Bitcoin block header

Once again this is a tamper resistance measure that provides security for the block, an important feature of a public, decentralized and trustless system. Any input in the block can spend an output which also appears in this block if it is a valid spend. The total length of 32 bytes is the result of the SHA hash function.

The input is converted from a variable-length value to a fixed value that is bits in length. The Merkle Root can always be traced back to the very root of the data tree structure i. The Merkle Tree and is a fast and efficient way to verify the data. This is an example of a Merkle Tree. The root of the tree is g which is the hash of all transactions from t 1 to t 8. The hash g of the root is a concatenation of all the transactions in the blockchain. Timestamp The timestamp is a 4 byte filed that is the time measured in Unix epoch time.

This is the number of seconds that have elapsed since January The timestamp begins when the miner started the hashing the header. This must be strictly greater than the median time of the previous 11 blocks. Full nodes will not be accepting blocks with headers that are more than 2 hours in the future according to their clock.

Unix time is not a true representation of UTC. According to the Bitcoin protocols, the block propagation time must be on average 10 minutes, so the timestamp must fall within the allowable time that is within this range. The variability, however, is due to the difficulty which will be discussed next.

Difficulty Target The difficulty target is a 4-byte file also referred to as the Bits. The Bits specify a value or target threshold that contains leading zeroes. This is the basis of the difficulty target, which is not the same as the Bits. The difficulty target is adjusted every 2, blocks on the Bitcoin network. This information is important for miners in particular. The difficulty target is coded as a Bitcoin protocol.

When there are too many miners, the hash difficulty increases in order to control the supply of Bitcoin. If the difficulty is too easy, the block propagation time falls below 10 minutes on average. This could exhaust the supply of BTC given very quickly, and hash rates do increases as more powerful mining devices are added to the network.

When the block propagation time is more than 10 minutes, the difficulty is too high, so the protocol code ensures that the difficulty must be decreased. Nonce The nonce is a 4-byte field that is an arbitrary number of miners change to modify the header hash in order to produce a hash less than or equal to the target threshold. The reward is then put into the coinbase field and it is given to the miner who discovered the nonce first. The nonce must be equal or less than the difficulty target.

Discovering the nonce is the main activity miners engage in during the consensus mechanism of validating blocks. The miners are competing with one another by trying to solve a cryptographic puzzle that must be below the difficulty target. It was coded in a protocol to be rather difficult so that not just anyone in a permissionless system can discover the nonce. The lower the value of the difficulty target, the harder it is to generate a block.

The difficulty value has a hexadecimal form that was converted to This has been hashed using the SHA algorithm. This is the number used to generate the block that was added to the blockchain. This is what links a block to the rest of the blockchain. The example uses a different format for the time, but it is originally the number of seconds based on the Unix epoch time.

Synopsis The block header summarizes the block. In order for the miner to get its block added to the blockchain, it must find a Block Hash that meets a certain requirement. More specifically, the Block Hash must be a number that starts with a certain number of zeros.

How many zeros exactly? It varies. The software of the Bitcoin system determines the minimum number of zeros that the Block Hash should begin with. Originally, the requirement was for only a small number of zeros, but as more and more miners joined, the Bitcoin software started requiring a greater number of zeros. The lower the threshold, the more zeros that are required. This is automatically adjusted up or down by the software running the Bitcoin network.

The difficulty changes based on the number of miners. The more computers that are mining, the greater the difficulty and the more zeros are needed at the start making it harder to find the winning nonce. A Block Hash can be interpreted as a very large number and must be below a certain threshold.

This is why block hashes start with a series of zeros followed by an alphanumeric string. Some blocks have as many as twenty leading zeros, while earlier blocks have as few as eight. The number of zeros required roughly demonstrates the difficulty of mining at the time the block was published. Fortunately, the miner can have multiple attempts. The miner needs to alter the data somehow before trying to hash the Block Header again.

Bitcoin provides a way! The miner is free to put any number it wants in. The nonce is completely separate from the transactions in the block. So if the first hashing attempt of the Block Hader fails, the miner changes the value of the nonce. You can think of this process as like trying to find the combination of a combination padlock. There are no shortcuts possible when trying to find the combination of numbers that unlocks it.

You have to try every possible combination until at some point, you find the correct one by chance. You have to guess over and over until you get lucky! The miner is also competing against other miners. The first miner to find a nonce that results in a valid Block Hash is granted the right to add its block into the blockchain and is rewarded for doing so.

The only way to find a Block Hash with the required number of initial zeros is to randomly pick a nonce value and run the Block Header through the hash function. If that fails to produce the desired result, then all the mincer can do is to try again with a different nonce. The miner changes the nonce, then starts over again until it manages to find a Block Hash with at least the required minimum number of zeros. In this case, four zeros. It then runs the Block Header through the hash function and sees if the Block Hash starts with four zeros.

As you can see, the Bloch Hash only starts with two zeros so it does NOT meet the criteria needed to add the block to the blockchain. If the output is not valid, the miner keeps trying with different nonce values. In this case, the Block Hash finally starts with four zeros and meets the criteria. Miners can mine for long periods of time with no guarantee that they will ever find a correct nonce.

Most never will! The nonce is used as a counter, where its value is just constantly incremented until either it finds the correct Block Hash or some other miner does. Miners do this repeatedly…. For every new block that gets added to the blockchain, new bitcoins are paid out to the miner. This is known as the block subsidy.

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See the merkle trees section below. Must be strictly greater than the median time of the previous 11 blocks. Full nodes will not accept blocks with headers more than two hours in the future according to their clock. See the nBits format described below. If all bit values are tested, the time can be updated or the coinbase transaction can be changed and the merkle root updated.

The hashes are in internal byte order; the other values are all in little-endian order. An example header in hex: Block version: 2 b6ff0b1baa30ca44dd9e8 dbeb48ca0c Hash of previous block's header 9d10aa52eecaf04ede2 70ddadecd12bc9baaab Merkle root 24d95a Version 2 was introduced in Bitcoin Core 0. As described in BIP34 , valid version 2 blocks require a block height parameter in the coinbase.

Also described in BIP34 are rules for rejecting certain blocks; based on those rules, Bitcoin Core 0. Version 3 blocks were introduced in Bitcoin Core 0. Transactions that do not use strict DER encoding had previously been non-standard since Bitcoin Core 0. Requirements for a Block Header The block header contains three sets of block metadata. It is an byte long string, and it is comprised of the 4-byte long Bitcoin version number, byte previous block hash, byte long Merkle root, 4-byte long timestamp of the block, 4-byte long difficulty target for the block, and the 4-byte long nonce used by miners.

Block Header Components Each of these components is vital to creating an accurate and reliable header. The primary identifier of each individual block is the cryptographic hash it contains. It is essentially a digital fingerprint, and it is created by hashing the block header through the applicable algorithm twice.

The Bitcoin version number is useful in keeping track of changes and updates throughout the protocol. The previous block hash links to the previous block, or its parent block, effectively securing the chain. The Merkle root is made up of all of the hashed transaction hashes within the transaction.

This is not as complicated as it sounds, each hashed is just further hashed. The timestamp is included so that everyone working on the project will be able to see a permanent, encoded record of when a particular event occurred. It typically provides the date and time of day for that particular event and is often narrow enough to be accurate within just a fraction of a second. The difficulty target is used, simply, to adjust how hard it is for the miners working to solve the block.

Lastly, the nonce is the value that miners can alter to create different permutations and generate a correct hash in the sequence. This compensation may impact how and where listings appear.