Page 32 - MSDN Magazine, March 2018

Block 0 (Meta Data)
H(Block 0)
(Genesis Block)
Figure 9 The Blockchain Is Composed of Blocks That, in Turn, Include Transaction Hash Trees; Blocks on the Blockchain Are Back-Linked to Previous Blocks and Are Validated Using a Proof-of-Work Algorithm
a new block is valid—they prove it to themselves by val- idating the block. To validate, a node simply verifies the PoW puzzle solution by computing the block’s SHA-256 hash concatenated with the nonce value and verifies that the answer produces a hash that has the number of lead- ing zeroes specified by that block’s PoW difficulty value.
Incidentally, on some blockchains, the PoW difficulty value is continually adjusted by the protocol so that new blocks are added to the blockchain at a prescribed time interval. This continual adjustment is necessary because nodes are constantly appearing and disappearing from the network and thus the average computational power of the nodes is always changing. Remember that in PoW, there is an incentive to add blocks to the blockchain, so
the key innovations of blockchain technologies. I also mentioned that blockchains are decentralized and therefore offer “collusion resistance.” Because PoW takes time and costs money in computa- tional power, it’s practically impossible for any single node or group of nodes to collude on the network and have a blockchain-making advantage over other peer nodes. (There is a “51 percent attack” risk that suggests collusion can occur if a group of nodes ends up with 51 percent of the computational power, but employing a PoW con- sensus algorithm makes the possibility of such an attack infeasible.)
To construct a block of transactions, a node grabs unprocessed transactions stored on the network and builds a Merkle tree in order to compute the Merkle root hash. Therefore, a candidate block contains a list of transactions along with a block header that includes the Merkle root hash value, a current timestamp andPoWdifficultylevel(andsometimesadditionalheaderdata). Then it must solve the PoW puzzle, which involves computing a double hash of the entire 256-bit block hash value, then finding a 32-bit number, called the nonce, which can be concatenated to it so that the hash of the resulting 288-bit number produces a result that has a certain number of leading zeroes. The 32-bit nonce has a range of 0 to 232 (4,294,967,295), so instead of just trying to guess the nonce, it’s typical to start with nonce 0, generate the SHA-256 hash, and determine if it has the target number of leading zeroes (that is, the resulting hash is below a target value); if it doesn’t, the node increments the nonce value and tries again. If the node attempts all nonce values without solving the puzzle, its recal- culates the block hash value. This guarantees the production of a different block hash value because a timestamp in the block header is included in the block-hash calculation. At any time, a node can select a different batch of pending transactions for inclusion in the new block (or add new pending transactions that might have appeared since it last checked), and this changes the Merkle root hash value, which, along with the timestamp, changes the newly computed block hash value. Each time the block hash is recomputed, the node iterates over all 4 billion-plus nonces again.
In time, some node on the network will solve its cryptograph- ic puzzle. When it does so, it adds the new block to the end of its copy of the blockchain (every node maintains a copy of the block- chain) and then broadcasts the new block to every other node on the network so that they can update their blockchain copy. When a node broadcasts a new block, other nodes don’t simply trust that
node administrators often beef up their hardware in order to com- pete for an award. On the Bitcoin blockchain, the difficulty value is tweaked every 2016 blocks so that blocks continue to be added at an average rate of 10 minutes per block.
Sometimes branches occur. That’s because on a large network, new-block propagation takes time. It’s possible that during prop- agation, another node solves its PoW puzzle, adds a new block to its copy of the blockchain, then broadcasts that blockchain on the network. Receiving nodes will always add a valid block to their copy of the blockchain—and because each block is cryptograph- ically tethered to the previous block, two new blocks published by two different nodes will result in a branch linked to the same block at the end of the chain. That’s OK, however. Over time, nodes will add new blocks to the end of what the protocol deems to be the“longestchain.”Forexample,givenabranch,thelongestchain could be defined as the one with the most-recent block timestamp. Over time, a single branch will prevail in length and blocks from abandoned (shorter) branches will be removed, returning their transactions to the pool of unprocessed transactions.
Wrapping Up
In this article, I’ve shown how to construct a public blockchain that’s composed of cryptographically linked blocks—each of which contains its own hash chain of cryptographically linked transactions—on a decentralized peer-to-peer network of nodes. I covered the basics of blockchain technologies, trying not to focus on any single implementation but instead concentrating on some of the more typical technical features they all share. If you want to explore the subject further, I suggest choosing a specific block- chain technology, such as Bitcoin, Ethereum or Ripple, and trying to master the details of its particular implementation. If you want to experiment with blockchains on your own, take a look at Azure-hosted blockchain offerings at bit.ly/2Gj2zaC. n
Jonathan Waldman is a Microsoft Certified Professional who has worked with Microsoft technologies since their inception and who specializes in software ergo- nomics. Waldman is a member of the Pluralsight technical team and he currently leads institutional and private-sector software-development projects. He can be reached at jonathan.waldman@live.com.
thanks to the following Microsoft technical expert for reviewing this article: James McCaffrey
26 msdn magazine
Blockchain
Block 1 (Meta Data)
Block 2 (Meta Data)
Block 3 (Meta Data)
H(Block 1)
H(Block 0)
Merkle Root
Nonce
Transactions Hash Chain
H(Block 2)
H(Block 1)
Merkle Root
Nonce
Transactions Hash Chain
H(Block 3)
H(Block 2)
Merkle Root
Nonce
Transactions Hash Chain
Merkle Root
Nonce
Transactions Hash Chain