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The Burn-to-Claim cross-blockchain asset transfer protocol

Authors:

Abstract

The future of multi-blockchain architecture depends on the emergence of new protocols that achieve communication between trustless cross-chain participants. However, interoper-ability between blockchains remains an open problem. Existing approaches provide integration through solutions using a middle-ware system, which makes it harder to gain confidence mainly in terms of security and correctness of the process. A cross-chain protocol needs to provide a self-verifiable state-proof that embeds trust in the transfer process. We propose a Burn-to-Claim cross-chain protocol to seamlessly exchange assets between networks. Our scheme transfers assets from one blockchain system to another in a way that the asset is burned from the source blockchain and claimed on the destination blockchain. Our mechanism employs combinations of crypto mechanisms such as digital signatures and time lock to operate the protocol in a distributed manner. We provide an analysis which proves that our cross-chain protocol transfers assets correctly and securely.
The Burn-to-Claim cross-blockchain asset transfer
protocol
Babu Pillai
School of ICT
Griffith University
Gold Coast, Australia
babu.pillai@griffithuni.edu.au
Kamanashis Biswas
Faculty of Law and Business
Australian Catholic University
Brisbane, Australia
kamanashis.biswas@acu.edu.au
Zh´
e H´
ou
School of ICT
Griffith University
Brisbane, Australia
z.hou@griffith.edu.au
Vallipuram Muthukkumarasamy
School of ICT
Griffith University
Gold Coast, Australia
v.muthu@griffith.edu.au
Abstract—The future of multi-blockchain architecture depends
on the emergence of new protocols that achieve communication
between trustless cross-chain participants. However, interoper-
ability between blockchains remains an open problem. Existing
approaches provide integration through solutions using a middle-
ware system, which makes it harder to gain confidence mainly
in terms of security and correctness of the process. A cross-
chain protocol needs to provide a self-verifiable state-proof that
embeds trust in the transfer process. We propose a Burn-to-
Claim cross-chain protocol to seamlessly exchange assets between
networks. Our scheme transfers assets from one blockchain
system to another in a way that the asset is burned from the
source blockchain and claimed on the destination blockchain.
Our mechanism employs combinations of crypto mechanisms
such as digital signatures and time lock to operate the protocol in
a distributed manner. We provide an analysis which proves that
our cross-chain protocol transfers assets correctly and securely.
Index Terms—blockchain, interoperability, asset transfer,
cross-blockchain protocol
I. INTRODUCTION
Blockchain technology offers an immutable, decentralized,
and transparent mechanism for transaction processing. In-
terestingly, beyond its role as a protocol for exchanging
values within a network, one might reasonably assume that
a blockchain system should be able to transfer assets between
networks. However, the current architecture of this technology
limits the transaction within the same network. A blockchain
application cannot uniformly use multiple networks and obtain
a composition of their guarantees [1]. A core reason of
this issue is that each blockchain network has its own state
assumptions of the proof-problem [3], and without restrictions,
one network cannot verify the information in a different
network. Therefore, currently, seamless composition between
two blockchain networks is difficult [12].
The cross-blockchain integration would enable the in-
teroperability among distinct, potentially heterogeneous,
blockchains [8]. However, with the current protocols of
blockchain systems, it is difficult to have direct interoperability
between systems [4]. There is no method in the system to
provide a cross-chain value transfer; therefore, external third-
party services are the preferred solution [1, 12]. However,
the blockchain system cannot recognise and verify any such
process carried out by a third-party provider. Today most en-
terprise blockchain applications reside on private blockchains
such as Hyperledger and Corda and unable to leverage the full
potential of the distributed ledger technology.
To address the above issues, we propose a built-in process
that is integrated with the protocol to carry out transactions and
facilitate interoperability between systems. We first formalise
the concept of interoperability from our previous work [13]
and then introduces the Burn-to-Claim cross-blockchain proto-
col – a built-in method to address cross-chain interoperability.
We define it as a protocol which consists of two components:
an exitTransaction to generate a self-verifiable proof and an
entryTransaction to verify the validity of the proof in order to
re-create the asset. The contribution of this paper are threefold:
Protocol: We propose a protocol for cross-blockchain
asset transfer where transactions are performed in a
decentralised and trustworthy manner.
Workflow: We propose a novel and simple workflow
which is flexible and can be adopted for digital asset
transfer among blockchain networks without violating the
key characteristics of the blockchain technology.
Analysis: We perform a preliminary analysis to show
the correctness, security and fairness of the proposed
protocol.
A. Assumptions
The following assumptions are made in the proposed protocol:
The underlying blockchain networks are secure with a
concept of transaction finality within finite time, after
which the transactions cannot be rolled back [2].
The participants involved in the cross-communication
process ‘trust’ each other to a required minimum level
and are willing to process the transaction if valid proof
is presented by the other party. The required level of trust
varies depending on the application and to be agreed by
both parties.
The network nodes are motivated to take part in the
respective proof-mechanism (mining process) on both the
chains.
A transaction output carry a single output which cor-
responds to a single asset and the networks involved
recognize and form a common understanding of the assets
they are transacting.
Under these assumptions, once a transaction is broadcasted,
the network nodes verify the transaction and include it in
a block. A blockchain system tends to synchronise using a
protocol in which the network nodes constantly try to produce
new blocks and broadcast their achievements to the entire
network. In the case of cryptocurrencies, for instance, this
behaviour is motivated by mining rewards. Here also we
assume there is an appropriate incentive mechanism to support
participation by nodes.
II. TH E BURN-TO -CLA IM P ROT OC OL
In this section we formally define the proposed Burn-to-
Claim protocol and its primitives. A summary of notations
used in this paper is available under this Git repository1.
A. Communication between networks
The Burn-to-Claim protocol consists of two main functions
for the communication between networks: exitTransaction
which locks the asset and serves as a transfer-proof in the
source network, entryTransaction which verifies the validity
of the transfer-proof in order to re-create the asset in the
destination network.
In our protocol, the sender who wants to transfer the asset
must present a proof that the asset is locked. To achieve this,
we adopt the proof-of-burn protocol [10, 14], which presents
a mechanism where the sender transfers the asset to a non-
spendable burn-address and is able to present that transaction
as a proof for the locked asset.
Definition 1 (Burn-address).A burn-address given as βis
an address to which one can send assets, but from where
they can never be recovered because there is no private key
corresponding to that address.
The process of burning consists of sending crypto asset to an
address where they become inaccessible and useless. Typically,
these addresses are randomly generated, where the addresses
do not have a corresponding private key therefore the asset
at those addresses are not spendable. The burn protocol [10]
presents proofs such that if the underlying cryptographic
scheme is secure then the probability of finding a private key
for a given burn address is nearly negligible.
1) Exit transaction: The exit-transaction must be initiated
on the source network by the sender. This execution checks the
validity of the transaction and generates a transfer-proof. The
transaction-validity process checks the authenticity of the asset
and the owner’s ability to spend. The transfer-proof generator
produces a proof that the asset exists and it is locked while
the asset is in transit. This transaction aims to create an exit
proof for that asset in the source blockchain network. Having
an exit proof created by the system as part of the protocol will
ensure the system’s security. Moreover, the network comes to
a consensus about the asset transfer, thereby the authenticity
of the information is satisfied.
In our protocol, exitTransaction uses a conditional time-lock
with a secret code – the former determines a time frame for
1https://github.com/b-pillai/burn-to-claim
the transaction, and the latter is used to claim the asset in the
destination network. A time-lock is defined as a function that
locks the output of a transaction for a period of time such that
the asset cannot be spent until the time has elapsed2.
Our protocol requires the sender to generate a secret code γ
using a random key generator function, keyGen(). Then the
secret code γis encrypted using the public key of the recipient
KR
pbefore sharing with recipient. The encrypted secret code
is denoted by Γ. We assume the public key information is
shared among users during the preparation stage.
Algorithm 1 exit-transaction-protocol
1: function exitTransaction(T x(KS
p,KS
adr,β,v,T x),H(γ),σ)
2: if exportVerifier(KS
adr,v,σ) is true then
3: KS
adr β:v
4: vis timelocked(v, t) at βin N1
5: T xt(T x,H(γ),σ)
6: else
7: invalid transaction
8: end if
9: return transaction receipt
10: end function
The function exitTransaction defined in Algorithm 1 takes a
tuple of (T x(KS
p,KS
adr,β,v,T x), H(γ),σ) as inputs where
T x includes the sender’s public key (KS
p) and address (KS
adr),
burn-address (β), asset (v) and the previous transaction (T x)
in which the asset vwas spent. H(γ)represents the hash value
of secret code γand σis the digital signature of the T x .
Definition 2 (exportVerifier).The exportVerifier function con-
sists of two sub-functions spendVerifier and assetVerifier, it
returns true when both sub-functions return true, and it returns
false otherwise.
spendVerifier:
returns true if for the given KS
adr from KS
p
the function verify(T x,KS
p,σ) returns true and
getAddress(KS
p) returns KS
adr;
returns false otherwise.
assetVerifier:
returns true if balance(KS
adr) is true and T xis
included in a valid state;
returns false otherwise.
If exportVerifier returns true then the transaction executes
the transfer of the asset to the given burn-address βwith
conditions such that the asset is time-locked within the source
network for a defined time-lock period tand the hashed secret
code H(γ)is added to the data structure of the transaction.
2) Entry transaction: The purpose of the entry-transaction
is to recreate the asset in the destination network. An entry-
transaction must be initiated in the destination network by
the recipient. Upon initiating the entryTransaction with the
transfer-proof from source chain, the network nodes verify the
validity of the transfer-proof and recreate the asset.
2https://bcoin.io/guides/cltv.html
Algorithm 2 entry-transaction-protocol
1: function entryTransaction((T x(KR
p,KR
adr,β,v,T x),Γ,σ)
2: if importVerifier(T x,T x,σ)and
3: (T x.time-lock is under the time limit) and
4: decrypt(Γ,KR
r) = T x.H(γ)) is true then
5: βKR
adr :v
6: T xe(T x,H(γ),σ)
7: else
8: invalid transaction
9: end if
10: return transaction receipt
11: end function
The function entryTransaction defined in Algorithm 2 takes
a tuple (T x (KR
p,KR
adr,β v,T x), Γ,σ) as input where
the T x includes the recipient public key KR
p, the recipient
address KR
adr, the burn-address β, the asset vand the previous
transaction T x;Γrepresents the encrypted secret code and σ
denotes the digital signature.
Definition 3 (importVerifier).The importVerifier consists of
two functions spendVerifier and proofVerifier, it returns true
when both sub-functions return true, and false otherwise.
spendVerifier:
returns true if for the given burn-address βfrom
KR
pthe function verify(T x,KR
r,σ) returns true and
getAddress(KR
p) returns KR
adr (assuming the node
use a source chain version of getAddress function);
returns false otherwise.
proofVerifier:
returns true if βis generated from KR
adr, balance(β)
=burn,T xis included in a valid state;
otherwise returns false.
A proofVerifier is an extended version of assetVerifier.
Here the nodes on the destination network need to verify the
proof from the source network. We assume that through the
gateway mechanism, the destination network nodes are able
to verify the proof. Through the T xany gateway node can
access the specific transaction in the source network. Once the
importVerifier returns true, the mining nodes need to check the
time-lock and the secret code. We assume that both the source
and the destination network run on a global clock. If the time
is under the time-lock period and the hash of decrypted secret
code matches with the hash value embedded in the transaction,
the network awards the asset to the recipient address KR
adr.
B. Workflow of the protocol
This section presents a walk-through of the workflow of
the burn-to-claim protocol. We begin with a use case of
two blockchain systems which are self-sufficient and secure.
The two networks run different applications but they want
to interoperate. These networks may have distinct consensus
participants that employ different agreement protocols. It is
assumed that the majority of consensus participants in both
networks are honest. Here the assumption is that even though
these systems are not connected, they have enough credibility
which is governed by a protocol and have some common
agreements. For example, they can be two different businesses
with a collaborative business interest, different branches of a
company or different departments in an organisation. The main
objective of this paper is to address the cross-blockchain trans-
action proof-problem. Therefore, we focus on the construction
of consensus on how the transactions are verified, and on what
conditions the transactions are valid.
1) Network assumptions: To address the state proof-
problem, we made some assumptions about the network
participants and their ability to mine. We assume that the
cryptographic primitives of the networks are secure. For the
underlying network, we make the same assumptions as pre-
sented in [5, 11]. One assumption is that the nodes in sender
and receiver networks are synchronized with a global clock.
In the network of our model, some nodes are elected
as gateway nodes. We envision each blockchain as an au-
tonomous system, which communicates with each other via a
cross-blockchain protocol. To facilitate communication among
blockchains, the nodes can rely on decentralised integration
mechanisms that can identify and address cross-chain inte-
gration. This decentralised integration mechanism acts as a
verification method for the cross-blockchain protocol. The
architecture of our model can be seen as a number of net-
works (based on the topology) connected through a gateway
mechanism to create a network-of-networks (NoN) [6].
There will be multiple nodes which function as gateways on
the same network; therefore, they will be competing against
each other for the mining reward. We assume that not all
the nodes will verify the transaction, but a sufficient number
of them must do to satisfy the security of the system. The
other nodes will accept a gateway node’s proposal based on
the credibility of the gateway node in the network. We leave
further discussions on a decentralised integration mechanisms
for future work.
2) Example: We set an example where Alice (the sender
Sin the blockchain network N1) wants to transfer an asset
to Bob (the recipient Rin the blockchain network N2). Alice
first submits an exitTransaction to network N1. Network N1
executes the transaction, effectively burns Alice’s asset in
network N1. Bob notices that the asset has been burned
successfully and submits the proof of burn to network N2.
Network N2verifies the proof and if successful, recreates the
asset and assigns it to Bob’s account. Figure 1 shows a brief
overview of the protocol construction.
3) Preparation stage: In the preparation stage, first, Alice
Sand Bob Rrequire to exchange their public keys via a key
exchange mechanism. Suses the public key of Rto encrypt
the secret code (γ) and share the encrypted secret code (Γ)
with Rto initiate entryTransaction. Then, they mutually agree
on the time-lock period t. After that, Alice Sgenerates the
burn address (β). Here, we describe the steps in detail.
In order to spend value in an address Kadr, a user needs to
prove the ownership of the public key that is used to generate
the address. That means any value sent to an address with
Figure 1. A high level overview of the Burn-to-Claim protocol workflow. The boxes represent chained blocks along the time steps in the source and destination
networks. At the preparation stage, Sgenerates and shares an encrypted secret code encrypt(γ) with R. At the commit stage, Sinitiates the exitTransaction,
burning the asset to βgenerated from R’s address, then time-locks the transaction output with the hash value of H(γ). After confirmation period Rgets
notified with the previous transaction T x. At the execution stage, Rinitiates exitTransaction, claiming asset v. To do that Rmust prove the relation with
βand reveal γwithin the time-lock period. If Rfailed to claim the v,Sreclaims the vafter the time-lock period.
no private key can be considered as burned. However, how
do we know that an address does not have a private key?
To address this issue and to make a more specific proof in
our protocol, Suses R’s address to generate the burn address
which is given as getAddress(KR
adr) and returns β. This will
guarantee that the address βdoes not have a private key in
the source network and βis more specific to the recipient R
because it is generated from R’s address. Therefore, we can
use such a transfer to burn address as a valid proof.
Now that we have the proof we need to add a provision
for retrieving the asset in case of an unsuccessful transfer.
For instance, the asset is burned on the source chain and
not recreated in the recipient network, or the asset is sent
to a wrong address. We name this property as reclaim-from-
burn and explain the process in Algorithm 4. To ensure
this correctness of this property, we impose a time-lock on
transaction output in the source network. This will stop the
sender from claiming the asset back before the recipient claims
the asset. The time-lock period is agreed based on factors such
as the network latency and consensus timing, which we refer
to as transit-time.
We also need to consider the double-spending problem
where a dishonest sender commits exit-transaction on the same
asset multiple times [9]. In order to prevent double-spending,
our protocol uses a secret code and a time-lock value. To claim
an asset, the recipient must reveal the secret code whereas the
sender is unable to reclaim an asset until the time-lock period
is expired.
4) Commit stage: In the commit stage, Screates and
broadcasts an exitTransaction to invoke Algorithm 2 in the
network N1.
Algorithm 3 exit-transaction-time-key-lock condition
1: while (time-lock is true) do
2: if (Rclaim v)then
3: Rreveal secret γto N2
4: N1.miner gets γfrom N2
5: N1.miner claims his γ-locked fee
6: .secret γis known to N1&N2
7: end if
8: end while (after the tlock)
9: if (Rfail to claim v)then
10: Sreveals γto N1
11: Sre-claim v
12: N1.miner claim his γ-locked fee
13: .secret γis known to N1
14: end if
Algorithm 3 shows the transfer condition logic. The con-
dition here is that anyone claiming the transaction output
(includes the asset and the miner’s fee) must reveal the secret
code. Therefore, the transaction will be mined by the miners
who has access to the destination network’s data because they
need to get the secret code to claim the fee. That means if
there is no valid gateway node then this transaction will not
go through in the first place.
The time-key-lock condition mechanism allows only one
party to claim the asset at a time. For example, while the
T xtoutput is time-locked in the source network, if Rreveals
the secret code γin her network, the miner who mined this
transaction in the source network will reveal it in source
network to claim his fees; thereby the γis known to both
networks. After that, Swill not be able to claim the asset.
Likewise, if Rfails to claim the asset within the time frame,
then Sreveals γafter tlock expired in the source network to
reclaim the asset. In our design γis not known to the network;
therefore, once the γis revealed nobody will be able to claim
the asset even after tlock the expiry of tlock.
5) Execution stage: The execution stage has two possi-
bilities: either Rclaims the asset within the transit time-
lock period or Sreclaims the asset after the time-lock period
expired.
a) Rclaims the asset: Rconstructs an entry-transaction
and broadcasts to N2. If Rcan prove the ownership of KR
r
and KR
adr, the network will be able to process the transaction.
However, our protocol requires a solid evidence that Shas
burned the asset von the source network. Here Rprovides a
burn-address βand a previous transaction T x, which form
the proof of commit in the source chain.
Our protocol requires a mining process where some nodes
are able to mine in multiple blockchains. With that require-
ment, among nnumber of nodes mgateway nodes propose
this transaction and eventually one proposal will get accepted
by the network. We assume that these gateway nodes are
able to validate the transfer-proof with T x. The nodes verify
that transaction occurred on the source chain by ensuring that
the transaction is contained in a block. This can be done by
checking the chain validity [7] of the block and the transaction.
If importVerifier returns true the network awards the asset v
to the recipient address KR
adr. That means by now the secret
code γis known to the network, and after that, the source
network miner will take it to the source network to claim his
fee, and then γis known to both the networks.
b) Sreclaims the asset: In case the recipient has not claimed
the asset within the time-lock period, the sender gets notified
by the network or the mining node who mined the T xt, and the
sender then invoke reclaimTransaction, which is is a variant
of entryTransaction and is defined in Algorithm 4, to claim
the asset back. The transaction first checks the signature via
verify(T x,KS
adr,σ), then checks the time-lock period and
secret code hash. As we stated earlier, our protocol requires
some miners to mine in both the chains. Therefore, we assume
that miners are able to check with the network N2before
approving this transaction.
Algorithm 4 reclaim-transaction-protocol
1: function reclaimTransaction((T x(KS
p,KS
adr,β,v,T x),Γ,σ)
2: if reclaimVerifier() is true and
3: (T x.tlock pass the time limit) and
4: decrypt(Γ,KS
p) = T x.H(γ)) is true then
5: βKS
adr :v
6: else
7: invalid transaction
8: end if
9: return transaction receipt
10: end function
The function of reclaimVerifier consists of two sub-
functions spendVerifier and proofVerifier. The function check
if verify(T x,KR
r,σ) returns true and getAddress(KR
r) returns
KR
adr. Then the proofVerifier checks the transfer proof of T x
referring to if the balance(β) = burn and T xBand B
Qreturn true else return false. The reclaimTransaction can be
included with the entryTransaction but for clarity, we present
it as a separate transaction.
III. ANALYSI S
In this section, we analyse the correctness and security of
the Burn-to-Claim protocol. Cross-chain data guaranty/trust
is one of the most important means to enable interoper-
ability among blockchain networks. The integration process
for exchanging information may be based on other existing
techniques. But to build trust about the shared information we
must resolve specific properties of the individual transactions
involved in the exchange process; that is, security: a cryp-
tographic assurance of transfer commitment of transactions;
correctness: each successful transaction commits only one
valid outcome and fairness: either the transfer executes the
transfer of an asset or return the asset [2, 10].
It should be noted that the burn-address in our proposed pro-
tocol is not generated from a regular keypair, rather a unique
address is generated each time by combining the recipient
address and other parameters. Therefore, it is not spendable
in the current network. This means, with the signature scheme
of the underlying cryptocurrency, the asset burned in the
proposed scheme would remain unspendable.
Lemma 1 (Unspendability).A burn-address βis unspendable
with respect to a blockchain address protocol [11].
The main functionality of entryTransaction is to recreate
asset but only after the asset is burnt on the source chain.
In our protocol the asset must be permanently burned at the
commit stage. With the burn protocol and its property of
unspendability the asset is permanently burned before claiming
on the source chain.
Lemma 2 (Burn before claim).An asset vwhich is transferred
from N1to N2must be burned in N1before a user can claim
it in N2.
The sender who initiates the exit-transaction must own the
asset he is being transferred. In exitTransaction the exportVer-
ifier() checks the transaction validity and owner’s ability to
spend. This process must be carried by each mining node and
must reach the consensus of the network. Therefore, as long
as the network is secure, the participants can only exchange
the asset of their own.
Lemma 3 (Ownership).The function exitTransaction can only
be successfully executed if the sender owns the asset.
Theorem 1 (Security).The recipient network can rely on the
Burn-to-Claim exit-proof guarantee provided by the source
network.
The function exitTransaction transfers the asset vto a burn-
address βthat is derived from the recipient Rs address KR
adr.
To claim v, the recipient must prove to the network that βis
derived from an address he owns. The function spendVerifier
checks the signature σto verify the Kpand checks the whether
βis derived from the given Kpusing the function getAddress.
Therefore, only the user who owns a private key associated
with KR
adr can make the claim of the asset in the destination
network.
Theorem 2 (Correctness).The exchange operation only ex-
change an asset to a correct recipient.
We assume that no adversary can obtain the private key of
a secure signature scheme, except with negligible probability.
As a result, the correctness of our protocol is dependent on a
probabilistic polynomial-time adversary can decipher the key.
Let σbe a secure signature scheme then the possibility of
a distribution entity to decipher the Krfrom a Kpor βis
unpredictable.
Theorem 3 (Fairness).The exchange operation should only
execute one outcome, either the exchange succeeds and the
asset is transferred to the recipient, or it is failed and the
asset returns to the sender.
Fairness in our context means that both parties receive the
item they expect or neither do. The fairness theorem only relies
on the fact that a standard hash function is collision resistant.
By using hash preimage as the secret code of the conditional
payment, the atomicity of the transactions can be guaranteed
without the participation of a trusted third party, so as to realise
fair cross-chain exchange.
IV. CONCLUSION
In this paper, we analyse the blockchain transaction protocol
and propose to add new features that help solve the problem
of interoperability. One of the critical features of our method
is that it presents an internal functionality for value/asset
exchange. An internal protocol provides a universal language,
and via such a protocol, blockchain users can communicate
directly and transfer various forms of data using standardised
pathways. Furthermore, we briefly showed that Burn-to-Claim
protocol is resilient to double-spending by its correctness and
fairness properties.
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... Cross-blockchain protocols are an active area of research that tries to provide certain guarantees when moving value/data between blockchains networks [15]. Several protocols have been proposed over the past few years, such as hash time locks, cryptographic proofs -rollups [16], Proof-of-Burn [17], [18], [1] and signature-based protocols [19]. Technically, these proposals built on the security aspects of the cryptography involved in the protocols. ...
... Gas is the measure of the amount of Ether that is required to run. D. The Burn-to-Claim protocol Figure 1 shows a high level overview of the Burn-to-Claim protocol [1], [18] workflow. The vertical lines represent actors and horizontal arrows represent message activity along the time line. ...
... At the very high level, we aim to construct a model that consists of a scenario where a user wishes to transfer an asset from a blockchain network to another blockchain network. The Burn-to-Claim [1], [18] protocol executes the transfer process so that the asset is being destroyed (removed) from one blockchain network and re-created on the other blockchain network. The networks involved in this process are source networks from where the asset is removed and destination networks to where the asset is moved. ...
... It gives the guarantee that the token is permanently destroyed before being transferred onto another network [43]. We refer to our previous work for details on a burn-to-claim protocol [44], [45]. ...
... Integration systems aim to address this problem by implementing different protocols to accomplish the composability of cross-blockchain transactions between integrating networks. Consequently, there exist a range of cross-blockchain protocols such as atomic swap, lock/unlock, burn/mint to be used based on the use-cases [10], [12], [13], [44]. ...
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