Page 54 - ITUJournal Future and evolving technologies Volume 2 (2021), Issue 1

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 1

Fig. 4 – Network types
3.3.1 Public blockchain
In a public blockchain, no binding contract or registration
is needed to be a part of the network. Users can join or
leave the network whenever they want. Consequently, the
PCN will be open to anyone who would like to use it.
Fig. 3 – The users and businesses independently come together and es‑
tablish payment channels between each other. Consequently, they form
a PCN of end users and relays acting as the backbone. Solid‑arrowed 3.3.2 Permissioned blockchain
lines represent channels between the nodes. Double lines represent
how they agreed in the blockchain to establish a channel (only some of Permissioned (i.e., Private) blockchain lays on the oppo‑
these are shown for simplicity).
site side of the public blockchain, where the ledger is man‑
3.2.2 Distributed architecture aged by a company/organization. Moreover, the roles of
the nodes within the network are assigned by the central
In distributed networks, there is no central node. As op‑ authority. Not everybody can participate or reach the re‑
posed to the centralized network, each user has the same sources in the permissioned blockchain. PCNs employing
connectivity, right to connect, and voice in the network. A permissioned blockchain will be “members‑only”.
sample architecture is shown in Fig. 4(b).

3.3.3 Consortium blockchain
3.2.3 Decentralized architecture
Contrary to the permissioned blockchain, in consortium
This type of architecture is a combination of the previous blockchain, the blockchain is governed by more than one
two types which is shown in Fig. 4(c). In this architec‑ organization. From the centralization point of view, this
ture, there is no singular central node, but there are in‑ approach seems more liberal but the governance model
dependent central nodes. When the child nodes are re‑
moved, central nodes’ connections look very much like a of the blockchain slides it to the permissioned side. PCNs
distributed architecture. However, when the view is con‑ utilizing consortium blockchain will be similar to permis‑
centrated around one of the central nodes, a centralized sioned blockchain in terms of membership but in this
architecture is observed. case, members will be approved by the consortium.

4. PRIVACY ISSUES IN PCNS: METRICS AND
3.2.4 Federated architecture
EVALUATION
Federated architecture sounds very much like the federa‑
tion of the states in the real world and arguably lies some‑ As PCNs started to emerge within the last few years, a lot
where between centralized and decentralized networks. of research has been devoted to making them ef icient,
In a federated architecture, there are many central nodes robust, scalable, and secure. However, as some of these
where they are connected in a P2P fashion. Then the re‑ PCNs started to be deployed, they reached a large number
maining nodes (children) strictly communicate with each of users (e.g., LN has more than 10K users), which is ex‑
other over these central nodes which very much looks like pected to grow further as long as users are satis ied with
a federation of centralized architectures. Moreover, each their services. Such growth brings several privacy issues
federation can come up with their local rules in addition that are speci ic to PCNs. In this respect, we observed that
to the protocol being used. strengthening the security in PCNs comes with weaker
privacy while strengthening the privacy in PCN makes the
3.3 Types of blockchain networks network less practical. We argue that very little atten‑
tion has been paid to these issues and there is a need to
In this section, we categorize the existing PCNs based on identify and understand privacy risks in PCNs from both
the blockchain type they employ. There are mainly three the users’ and businesses’ perspectives. Therefore, in this
types of blockchains employed by PCNs. section, we irst de ine these privacy metrics and explain

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