Proof Of Work

The block chainblock chain - A chain of blocks with each block referencing the block that preceded it. The most-difficult-to-recreate chain is the best block chain. is collaboratively maintained by anonymous peerspeers - A computer that connects to the Dash network. on the networknetwork - The Dash P2P network which broadcasts transactions and blocks., so Dash requires that each blockblock - One or more transactions prefaced by a block header and protected by proof of work. Blocks are the data stored on the block chain. prove a significant amount of work was invested in its creation to ensure that untrustworthy peers who want to modify past blocks have to work harder than honest peers who only want to add new blocks to the block chain.

Chaining blocks together makes it impossible to modify transactionstransactions - A transaction spending satoshis. included in any block without modifying all following blocks. As a result, the cost to modify a particular block increases with every new block added to the block chain, magnifying the effect of the proof of work.

The proof of workproof of work - A hash below a target value which can only be obtained, on average, by performing a certain amount of brute force work---therefore demonstrating proof of work. used in Dash takes advantage of the apparently random nature of cryptographic hashes. A good cryptographic hash algorithm converts arbitrary data into a seemingly-random number. If the data is modified in any way and the hash re-run, a new seemingly-random number is produced, so there is no way to modify the data to make the hash number predictable.

To prove you did some extra work to create a block, you must create a hash of the block headerblock header - An 80-byte header belonging to a single block which is hashed repeatedly to create proof of work. which does not exceed a certain value. For example, if the maximum possible hash value is 2256 − 1, you can prove that you tried up to two combinations by producing a hash value less than 2255.

In the example given above, you will produce a successful hash on average every other try. You can even estimate the probability that a given hash attempt will generate a number below the target thresholdtarget threshold - The target is the threshold below which a block header hash must be in order for the block to be valid, and nBits is the encoded form of the target threshold as it appears in the block header.. Dash assumes a linear probability that the lower it makes the target threshold, the more hash attempts (on average) will need to be tried.

New blocks will only be added to the block chain if their hash is at least as challenging as a difficultydifficulty - How difficult it is to find a block relative to the difficulty of finding the easiest possible block. The easiest possible block has a proof-of-work difficulty of 1. value expected by the consensusconsensus - When several nodes (usually most nodes on the network) all have the same blocks in their locally-validated best block chain. protocol. Every block, the network uses the difficulty of the last 24 blocks and number of seconds elapsed between generation of the first and last of those last 24 blocks. The ideal value is 3600 (one hour).

  • If it took less than one hour to generate the 24 blocks, the expected difficulty value is increased so that the next 24 blocks should take exactly one hour to generate if hashes are checked at the same rate.

  • If it took more than one hour to generate the blocks, the expected difficulty value is decreased for the same reason.

This method of calculating difficulty (Dark Gravity Wave) was authored by Dash creator Evan Duffield to fix exploits possible with the previously used Kimoto Gravity Well difficulty readjustment algorithm. For additional detail, reference this Official Documentation Dark Gravity Wave page.

Because each block header must hash to a value below the target threshold, and because each block is linked to the block that preceded it, it requires (on average) as much hashing power to propagate a modified block as the entire Dash network expended between the time the original block was created and the present time. Only if you acquired a majority of the network's hashing power could you reliably execute such a 51 percent attack51 percent attack - The ability of someone controlling a majority of network hash rate to revise transaction history and prevent new transactions from confirming. against transaction history (although, it should be noted, that even less than 50% of the hashing power still has a good chance of performing such attacks).

The block header provides several easy-to-modify fields, such as a dedicated nonce field, so obtaining new hashes doesn't require waiting for new transactions. Also, only the 80-byte block header is hashed for proof-of-work, so including a large volume of transaction data in a block does not slow down hashing with extra I/O, and adding additional transaction data only requires the recalculation of the ancestor hashes in the merkle treemerkle tree - A tree constructed by hashing paired data (the leaves), then pairing and hashing the results until a single hash remains, the merkle root. In Dash, the leaves are almost always transactions from a single block..


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