blockchaindistributed-systemfault-toleranceconsensus

PBFT consensus algorithm and double spending

I am trying to figure out how PBFT consensus algorithm deals with the problem of double spending. I've read lots of literature but cannot seem to find an answer

Solution

  • pbft is a consensus algorithm given by Barbara Liskov and Miguel Castro in 1999 in order to prevent malicious attacks as malicious attacks and software errors can cause faulty nodes to exhibit Byzantine (i.e., arbitrary) behavior. pBFT was designed to work efficiently in asynchronous systems as compared to previous bft algorithms which only worked on synchronous systems.

    here is the research paper which states that

    Practical algorithm for state machine replication that tolerates Byzantine faults. The algorithm offers both liveness and safety provided at most āŒŠn-1 / 3āŒ‹ out of a total of replicas are simultaneously faulty. This means that clients eventually receive replies to their requests and those replies are correct according to linearizability. The algorithm works in asynchronous systems like the Internet and it incorporates important optimizations that enable it to perform efficiently

    Double-spending is a potential flaw in a digital or electronic cash scheme in which the same single digital token can be spent more than once. Unlike physical cash, a digital token consists of a digital file that can be duplicated or falsified.

    A double-spending attack is a potential attack against cryptocurrencies that has happened to several cryptocurrencies, e.g. due to the 51% attack.

    But this problem can be prevented using consensus algorithms and blockchain

    If two transactions attempt to spend the same tokens, each node will consider the first transaction it sees to be valid, and the other invalid. Once the nodes disagree, there is no way to determine true balances, as each node's observations are considered equally valid , a way to bring the nodes back in sync is using consensus algorithms and with blockchain the transactions in this system are never technically "final" as a conflicting chain of blocks can always outgrow the current canonical chain, however as blocks are built on top of a transactions, it becomes increasingly unlikely/costly for another chain to overtake it and hence preventing the double spending problem.