Blockchains: Guardians of the Gates

November 6, 2015
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2 min read

Bitcoin and other cryptocurrencies have been a hot topic ever since the seminal white paper “Bitcoin: A Peer-to-Peer Electronic Cash System” was written by someone (or a group of someones) using the pseudonym Satoshi Nakamoto.

Cryptocurrency is built on methods for blockchains used on a peer-to-peer network of independent nodes. Nakamoto (or whoever) figured out that to decentralize a currency, one had to overcome the double spending case where a coin is used twice. This was accomplished by time stamping all digitally signed transaction records and taking their hash in a specific way.

The Basic Method of Blockchains

The method seems complex on first viewing, but it’s actually recursive and simple. As Nakamoto put it, “The network time stamps transactions by hashing them into an ongoing chain of hash-based proof of work, forming a record that cannot be changed without redoing the proof of work. The longest chain not only serves as proof of the sequence of events witnessed, but proof that it came from the largest pool of CPU power. As long as a majority of CPU power is controlled by nodes that are not cooperating to attack the network, they’ll generate the longest chain and outpace attackers.”

Proof of Work

OK, what exactly is this proof of work? First introduced in Adam Beck’s Hashcash system in 2002, proof of work is the idea tjat each node in the network has to show it has done some computational work on each block it creates before that block can be added to the chain. The specific method of this involves scanning for a value that, when hashed with a hash generator like SHA-256, will begin with a number of zero bits.

The average work required for the number of zero bits is exponential and can be verified by executing only a single hash. This means that as blocks are added to the chain, the work to change a past block would include redoing the computational work in all the blocks that follow it.

A node shows that it worked to create a block by creating a hash of its block header. This hash does not exceed a certain value.

How Do We Establish if a Node Is Honest?

Honest nodes don’t have to do the computational work needed to change the blocks in a blockchain. That means that they will be able to create longer blockchains than dishonest ones in the same length of time. As the blockchain gets longer, the probability of a slower attacker catching up to it grows exponentially.

New blocks will only be added to the blockchain if their hash is at least as challenging as a difficulty value expected by the network’s consensus protocol. The difficulty is established by checking the difference in the time stamps (and hence how long it took to generate the last blocks in the chain) every 2,016 blocks.

The amount of computational power needed to create multiple hashes on blocks both identifies good players and bad ones in a blockchain net. Good ones get their chains longer, faster — and all of this work helps blockchains secure themselves from malicious actors.

Larry Loeb
Principal, PBC Enterprises

Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE mag...
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