A blockchain is a linear collection of data elements called block, where all blocks are linked to form a chain and secured using cryptography, and newly generated blocks are continuously chained to the blockchain in an untrusted environment. To date, there is still lack of formal definitions on the blockchain that can be accepted by both the academia and industry.
The Blockchain technique is derived from the Bitcoin which is first proposed by Nakamoto in 2008. The primary objective of Bitcoin is to propose a secure payment system without a trusted party such that online payments can be sent directly from one user to others without going through a financial institution. This is achieved by a distributed ledger which accounts for the ownership of coins. The key challenge is to resist double spending attacks, where an adversary could issue two transactions in parallel so as to transfer the same coin to different recipients. Essentially, enabling the distributed ledger to be secure against double spending attacks boils down to the Byzantine Generals problem (BGP). Existing solutions to solve BGP mainly focus on the permission model, where the participants in the system are predetermined (or participants know each other). However, the participants who maintain the ledger in a secure digital payment system are under a permissionless model, where anyone can join or leave the system without permitted by a centralized or distributed authority.
Bitcoin first achieved consensus on the distributed ledger in the permissionless model, where it cleverly integrates existing primitives and solutions from decades of research into one system to solve the fundamental problems existing in digital currencies in a practically viable way. Bitcoin is also the first cryptocurrency that is widely applied in large-scale networks. Due to the success of Bitcoin, its underlying technique called “blockchain” is extracted, which is of independent interest. Intuitively, the blockchain is a technique that maintains a public, immutable, and ordered ledger of records among all participants. It enables the participants to securely and periodically add new records into the ledger without trusting each other. It also guarantees that the records that have been added into the ledger cannot be removed or modified such that all honest participants have a consistent view of the ledger. More recently, the blockchain technique has been further investigated and one of the most prominent manifestations is to enable smart contracts. As a disruptive technique with profound implications, blockchain is transforming the very nature of how users including individuals and enterprises conduct transactions and perform smart contracts.
Generally, the blockchain technique can be classified into two types: public blockchain and private blockchain. For a public blockchain, the participants who maintain the blockchain are subject to the permissionless model, which means that anyone can join or leave the blockchain system without getting permission from a centralized or distributed authority. The most prominent manifestation of public blockchain is blockchain-based currencies, such as Bitcoin and Ethereum. The private blockchain (including the consortium blockchain) is derived from its public counterpart, where the participants who maintain the blockchain are authorized by blockchain managers or the managers themselves. At this point, the key difference between the private blockchain and distributed backup systems is that once a record is added to the blockchain, as long as the majority of participants is in accessible to the adversaries, the record cannot be removed or modified, even if the blockchain manager becomes the adversary.
In the PoW-based blockchain, the participants need to solve the PoW puzzle to fight for generating the new block; this results in striking energy demands. Actually, the PoW-based blockchain has even been labeled an “environmental disaster (Dziembowski et al. 2015).” As such, a blockchain system that is constructed on another mechanism, rather than PoW, to obviate the huge energy requirements is urgently needed. Essentially, PoW in blockchain facilitates a practical randomized “leader election” process that elects one of the participants to issue the next block, where the probability to be the leader for a participant is proportional to her/his hashrate.
Except for PoS, some other mechanisms have been proposed to replace PoW, for example, proofs of space. In the proofs of space setting, a prover demonstrates to a verifier that she/he has significant amounts of disk space.
Applications in Wireless Networks
Blockchain-based identity management for wireless networks. Blockchain can be utilized as a database to record all users’ identities in wireless networks without a trusted authority (Garman et al. 2014). Since the blockchain is inherently resistant to forgery and modification, after the users’ identities are recorded into the blockchain, anyone cannot tamper with the identities. Compared with existing systems, the blockchain-based identity management system does not require a trusted authority and thereby can resist the single-point-of-failure problem.
Blockchain-based random number generator for wireless networks. In wireless networks, random numbers play an important role to construct systems and protocols. In a PoW-based blockchain system, the hash value (Block Hash in Fig. 1) of each block on the chain only can be determined after a valid nonce is computed and the block is chained to the blockchain. Due to the inherent resistance against modification of blockchain, once the block is chained to the blockchain, its hash value is deterministic and would never be changed. As such, given a time t, if t is a past time, the hash value of the latest block that has appeared since t in the PoW-based blockchain is deterministic and can be extracted efficiently; if t is a future time, the hash value is unpredictable (Zhang et al. 2015, 2016). Therefore, the PoW-based blockchain can be utilized to construct a time-dependent random number generator.
Blockchain-based data sharing for wireless networks. In wireless networks, multiple network systems need to share data with each other. However, due to the differences on the data form, software version, network delay, or even node location, sharing data is always a challenging problem for different wireless network systems (Kosba et al. 2016). Since the blockchain is publicly accessible to all participants, it enables the data sharing among different systems to be feasible and efficient. Multiple systems can joint maintain a blockchain to record the data to be shared, which build bridges between these systems to share data.
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