera has two
key underlying technologies: cross-chain technology and on-
chain and off-chain technology. The former can support
MetaOpera to interoperate with the decentralized metaverse
based on blockchain. The latter can achieve MetaOpera to
interoperate with the centralized metaverse built at top of
centralized servers. these underlying technologies adopted in
the MetaOpera are resilient. To reach high efficiency across
decentralized metaverse, for instance, the cross-chain scheme
may be a notary technology as described in Table I.
There are five components in MetaOpera listed as follows:
Metaverse. A metaverse is a virtual world. Metaverses
are classified into two categories: decentralized meta-
verse (DM) and centralized metaverse (CM) based on
underlying technologies. Here, DM is built at the top of
decentralized blockchain technology. while CM is built
at the top of centralized serves.
Object. An object is a virtual digital item in the meta-
verse, such as cryptocurrency, avatar, skin, pet, fashion,
etc. These objects can be transferred, sold, rented, and
staked across metaverses. Each metaverse includes out-
bound objects and inbound objects. An object is referred
to as an outbound object if it had been locked in the
metaverse it belongs; subsequently, a corresponding ob-
ject will be minted in another metaverse. On the contrary,
an inbound object denotes the object moved from another
metaverse.
Owner. An owner represents a metaverse user, who holds
some objects and wishes to transfer them from one
metaverse to another metaverse.
Customizer. A customizer is a MetaOpera worker who
assists in object transformation from MetaOpera to
another metaverse in exchange for a reward. For example,
the 2D object of MetaOpera can be tailored into the 3D
object residing on a metaverse through a customizer.
NFT. NFT (non-fungible token) is a unique and ex-
changeable digital token. NFT has the characteristics
of rareness, indivisibility, and uniqueness. Thus, it can
denote the ownership of an object from metaverse, specif-
ically for the centralized metaverse.
C. MetaOpera Workflow
In this subsection, we demonstrate the interoperability work-
flow of the MetaOpera by depicting two across-metaverse
interoperability scenarios, see in Figure 1. The first workflow is
DM to DM, e.g., Axie Infinity metaverse [5] to MetaOpera to
Sandbox metaverse [4]. The other workflow is CM to CM, e.g.,
Minecraft metaverse [80] to MetaOpera to Roblox metaverse
[3]. It is worth noting that MetaOpera can support any two
metaverses to interoperate regardless of the CM or DM models
of the metaverse.
1) DM to MetaOpera to DM: As depicted in Figure1,
two decentralized metaverses (DM), Axie Infinity and Sand-
box, can communicate with each other through MetaOpera.
Specifically speaking, the owner of Axie Infinity wishes
to transfer assets into Sandbox. First, the owner issues a
transaction txx1that locks its asset Axies. Once it has been
locked, Axies will become an outbound object in Axie Infinity.
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X1: Lock NFT
Metaverse #1: Axie Infinity
Outbound object
Object
Axies AXSs
Ronin blockchain
Axies
3: Staking NFT
Y1: Lock Asset
OwnersOwners
Metaverse #3: Minecraft
Avata rs UGC
Avata rs
… …
Outbound object
Centralized server
Object
MetaOpera Protocol
MetaOpera blockchain
Avata rs
Axies
Outbound object
Axies
Inbound object
X2: Mint NFT Y2: Mint NFT
Metaverse #2: Sandbox
Ethereum blockchain
Axies
SAND NFT
Metaverse #4: Roblox
Avata rs
… …
Centralized server
Object
Avata rs
Robux
X4: Mint NFT Y4: Mint NFT-Derivatives
Customizer Customizer
Avata rs
Avata rs
… …
… …
Inbound object
Object Inbound objec t
Fig. 1. The design of MetaOpera illustrated with the example of four metaverse interoperability. The blue arrows denote the direction of cross-metaverse
object transfer. For clarity we only show two workflows of MetaOpera and the opposite direction is symmetric.
Meanwhile, the proof attesting to the validity of txx1is
generated by a committee of MetaOpera. Here the committee
is a notary scheme described in Figure I, The members of
the committee select from the maintainer of MetaOpera
according to their weights (i.e., coins). If the above txx1and
its proof are considered valid, MetaOpera will mint an NFT
corresponding to Axies, The minting action is achieved by a
mint transaction txx2generated by the owner. Until now, the
owner of Axie Infinity has transformed its Axies into the NFT
of MetaOpera.
Further, the owner sequentially transfers the NFT corre-
sponding to Axies into Sandbox. This process is similar to the
transfer from Axie Infinity to MetaOpera. The main differ-
ence is that a customizer may participate in the transformation
of object format, such as casting 2D NFT into 3D NFT, The
reason behind the difference is the heterogeneity of metaverse
model so that an object of a metaverse is incompatible with
another metaverse version. The owner first stakes its NFT
corresponding to Axies in MetaOpera by issuing a staking
transaction txx3. According to the NFT format, a customizer
tailors it into another NFT format supported by Sandbox.
Subsequently, proof testifying that txx3is valid is produced
by the committee of MetaOpera. Finally, Sandbox mints
corresponding NFT to the owner based on the proof and NFT
format cast by the customizer, completing the interoperability
for Axie Infinity and Sandbox.
2) CM to MetaOpera to CM: Upon connecting to the
MetaOpera, any two centralized metaverse (CM) can inter-
operate with each other. Figure 1 shows such an example
that an avatar of Minecraft travels to Roblox. The owner of
Minecraft generates a lock transaction txy1to lock its avatar. A
committee of MetaOpera observes the Minecraft state. Here
the committee is a multi-signature oracle described in Table
II and consists of MetaOpera users. If txy1is confirmed in
Minecraft. The committee will generate a proof for txy1to
convince all MetaOpera maintainers that the avatar has been
locked successfully. Once the proof is generated, the owner
issues a mint transaction txy2to mint an NFT corresponding
to the avatar into its MetaOpera address. The action of
minting the NFT will be implemented if the txy2and proof
are considered valid. This means that the avatar has transferred
from Minecraft to MetaOpera.
Next, the owner further transfers the avatar from Meta-
Opera to Roblox. The owner first generates a staking trans-
action txy3to lock the NFT above. Similarly, the proof for
txy3is also generated. A customizer issues a mint transaction
txy4in Roblox to cast an NFT-derivative corresponding to
the locked NFT. Where NFT-derivative is not NFT but an
object compatible with Roblox, since Roblox does not support
NFT. To keep the format persistence of the NFT-derivative
and the Minecraft avatar, casting NFT-Derivative is essential.
This is done by the customizer. The above casting action will
be executed eventually if the txy4and proof are considered
valid. Until now, the avatar of Minecraft has been transferred
into Roblox, completing the interoperability for Minecraft and
Roblox.
3) DM/CM to MetaOpera to CM/DM: As discussed ear-
lier, MetaOpera is a decentralized metaverse built at the top
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of the blockchain. Objects are able to interoperate from DM or
CM to MetaOpera and then interoperate to DM or CM again.
Alternatively, MetaOpera can support objects interoperating
from DM to CM or CM to DM.
IV. PERFORMANCE EVALUATION
In this section, we implement the proposed MetaOpera
protocol and evaluate its performance. We first measure the
cross-metaverse proof size and the average time of cross-
metaverse transactions. We then compare them with a state-
of-the-art cross-metaverse scheme based on Sidechains [6].
Implemention Setting. To simulate the performance of
Metaopera prorocol from Section III, we make a proof-
of-concept (PoC) implementation for Metaopera and Sand-
box. Without loss of generality, the PoC implementation is
also applicable to both Metaopera and anyone metaverse
connected to Metaopera. In our PoC implementation, the
blockchain of Metaopera is instantiated as Cardano [81].
While the blockchain of Sandbox is based on Ethereum [82],
which follows the Sandbox implementations of [4]). The PoC
implementations for MetaOpera and Sandbox are executed
in a personal platform installed with Windows 11 operating
system and equipped with Intel(R) Core(TM) i7-12700KF
CPU 3.60 GHz 32.00GB RAM. We carry out our MetaOpera
blockchain in standard C language. To achieve committee
members’ vote, the multi-signature scheme [83], a crypto-
graphic primitive, is employed in our PoC implementations.
Here the size of public key |vki|= 272 bits, the size of
signature |σi|= 528 bits, the size of proof-of-possession
|P OP |= 528 bits. A 256-bit hash function H(·)is also
applied in the PoC implementations, meaning that |H(·)|=
256 bits. Following Cardano implementation, we denote by ka
common prefix parameter. To form a decentralized Metaopera
committee, the chain quality, a blockchain fundamental secu-
rity property, is adopted in the committee selection. In short,
the committee members are selected from nodes that have
generated any kconsecutive blocks of Cardano. Moreover, we
set that the transaction size |tx|= 250 bytes, block height size
|h|= 32 bytes, and the random number size |r|= 32 bytes,
these parameters are important components of the above proof.
On the other hand, we use k0to indicate a common prefix
parameter of Ethereum. In addition, both the block-generating
times of Cardano and Ethereum are set to 5 seconds.
Implementations and Evaluations. In our Metaopera, the
proof attesting to the validity of cross-metaverse transactions
is generated by a committee of Metaopera. Its size affects
the efficiency of cross-metaverse transactions, For example,
The larger the proof size is, the longer its verification time
will be. Thus, we evaluate it. A proof is composed of public
keys, signatures, and hash values. Based on the principle of
the adopted multi-signature scheme, the proof can be further
improved by the optimized tips used in [6]. Formally speaking,
the improved proof can be denoted as |tx|+|h|+|r|+ 0.1·
k· |vki|+|σi|. As depicted in Figure 2, the proof size of
Metaopera increases linearly in the size of committee c.
This is because the proof size increases as the number of
committee members increases. More precisely, in the case of
the committee size of 400, the proof size is roughly 1.7 KB,
which is 8 times smaller than the proof size of PoS sidechains
[6].
100 200 300 400 500 600
Committee size (c)
0
5
10
15
20
25
PoS sidechains proof size (KB)
PoS sidechains (the left Y-axis)
MetaOpera (the right Y-axis)
0
1
2
3
4
5
MetaOpera proof size (KB)
X 400
Y 13.6857
X 400
Y 1.69922
Fig. 2. The proof size at different committee sizes.
In our PoC implementation, cross-metaverse transactions
consist of two transactions: tx included in Metaopera and tx0
included in Sandbox. To verify the validity of tx, the proof is
essential. Therefore, the time of cross-metaverse transactions
includes both the confirmed time of tx and tx0, as well as
the proof generating time. Let kand k0be 400 and 500
respectively. We have that the confirmed time of tx is roughly
5k= 5 ×400 = 33.33 minutes and the confirmed time of k0
is roughly 5k0= 5 ×500 = 41.67 minutes. In the case of
a committee size of 400, we measure the time tthat these
committee members collectively generate the proofs for 200
different cross-metaverse transactions. Measurement results
show that the average time for generating a proof is roughly
1.5 minutes. To this end, the average time of cross-metaverse
transactions is roughly 76.5 minutes, as described in Table
III. As a comparison, the average time of transactions of
Sidechains [6] is 5.251 hours, which is 3 times more than
our MetaOpera.
V. OPEN ISSUES A ND CHALLENGES
Although we have proposed the MetaOpera protocol,
cross-metaverse interoperability is still in its initial stage from
the perspective of either industry or academia. In this section,
we summarize open issues and challenges in cross-metaverse
interoperability and suggest potential future research direc-
tions.
A. Economic System
To date, most metaverse builds their economic system using
smart contracts on the Ethereum blockchain. This raises the
risk that a single security hazard in Ethereum will affect
a majority of metaverses. To this end, diversified economic
systems are desired in co-existing metaverses, to enhance their
robustness against single-system crashes. This also requires
cross-metaverse interoperability of these systems.
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TABLE III
THE AVER AGE T IM E OF CR OSS -METAVERSE TRANSACTIONS.
Time of cross-metaverse transaction MetaOpera time MetaOpera time/h Sidechains [6] time Sidechains [6] time/h
Confirm Time of tx in MetaOpera 2k 1.112 10k 5.556
Proof Generating Time t0.025
Confirm Time tx0in Sandbox k00.695 k00.695
Average time of cross-metaverse transaction 2k +t+k01.832 10k +k06.251
B. Applications
At the current stage of metaverse development, almost all
the applications provided by the blockchain-based metaverses
are the circulation of cryptocurrencies and NFT. However,
the metaverse has more potential applications other than this.
For example, the immersive experience can be used for a
deep interaction between humans and multiple physical or
virtual environments. This needs further development of cross-
metaverse interoperability technologies to boost the applica-
tion scenarios of the metaverse.
C. Physical/Virtual Interoperability
Most research and implementation of metaverse focus on
mapping physical objects to the virtual metaverse. There are a
number of technologies that can be used, such as 3D scanning.
However, how the actions in the virtual world reflect in the
physical world is also a key concern. This is important for
production metaverse applications such as surgery and driving.
Such interoperability requires trustworthiness, low latency, and
accuracy of the reflection of the action.
D. Security
The security issues in cross-metaverse interoperability con-
cern with authentication and permissions for assets and data
exchanges. In particular, when an asset or object data is
crossing metaverses, both metaverses need to confirm their
ownership. Moreover, these metaverses need to ensure the
asset or data is only valid in only one metaverse. For the
objects existing in a metaverse, permission is important to
specify who and what actions can be interacting with the
objects.
E. Off-line Trustworthiness
Even though the Internet is universal existence in the
physical world, it is inevitable that some locations still have
no Internet connection, such as in a desert. After an object lost
connection with the metaverse, its properties may be changed
before back online. In such a situation, cross-metaverse inter-
operability needs to ensure the trustworthiness of the changes
from a tiny centralized server, i.e., an offline data collector, to
the centralized or decentralized online metaverse.
F. Universal Data Structure
Physical objects have common properties, such as shape,
weight, and color. However, some of their properties are
unique and independent. For example, a cup has the capac-
ity property, but a pencil does not. Hence, cross-metaverse
interoperability needs to design a universal data structure to
describe the physical objects in the metaverse. In addition,
this data structure needs to be a standard for all metaverses to
enable interoperability.
G. Universal Policy
The challenges of the universal policy include two per-
spectives: governance and metaverse platform. In terms of
governance policy, a universal law or rule is required to
restrain criminal behavior. This is because metaverses have
independent rules, and their evaluation criteria for improper
behavior are different. Hence, a universal governance policy
is vital to prevent cross-metaverse crimes. On the other hand,
existing metaverse platforms usually have different user poli-
cies. Hence, users are hard to control the privacies, copyrights,
and exchange of their assets in multiple metaverses. Therefore,
a universal metaverse platform policy is also critical for cross-
metaverse interoperability.
VI. CONCLUSION
Cross-metaverse interoperability is a fundamental require-
ment when users move their digital assets between diverse
metaverses. This paper first introduces the preliminaries and
basics of interoperability of cross-metaverse. It then proposes
the MetaOpera protocol for cross-metaverse interoperability
and evaluated its performance. This paper also discusses the
challenges and open issues of cross-metaverse interoperabil-
ity. Furthermore, we hope this article is capable to inspire
researchers, engineers, and educators to explore more coop-
erative metaverse applications to establish a better metaverse
society.
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Taotao Li received the Ph.D. degree in cyber security from the Institute of
Information Engineering, Chinese Academy of Sciences and University of
Chinese Academy of Sciences, China, in 2022. He is currently a postdoc
with the School of Software Engineering, Sun Yat-Sen University, Zhuhai,
China. His main research interests include blockchain, Web3, and applied
cryptography.
Changlin Yang [M’17] obtained his PhD degree from the University of
Wollongong in 2015. He was a Senior Wireless Engineer at Huawei Australia
from 2015 to 2017, a lecturer at the School of Computer Science, Zhongyuan
University of Technology, China from 2017 to 2018 and 2021 to 2022,
and a postdoctoral researcher at the Department of Electrical Engineering,
Columbia University, USA from 2019 to 2021. He is now a research
fellow with the School of Software Engineering, Sun Yat-Sen University. His
research interests include coverage problems in Internet of Things, blockchain
scalability and trustworthiness metaverses.
Qinglin Yang [M’21] received his B.S. degree from the School of Ge-
ographical Sciences in 2014 and received his M.S. degree of computing
mechanism from College of Civil Engineering, Kunming University of Science
and Technology in 2017, and a Ph.D. degree from the University of Aizu,
Japan in computer science and engineering and 2021. He is a research fellow
at School of Intelligent System Engineering, Sun Yat-sen University, China.
His research interests include edge computing, federated learning, and Web3.
Siqi Zhou is now pursuing a Ph.D. degree in mathematics from the School
of Mathematical Sciences, Shanghai Jiao Tong University, China. Her main
research interests include quantum information, quantum algorithm, and
blockchain.
Huawei Huang [SM’22] (corresponding author, huanghw28@mail.sysu.
edu.cn) is an Associate Professor at Sun Yat-Sen University. He received his
Ph.D. degree from the University of Aizu (Japan) in 2016. He has served as a
research fellow of JSPS, and a program-specific Assistant Professor at Kyoto
University, Japan. His research interests include blockchain and distributed
computing. He has served as a lead guest editor of blockchain special issues
at IEEE JSAC and IEEE OJ-CS. He also served as a TPC chair for multiple
blockchain conferences and workshops.
Zibin Zheng ( Fellow, IEEE) is a Professor and Deputy Dean of the School of
Software Engineering, Sun Yat-sen University, China. He published over 200
international journal and conference papers. According to Google Scholar,
his papers have more than 26,000 citations. His research interests include
blockchain, software engineering, and services computing. He was a recipient
of several awards, including the IEEE TCSVC Rising Star Award, IEEE Open
Software Award, Top 50 Influential Papers in Blockchain, the ACM SIGSOFT
Distinguished Paper Award of ICSE, and the Best Student Paper Award at
ICWS.
Article
With the advancement of blockchain technology, hundreds of cryptocurrencies have been deployed. The bloom of heterogeneous blockchain platforms brings a new emerging problem: typically, various blockchains are isolated systems, how to securely identify and/or transfer digital properties across blockchains? There are three main kinds of cross-chain approaches: sidechains/relays, notaries, and hashed time-lock contracts. Among them, notary-based cross-chain solutions have the best compatibility and user-friendliness, but they are typically centralized. To resolve this issue, we present Bool Network – an open, distributed, secure cross-chain notary platform powered by MPC-based distributed key management over evolving hidden committees. More specifically, to protect the identities of the committee members, we propose a Ring verifiable random function ( Ring VRF ) protocol, where the real public key of a VRF instance can be hidden among a ring, which may be of independent interest to other cryptographic protocols. Furthermore, all the key management procedures are executed in the TEE, such as Intel SGX, to ensure the privacy and integrity of partial key components. A prototype of the proposed Bool Network is implemented in Rust language, using Polkadot Substrate.
Article
With the rapid evolution of the blockchain technologies, the interoperability of different blockchain systems is emerging as one of the essential features of blockchains. Sidechains, a mechanism providing communications between different blockchains, have been heralded as the crucial factor of blockchain interoperability. However, there are still issues that need to be addressed in terms of security and feasibility. In this paper, for proof-of-stake (PoS) and proof-of-work (PoW) blockchains, we propose efficient sidechain constructions with fast cross-chain transfers and small proof size by novel cross-chain certificate generation process and committee selection methods. Moreover, we also provide an extra functionality of supporting instant cross-chain transfers, such that emergent cross-chain transactions can be processed immediately. Compared to prior sidechains, our PoS sidechain construction can achieve faster cross-chain transfers, which improves the promptness of cross-chain transfers. While our PoW sidechain construction is more efficient with smaller proof size, reducing the storage and bandwidth overhead. Furthermore, we formally prove our sidechain constructions satisfying the properties of atomicity and timeliness. Finally, we develop a proof-of-concept implementation of our sidechains, and the experimental results show our constructions is not only faster, but also efficient with low storage and bandwidth overhead.
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