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Instead of being directly sent, data availability is announced to the selected peers, and in case a peer lacks some of the announced information, he requests it back to the announcer. Two types of data structures are propagated through the network in that way: transactions and blocks.
Transactions are the basic data structure flowing though the Bitcoin network and the one most usually seen. Every single node can take part in a transaction by simply using a wallet, no matter of its type. Transactions flow though the network aiming to reach every single node to, eventually, be included in a block.
Blocks are the data structure the blockchain is built from and include some of the transactions that have been created during the block mining process. Unlike transactions, blocks require a tremendous hashrate to be generated, which virtually limits their creation to mining pools.
Moreover, the block generation throughput is set by design to 6 blocks per hour, periodically adjusting the block mining difficulty according to the total network hashrate. Nonetheless, a pull data synchronization mechanism is also performed in the network, and while having a quite specific use, it is fundamental for its proper operation. Its main purpose is to synchronize the blockchain of outdated nodes, that have been off-line when data have been propagated.
Outdated nodes request an on-demand synchronization to their peers during the bootstrapping phase, obtaining all the missing blocks in their local blockchain. Such a request does not refer to specific block values but to all blocks above the last block the enquirer is aware of.
Besides blocks, on-demand propagation of other types of data, such as transactions, is not set by default. Only nodes that have built a full index of transactions along the blockchain, like block explorer services , can provide this type of data since normal nodes only track transactions bounded to their addresses. Different P2P network overlays require a set of system parameters for the overlay system to operate.
For instance, structured P2P networks require to store information on the distribution of peers in the network in order to improve routing performance. However, the Bitcoin P2P network, in line with other unstructured P2P overlays, does not require any special system parameters for the normal behavior of the network.
Every single node could join the network with no prior knowledge of it. Apart from that, some default parameters are used by nodes, such as the maximum connection limit set to , although such a value is not a restriction and each node could select the number of connections it wants to maintain.
Differing from traditional P2P networks such as Gnutella [ 11 ] , Bitcoin does not follow a multihop routing scheme. As already pointed out before, peers in the network store a replica of all the information that has been flowing through the system up to the date, namely, the blockchain. In that way, no queries are forwarded between peers since all information is supposed to be located at one hop peer at most.
Therefore, data are guaranteed to be located if the network is synchronized, and no routing protocol is needed nor used, apart from the synchronization protocol. Propagation delay is therefore a fundamental factor for the Bitcoin network in order to achieve synchronization at any given time. In that way, Decker and Wattenhofer [ 12 ] analyzed the block propagation time for 10, blocks and discovered that it followed an exponential curve, being the median block propagation time 6.
Furthermore, an analysis of how block size influences on the propagation delay was also performed. Despite being a content distribution network, the routing state of Bitcoin cannot be directly defined due to the randomness and dynamism of its topology and to the fact that it is not known. Moreover, as we have pointed out before, no multihop routing is performed since data could be found at one hop peer at most.
How to build the network is a classic problem P2P networks have to deal with. From building the network from its roots to how nodes deal with peer disconnection, P2P networks need to be highly adaptable to avoid partitioning. In order to deal with this problem and also provide a fair and secure way to choose the peers a node is going to be connected to, the Bitcoin network performs a particular network discovery mechanism.
By default, all peers maintain up to connections with other peers. Each node will start 8 of those connections with other peers namely, outgoing connections and will accept up to from potential peers namely, incoming connections. Despite the name, all connections are bidirectional. In order to pick the outgoing connections, every single node will look for a subset of nodes it stores in a local database.
This database is formed by two different tables: tried and new. Tried table contains addresses from peers the node has already connected to, and new table contains addresses the node has only heard about. Additionally, when the node tries to establish a connection to the network for the first time, it queries a well-known list of DNS nodes, that will provide a set of online potential peers further information about how peers are stored and selected can be found in [ 13 ].
Nodes try to always maintain their 8 outgoing connections, selecting new peers from the database if any of the established connections is dropped. Peers are stored and selected from the database following a pseudorandom procedure that gives the network high dynamism and keeps its structure unknown. Peer information can be obtained by a node following two ways. First of all, a node could request such data to its neighbors, in order to fill up its database, through sending a getaddr message, or could receive such information spontaneously from one of its peers without any kind of request.
On the other hand, an addr message containing a single address could be sent to a node when a node wants to start a connection with a potential peer. By sending its address, the node notifies the receiver that it has been picked as a peer, and if the latter has room for more incoming connections, the communication is established. Peer addresses received in that way are stored in tried table. All addresses are stored in the database together with a timestamp that helps the node to evaluate the freshness of such an address when selecting a peer.
Security in P2P networks has always been a broad topic since multiple security threats can be identified in different P2P implementations. However, in P2P cryptocurrency networks, security takes a different twist. At first sight, one could believe that the threats P2P cryptocurrency networks face are a subset of the threats found in standard P2P networks. However, as we will see in detail in the next section, most of the threats encountered in general P2P networks do not apply directly to P2P cryptocurrency networks due to the cryptographical mechanisms used by the currencies and the level of security offered by their protocols.
Additionally, one can also believe that multiple new threats will also arise in cryptocurrencies due to the sensitivity they have as money transfer networks. However, as we will see in the next section, this is not also the case. In the next section Section 5 , we provide a detailed review of the most common security threats identified for typical P2P networks and discuss to what extent they affect the Bitcoin network. Reliability and fault resiliency analyze how robust the overlay system is when subjected to faults.
Typically, such robustness measurements are related to nonintentional failures, for instance, by a massive disconnection of peers of the network or an increasing volume of information being transferred through the network, but do not include intentional attacks that would be categorized inside the security properties of the network. Bitcoin implements a distributed consensus protocol resilient to Byzantine faults. That is, the protocol is resistant to arbitrary faults produced in the participating peers, from software errors to adversary attacks.
The main idea behind this protocol is to use a proof-of-work system to build the public ledger where transactions are stored. Appending new information to the public ledger requires a huge amount of computer power, thus preventing attackers to monopolize ledger expansion and censuring transactions.
In a similar way, changing the content of the blockchain is also computationally expensive, up to the point that transactions are considered secure when they have 6 confirmations i. Additionally, the blockchain is replicated on all full blockchain nodes, contributing to the fault resiliency of the system and providing high availability of the ledger data.
Assuming that categorization, the Bitcoin P2P network has been designed with a high level of reliability, thanks to the redundancy that implies the storage of all the relevant information of the network in every peer of the network. With this approach, the high inefficiency level in terms of storage space is translated into a high resilience of the network since the availability of a single node in the network contains the information to keep the system alive.
Moreover, the proof-of-work system allows peers to eventually reach a consensus state, even in the presence of attackers trying to subvert the system. As a drawback, the consensus protocol is somehow slow, with transactions needing 9 minutes median confirmation time as of October 13, [ 17 ] to confirm, and expensive, requiring the consumption of lots of energy for each mined block.
Security in P2P networks has been extensively studied in the literature. In this section, we provide a broad overview of the main security problems that arise in P2P networks, we review how each of the security problems may affect the Bitcoin network, and if it is the case, we explain the specific countermeasures Bitcoin provides in order to defend from each attack.
The list of reviewed attacks goes over the most typical types of attacks and security flaws found in common P2P networks. It is clear that specific networks and applications might present specialized attacks, but in most cases, they can be seen as a specification of the attacks presented here. So as to provide a clear picture of how common P2P attacks affect Bitcoin, we first review the three attacks that have been shown to be clearly applicable to Bitcoin.
After that, we include a list of attacks identified for common P2P networks, but this does not have such a high impact on Bitcoin, reviewing why the attacks do not apply to the specific Bitcoin network and detailing the particular cases where those attacks or some variation may somehow relate to Bitcoin. In favor of a clear and concise presentation, we have not explicitly covered some recent attacks such as [ 18 ], which do not directly affect or involve the Bitcoin network, or network-related attacks such as [ 19 ], which rely on BGP hijacking and are thus out of the scope of our study.
Denial-of-service DoS attacks are possible in most P2P scenarios and are especially relevant, for example, in P2P streaming applications [ 20 — 22 ]. Given their dynamic nature, P2P networks are usually more resilient against generic DoS attacks than more static networks. Targeted DoS attacks to specific parts of the P2P network a given node or services are usually more important.
There exist several potential DoS flooding attacks in Bitcoin, but the system has countermeasures in place. Transaction flooding is prevented by not relaying invalid transactions and imposing fees to valid transactions. On one hand, transactions are signed by the senders in order to demonstrate that they are authorized to transfer those bitcoins.
If the signatures of a transaction are not correct, the transaction is considered invalid and is not relayed to the network. On the other hand, the default protocol does not relay transactions without fees except for a few very specific cases that would also result in very expensive attacks.
Block flooding is prevented by only relaying valid blocks, which must contain a valid proof of work. In order for a block to contain a valid proof of work, its hash must be lower than a given target. Obtaining a block with such a hash is a computationally expensive task, thus performing DoS attacks with block data unfeasible. Network data flooding is easier than the previous two cases because it is indeed possible to create valid network messages without paying fees nor spending computation cycles.
However, Bitcoin has a banning protocol: peers may ban other peers for one entire day if their misbehavior score crosses a certain threshold. The misbehavior score is increased for sending duplicate version messages, sending large messages, and sending invalid blocks. Given the nature of Bitcoin, cpu usage DoS is possible by trying to make peers spend lots of time validating a transaction or a block.
In order to prevent this kind of attacks, Bitcoin tries to catch errors before starting to validate a transaction, limits the number of signature operations per transaction and per block, and limits the size of the script. Finally, previous versions of the Bitcoin client were also susceptible to continuous hard disk read attacks , where an attacker repeatedly sent double-spend transactions that passed the initial checks and required to retrieve data from disk in order to be fully validated.
This attack is now prevented by checking that the inputs of the transaction that is being validated are in the UTXO set i. An eclipse attack occurs when an attacker creates or has control of a large number of distinct nodes that populate the whole neighborhood of the victim node [ 23 ]. The attacker can then eclipse the view of the network that has the victim. Common solutions for sybil attacks are usually insufficient to defend against eclipse attacks [ 24 ].
In a cryptocurrency network, isolating a node from the rest of the network may enable two other attacks to the eclipsed peer. Second, if the eclipsed victim is a miner, the attacker can drop or delay the propagation of the new blocks found by the rest of the network. As a consequence, the victim wastes computation time trying to mine on the top of old blocks.
Bitcoin has many defense mechanisms to prevent eclipse attacks, some of which were added recently, after a study pinpointed some of the flaws of the then vigent implementation [ 13 ]: the client restricts the amount of outgoing connections to addresses in the same network, randomizes the address selection procedure, and maintains a big list of peers, among others.
In some P2P networks, it is easy to record all the activities of a giving node allowing attackers to easily create identifying profiles of users and their activities. This is relevant in anonymous systems, or systems that want to guarantee a certain degree of anonymity [ 16 , 25 ]. Bitcoin provides pseudonymity by allowing users to receive payments to their addresses, which are not initially linkable to their identities.
The usage of new addresses for each transaction in the system is intended to provide unlinkability between the different actions a single user performs through Bitcoin. Therefore, user profiling in Bitcoin usually consists in attacking the unlinkability between different addresses a single user has. Three different approaches have been taken to perform address clustering: using network layer data [ 26 ], performing analysis over the transaction graph [ 27 — 29 ], and analyzing Bloom filters [ 30 ].
The idea of using network layer data to cluster addresses is straightforward: if an attacker is able to connect to all the peers of the network, the first node that sends him a given transaction should be the creator of that transaction. Therefore, if the attacker first receives two different transactions from the same peer, he can infer that the source addresses of both transactions belong to the same user. However, as simple the attack may seem conceptually, it is not that easy to perform in practice.
It is not trivial to connect to all nodes of the network since most of them do not accept incoming connections. Moreover, some peers anonymize their connections using Tor. Finally, collected data are very noisy, and therefore, it is not easy to make strong claims when analyzing it. Regarding transaction graph analysis, there exist mixing services that are able to effectively break the relationship between an address and its past.
Additionally, the use of a secure wallet that tries to minimize the leaked information about addresses clusters helps mitigate the consequences of this kind of analysis. Finally, concerning the usage of bloom filters, users must be very careful when choosing the parameters of the filter and when generating different filters that match the same set of addresses and public keys. Additionally, new protocols are being designed to allow lightweight clients to retrieve their transactions of interest while maintaining privacy.
Several proposals provide mechanisms to create off-chain payment channels, such that secure transactions between Bitcoin users may be performed without needing to include all the transactions into the blockchain. In turn, these solutions may also entail privacy problems that are yet to be carefully studied [ 5 ]. After analyzing the three main attacks that have threatened the Bitcoin network over the last years, we summarize other common P2P attacks that have a lesser impact on Bitcoin.
We will show how some of those attacks could be used as a preliminary phase to achieve one of the three previously introduced ones, while others are not harmful for the Bitcoin network due to its design. As an example, in DHT-based P2P networks, a node can gain control over given resources by changing its own identifier [ 31 ].
These kinds of attacks are more difficult in networks where the identifier is derived from a public key [ 32 ]. ID collision attack : similar to previous attacks, here the attack is considered to happen when there are duplicated identifiers. The problem is usually prevented by ensuring the uniqueness of identifiers [ 16 ]. There is no clear concept of a peer identifier in Bitcoin. Two different properties could be considered identifiers in Bitcoin, depending on the exact entity one wants to identify: IPs and Bitcoin addresses.
IPs allow to identify peers, whereas addresses are linked to users. A malicious peer may benefit from a change of IP if it is banned for misbehavior. Each peer maintains a banscore for each of its neighbors. This banscore is increased whenever the peer misbehaves.
If the banscore surpasses a certain threshold, the neighbor is banned for 24 hours. Therefore, being able to change the IP allows a peer to effectively reset its banscore. Regarding the second kind of identifiers, Bitcoin addresses, the recommended behavior for users is indeed to change them frequently. In fact, the suggestion is to not reuse addresses, that is, to create a new address for each transaction made in the system.
This allows to protect user privacy. A sybil attack is a well-known attack in P2P networks, where a malicious user creates multiple identities in order to control the system or parts of the system [ 33 ]. This has been very extensively studied in the literature in the context of several P2P technologies [ 34 , 35 ]. Sybil attacks may be a problem in Bitcoin if they are able to eclipse all the connections from a peer see Section 5.
However, besides its extension to an eclipse attack, a peer with multiple identities cannot harm the system regarding the main content of the network: transactions and blocks. Blocks cannot be counterfeit without the corresponding proof of work, and transaction generation entails an associated fee in a similar way that was described in flooding attacks in Section 5. Nevertheless, if lots of sybil nodes start performing a huge amount of connections to the existing network, they may monopolize all available incoming connection slots, and the system decentralization could be reduced.
Network access in P2P environments starts by connecting to one or multiple nodes of the network. This first contacted node is known as the bootstrap node. A malicious bootstrap node can influence the view of the network for the new user [ 23 ]. Several solutions already exist for this problem such as not relaying in a single bootstrap node, use of cached peers for subsequent connections, random address probing, using external mechanisms, using specific bootstrapping services, or using network layer solutions e.
Bitcoin deals with bootstraping issues by defining a local peer database on every single node, that is queried following a pseudorandom protocol to obtain a subset of potential peers see Section 4. In that way, Bitcoin applies most of the solutions for the fake boostrapping protocol, such as not relaying in a simple bootstrap node , by establishing 8 outgoing connections on every bootstrap, use of cached peers for subsequent connections , by using peers stored in tried table, random address probing , by using a pseudorandom protocol to store and retrieve peer addresses from the database, and using external mechanisms by quering a list of well-known DNS nodes or even using a list of hardcoded nodes, if the DNS cannot be reached.
P2P networks often use some sort of private data that have to be protected from unauthorized access. Common solutions are those typically employed for distributed access control [ 38 , 39 ]. Bitcoin is based on public key cryptography, where private keys are needed to authorize payments. Therefore, private keys must be kept secret, and two methods are usually employed: encryption and off-line storage.
By using encryption, private keys remain secure even if an attacker is able to retrieve the key file as long as the encryption key remains secret. As for off-line storage, different approaches can be taken with different technical sophistication levels, from the usage of dedicated hardware devices to paper wallets. Notice that unlike other uses of public key cryptography where private keys need to be online for instance, in the handshake process in TLS , Bitcoin network operation does not involve private information since validations are performed using public information.
For that reason, off-line storage of public keys does not impact the network performance. A malicious node can deny the existence of a given resource under its responsibility, or claim to have a resource it does not have. This is specially relevant in content distribution applications, and common solutions are replication of resources [ 40 ], or use of error-correcting codes to reconstruct missing parts of the resource [ 41 ].
Bitcoin network is protected against malicious resource management by, on one hand, the high amount of data redundancy information of the network and, on the other hand, the multiple neighbors a node of the network is connected to. Thanks to the fact that peers establish connections by default to 8 other peers, if a given neighbor denies the existence of a certain resource, the peer can learn it from his other neighbors.
Moreover, if a neighbor says he has some resource he actually does not have, peers will notice when they try to retrieve it since transactions and blocks are identified by their hash. A free-rider user or node in a P2P network is a node that attempts to benefit from the resources of the network provided by other users without offering their own resources in exchange [ 42 , 43 ].
Depending on the application, this might not be an issue or even might not be considered a security problem. It is usually described in content distribution applications, and the main solutions proposed rely on incentive- or penalty-based mechanisms [ 44 ]. Bitcoin is sustained by an equilibrium of economic incentives. Miners are remunerated for their work by obtaining a reward for each block they successfully mine. Additionally, transaction senders and, although indirectly, also transaction recipients may include a fee to their transactions, which is also collected by the miner of the block that contains the transaction.
As a consequence, miners are encouraged not only to create blocks but also to include transactions on those blocks. There is, however, a set of nodes whose role is important in ensuring the decentralization of the network and that do not directly receive economic incentives for their work: full clients. While these clients store the blockchain and perform validation and relaying of transaction and blocks, they do not get a direct economic reward in return for their work.
P2P networks, which require multihop routing, will need to include measures similar to onion routing in order to secure connections between all nodes along the path [ 45 , 46 ]. MITM attacks in Bitcoin are not a problem for transaction and block integrity because transactions are cryptographically signed and blocks must contain a valid proof of work.
Transaction malleability may be a problem refer to Section 5. Censorship is neither a problem because a single peer maintains different connections. An attacker must be in the middle of all of them to hide information to the peer thus resorting in eclipse attacks. A replay attack is produced when a legitimated transmission is delayed or lately replayed with malicious purposes. This is a very common network attack that can affect P2P networks in several ways but is usually solved at a protocol level.
Replaying transactions or blocks that have been sent to the network does not have any effect on the Bitcoin network. Nevertheless, delaying block propagation may be a beneficial strategy for miners [ 47 ]. By not immediately propagating a block the miner has just found, the miner can start working on top of this newly found block while making other miners lose time working on the previous block. This strategy is known as selfish mining and reduces the bound on the percentage of hashing power an attacker must have in order to successfully control the information appended to the ledger.
Routing dysfunction can be presented in different aspects. On one hand, incorrect routing involves attacks where a node routes messages incorrectly or drops them [ 23 ]. These attacks might not be relevant in P2P networks that do not provide multihop routing. Due to the flooding mechanism used to propagate information through the network, the consequences of a single node dropping messages are negligible.
On the other hand, in a fake routing update , the attacker tries to corrupt a given route equivalent to corrupting a routing table for a given node [ 23 , 32 , 48 ]. As we have mentioned previously in Section 4 , there are no routing tables in the Bitcoin network. The most similar information a peer stores is addresses from other peers. Note that no information about where is this peer in the network nor its connections are stored by the Bitcoin client, just the address and a timestamp.
Therefore, the attack that better resembles fake routing updates in Bitcoin is to send fake addresses. These kinds of attacks are usually performed as a first step in eclipse attacks, attacks already described in Section 5.
When using multihop routing, intermediate nodes can modify the content of the relaying packets. End-to-end integrity has to be provided in order to, at least, detect these types of attacks [ 16 ]. Tampering with the content of a block changes its hash and, with very high probability, invalidates its proof of work. Therefore, tampering with block data is not a feasible attack on Bitcoin. On the other hand, transactions are a signed data structure, with the signature cryptographically protecting its integrity.
Therefore, an attacker can not tamper with a transaction to its will, for instance, by changing the destination address of the bitcoins transferred on the transaction. There is, however, a very specific situation where this kind of attack would be possible although the countermeasures that prevent this attack are already implemented and ready to be deployed.
Because Bitcoin transactions are malleable, it is indeed possible for an attacker to change some part of the transaction while keeping the signature valid. This happens mainly because not all parts of the transaction are signed e. The aforementioned situation where malleability is a problem for Bitcoin happens when a user is dealing with 0-confirmation transactions, that is, transactions that have been sent to the network but have not yet been included in a block.
Because transactions are not yet in a block, an attacker may change some of the unsigned part of the transaction, creating another valid transaction that spends the same inputs but has a different identifier recall that transactions are identified by their hash. Then, if this transaction is part of a protocol where transactions are identified by their hash, the attacker may be able to use it at his advantage.
Cryptocurrencies are indeed a powerful tool for the development of new decentralized applications currently, the best well-known application for P2P networks is as a content distribution technology [ 49 ] , thanks to the distributed trust mechanism in which they are based on.
Three relevant properties of cryptocurrencies can be used as a building block for such applications: i Secure distributed payment mechanism ii Distributed storage with integrity by design iii Secure transfer and distribution of digital assets.
The obvious use case of cryptocurrencies is, of course, to adopt them as the payment layer in any system where there is the need to transfer money from a payer to a payee in a totally distributed and uncensored fashion.
Multiple applications could benefit from a flexible payment system, from P2P distributed storage schemes, where users could hire local disk space for an economical incentive, to more sophisticated scenarios, like mobile crowdsensing [ 50 ]. Mobile crowdsensing MCS is a distributed application where the power of the crowd, jointly with the sensing capabilities of smartphones they wear, provides a powerful tool for data sensing, especially in those scenarios involving user behavior or those that rely on user mobility, where standard sensor networks may not be suitable.
However, including human participation in sensing tasks carries, at least, three critical challenges [ 51 ]: user participation, data sensing quality, and user anonymity. Therefore, incentive mechanisms are of utmost importance in MCS scenarios to engage as many crowd sensors and provide the data collection center with a considerable wealth of data.
User participation can be promoted by providing a pay-per-sense mechanism. However, standard payment schemes have multiple drawbacks in a pay-per-sense application. First of all, user enrollment in the payment system entails a burden step for user participation.
Second, collateral costs of standard payment systems mainly in the form of fees prevent their use in a pay-per-sense scenario. Finally, standard payment mechanisms do not provide privacy-preserving properties, specially relevant when such payments could identify sensed data from a particular individual whose identity should not be disclosed. Cryptocurrencies can be successfully used in mobile crowdsensing scenarios as a rewarding mechanism since they allow an affordable pay-per-sense scheme with relevant privacy-preserving properties, as it has been proposed in the Paysense system [ 52 ].
Distributed storage is also a very interesting property offered by blockchain-based cryptocurrencies, but despite other distributed proposals, its main advantage is the integrity-by-design property that makes it so attractive for multiple applications. An example of such an idea is the cryptocurrency Namecoin [ 53 ]. Namecoin is a blockchain-based cryptocurrency whose purpose is to provide network address resolution for network identifiers, normally human readable.
In such a cryptocurrency, transactions can store data for tying the network address with other identifiers, and such transactions are stored in the blockchain inhering its integrity properties. Keys used for creating the transaction provide an authentication token for the owner who registered the tie. Namecoins could be used as a DNS replacement in a P2P network or even for node authentication when such authentication needs a tie between identity and keys for instance, using standard public key infrastructure, PKI.
Cryptocurrencies were designed to transfer money, but its use can be extended to transfer other types of digital assets. By using a cryptocurrency as a transport layer, digital assets can be associated with cryptographic keys and can be traded, using the secure information included in the blockchain to determine the legitimate owner of every asset at each specific time. Multiple examples of such digital assets can be found, from shares of a company to DRM where the property of the media object can be determined [ 54 ].
Furthermore, extending a bit the concept of asset, cryptocurrencies can also be used to store reputation, conceptualized as an asset that users can store and transfer. Revisiting again the example of mobile crowdsensing, we recall the fact that data-sensing quality was one of the important challenges of such a scenario. In MCS systems, there is no control over the crowd sensors, and it cannot be assumed that all individuals will behave in the exact same manner or will be equally honest.
Therefore, the overall quality of the sensor readings can see itself deteriorated if counterfeit data are received from malicious users. Hence, data validation techniques should be properly deployed, and a commonly used approach is to validate the data depending on the trust level of the crowd sensor that reports it. In this particular scenario, cryptocurrencies can be used as an annotation mechanism [ 52 ], by which users earn or lose reputation depending on the correctness of previous actions, accounted by the amount of rewards that they previously obtained.
Notice that, in this section, we have pointed out some possible uses of cryptocurrencies in distributed applications to show the broad intersection between both fields. However, an in-depth study on how interaction could be optimally performed between cryptocurrencies and particular scenarios is left for future work. In this paper, we have characterized P2P cryptocurrency networks by providing a deep analysis of the most relevant cryptocurrency nowadays: Bitcoin. By characterizing P2P cryptocurrency networks using well-known taxonomy in the field of P2P networks, we can conclude that such networks present a new paradigm due to the main properties that a cryptocurrency has to provide: reliability and security.
P2P cryptocurrency network reliability stands on top of a strong redundant mechanism regarding system information. As a result, every peer of the network stores all the relevant information of the system. With this approach, the availability of a single node in the network contains the information to keep all the systems alive.
Notice that this approach turns out to a high inefficiency level regarding storage space, so this strategy is not followed by any other P2P network paradigm. Furthermore, such an approach also demands new synchronization mechanisms to provide all nodes with the same correct information. Information redundancy is also used in the security plane for network topology protection. As we have seen, the main attacks to cryptocurrencies are eclipse attacks, where a victim or part of the network can be isolated.
Such attacks can be performed when an attacker takes advantage of his position on the network topology. To avoid such possibility, the network topology has to be protected, and cryptocurrency networks use two different measures for such protection. On one hand, routing information should not be disclosed, so cryptocurrency networks are not multihop networks, and network nodes only are aware of one-hop neighbors. Using this approach, no routing information has to be provided to network nodes, and there is no restriction regarding information availability since, as we have pointed out above, information is replicated in every network node.
On the other hand, network topology disclosure also has to be protected when nodes access the network. In such a phase, cryptocurrency P2P networks use a pseudorandom approach to determine each node connection to hinder the topology structure of the network. Notice that this topological secrecy property of P2P cryptocurrency networks is not so relevant in other P2P network paradigms, and for that reason, the mechanisms to achieve it are also particular of such environments.
Furthermore, some mechanisms specifically designed for other P2P network paradigms are not needed in cryptocurrency networks due to the characteristics of the information flowing in such networks. This is the case of multiple secure protections that try to prevent different attacks. For instance, intrinsic cryptographic properties of blocks and transactions can directly prevent DoS attacks, replay attacks, or tampering with message bodies.
As a new paradigm, P2P cryptocurrency networks open new research opportunities both as a direct field of study and also as a tool for other applications. For instance, a more formal analysis should be performed towards the pseudorandom mechanisms used in this kind of networks for selecting the nodes to connect to verify that network topology is both unknown and uniform.
Furthermore, the development of a global P2P cryptocurrency network that could provide service to multiple cryptocurrencies, taking into account different particularities of each cryptocurrency, could also be another interesting research line.
On the other hand, analyzing how such a new network paradigm could be efficiently combined and integrated with other distributed applications could also be a relevant topic for future work. This work is partially supported by the Spanish Ministry under Grant no. This is an open access article distributed under the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Article of the Year Award: Outstanding research contributions of , as selected by our Chief Editors. Read the winning articles. Journal overview. Special Issues. Academic Editor: L. Received 18 May Accepted 03 Dec Published 01 Mar Abstract P2P networks are the mechanism used by cryptocurrencies to disseminate system information while keeping the whole system as much decentralized as possible.
Introduction Since , when the Bitcoin cryptocurrency [ 1 ] was released, a plethora of more than different cryptocurrency proposals have appeared. A Basic Description of the Bitcoin System In this section, we point out the main ideas to understand the basic functionality of the Bitcoin cryptocurrency. Figure 1. Figure 2. Figure 3. Table 1. Properties of archetypal Bitcoin nodes. The properties described in the table refer to the most common nodes of each type, but due to the vast heterogeneity of Bitcoin nodes, some differences may be found in the real network.
Figure 4. Abstraction of the defined network subsets: nodes in the reachable network accept incoming connections; nodes in the nonreachable network just create outgoing connections; nodes in the extended network do not implement the Bitcoin protocol for instance, miner clients using the stratum protocol. References S. Narayanan, J. Bonneau, E. Felten, A. Miller, and S. Donet and J.
Ferrer and M. Payeras, Eds. View at: Google Scholar M. Lischke and B. View at: Google Scholar A. Bitcoin Core, Bitcoin Core Version 0. Lua, J. Crowcroft, M. Pias, R. Sharma, and S. Kirk, Gnutella Protocol Development , These cookies allow you to share your favourite content of the Site with other people via social networks. Some sharing buttons are integrated via third-party applications that can issue this type of cookies.
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