Page 58 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 1

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

path discloses the channel balance availability for the re‑ problem for the privacy and relationship anonymity be‑
quested transfer amount to the landmarks. Then land‑ tween the sender and the recipient. To solve that problem
marks decide on the feasibility of the transaction by do‑ they introduce multi‑hop HTLC contracts. In their non‑
ing multi‑party computation. During the transfer phase, blocking approach, Rayo a non‑blocking payment routing,
when an intermediary node realizes the transaction to there is a global payment identi ier system that helps the
the next node, it informs the landmark. Landmarks ac‑ nodes to order the payments with respect to their identi‑
knowledge the transactions and when all of the transac‑ ier number. For that reason, Rayo is prone to relation‑
tions are executed on the intended path, the transaction ship anonymity attacks if the attacker is located on the
is marked successful. In SilentWhispers, the sender and payment path. Fulgor aims for guaranteed privacy. The
the receiver are kept private but the landmarks know the multi‑hop HTLC contract offered in Fulgor is fully com‑
sender‑recipient pair. The payment amount is also pri‑ pliant with the Bitcoin scripts. Thus, it is only usable in
vate for the nodes who do not take part in the transaction. LN or Bitcoin‑like cryptocurrency backed PCNs. Fulgor’s
Moreover, the balances of the channels within the net‑ motivation is that in LN the same hash of the pre‑image
work are kept private. Although centralization is possible, is distributed on the payment path. This creates a privacy
the approach is decentralized and landmarks are trusted problem which by comparing the collected hashes collud‑
parties. ing nodes can learn about the path of the payment. Ful‑
gor introduces one more phase of messaging with zero‑
4.3.5 SpeedyMurmurs knowledge proof‑based communication. The sender dis‑
tributes unique hashes to the intermediary nodes. It sat‑
SpeedyMurmurs [10] is a routing protocol, speci ically an is ies balance privacy, business privacy, sender and recip‑
improvement for LN. In SpeedyMurmurs, there are well‑ ient anonymity.
known landmarks like in SilentWhispers. The difference
of this approach is that the nodes on a candidate path 4.3.8 Bolt
exchange their neighbors’ information anonymously. So
if a node is aware of a path closer to the recipient, it Bolt [12] is a hub‑based payment system. That is, there is
forwards the payment in that direction, called “shortcut only one intermediary node between the sender and re‑
path”. In a shortcut path, an intermediary node does cipient. Bolt assumes zero‑knowledge proof ‑based cryp‑
not necessarily know the recipient but knows a neighbor tocurrencies. It does not satisfy privacy in multi‑hop
close to the recipient. SpeedyMurmurs hides the identi‑ payments, however, it satis ies very strong relationship
ties of the sender and the recipient by generating anony‑ anonymity if the intermediary node is honest. On the
mous addresses for them. Intermediary nodes also hide other hand, being dependent on a single node makes this
the identities of their neighbors by generating anony‑ approach a centralized one.
mous addresses. Although it may be complex, applying
de‑anonymization attacks on the network will turn it into 4.3.9 Permissioned Bitcoin PCN
SilentWhispers. This is because, while the algorithm is a
In PCNs, if the network topology is not ideal, e.g., star
decentralized approach, with unfair role distribution, it
topology, some of the nodes may learn about the users
may turn into a centralized approach.
and payments. To this end, the authors in [14] propose a
new topological design for a permissioned PCN such that
4.3.6 PrivPay
the channels’ depletion can be prevented. They come up
PrivPay [11] is a hardware‑oriented version of Silen‑ with a real use case where a consortium of merchants
tWhispers. The calculations in the landmark are done in create a full P2P topology and the customers connect to
tamper‑proof trusted hardware. Hence, the security and this PCN through merchants which undertakes the inan‑
privacy of the network are directly related to the sound‑ cial load of the network to earn money. The privacy of
ness of the trusted hardware which may also bring cen‑ the users in the PCN is satis ied by LN‑like mechanisms.
tralization. In PrivPay, sender privacy is not considered. The authors also investigate how initial channel balances
Receiver privacy and business volume privacy is achieved change while the sender/receiver privacy and the rela‑
by misinformation. When an attacker constantly tries to tionship anonymity can be satis ied by enforcing at least
query data from other nodes the framework starts to pro‑ 3‑hops in a multi‑hop payment.
duce probabilistic results.
4.3.10 Anonymous Multi‑Hop Locks (AMHL)
4.3.7 Rayo and Fulgor
In the AMHL proposal [15], the authors offer a new HTLC
: Rayo and Fulgor [13] are two multi‑hop routing proto‑ mechanism for PCNs. On a payment path, the sender
cols for PCNs (Fulgor is suitable for LN only). They de‑ agrees to pay a service fee to each of the intermediaries
velop these protocols against the security law coming for their service. However, if two of these intermedi‑
from hash distribution in LN. Speci ically, the same hash aries maliciously collude they can eliminate honest users
of the pre‑image is distributed on the path when a pay‑ in the path and consequently steal their fees. In order to
ment takes place so the authors argue that this creates a solve this, they introduce another communication phase

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