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be an even number of “leaf” nodes in the tree, so the last transaction is duplicated, creating 420 nodes, each containing the hash of one transaction. The transaction hashes are then combined, in pairs, creating each level of the tree, until all the transactions are summarized into one node at the “root” of the tree. The root of the merkle tree sum‐ marizes all the transactions into a single 32 byte value c91c008c26e50763e9f548bb8b2fc323735f73577effbc55502c51eb4cc7cf2e which you can see listed as “merkle root” in Example 8-3 The mining node will then add a 4-byte timestamp, encoded as a Unix “Epoch” time‐ stamp, which is based on the number of seconds elapsed from January 1st, 1970, mid‐ night UTC/GMT. The time 1388185914 is equal to Friday, 27 Dec 2013, 23:11:54 UTC/ GMT. The node then fills in the difficulty target, which defines the required Proof-of-Work difficulty to make this a valid block. The difficulty is stored in the block as a “difficulty bits” metric, which is a mantissa-exponent encoding of the target. The encoding has a one-byte exponent, followed by a 3 byte mantissa (coefficient). In block 277,316, for example, the difficulty bits value is 0x1903a30c. The first part 0x19 is a hexadecimal exponent, while the next part 0x03a30c is the coefficient. The concept of a difficulty target is explained in “Difficulty Target and Re-Targeting” on page 199 and the “difficulty bits” representation is explained in “Difficulty Representation” on page 198. The final field is the nonce, which is initialized to zero. With all the other fields filled, the block header is now complete and the process of mining can begin. The goal is now to find a value for the nonce that results in a block header hash that is less than the difficulty target. The mining node will need to test billions or trillions of nonce values before a nonce is found that satisfies the requirement. Mining the Block Now that a candidate block has been constructed by Jing’s node, it is time for Jing’s hardware mining rig to “mine” the block, to find a solution to the Proof-of-Work al‐ gorithm that makes the block valid. Throughout this book we have studied crypto‐ graphic hash functions as used in various aspects of the bitcoin system. The hash func‐ tion SHA-256 is the function used in bitcoin’s mining process. In the simplest terms, mining is the process of hashing the block header repeatedly, changing one parameter, until the resulting hash matches a specific target. The hash function’s result cannot be determined in advance, nor can a pattern be created that will produce a specific hash value. This feature of hash functions means that the only way to produce a hash result matching a specific target, is to try again and again, randomly modifying the input until the desired hash result appears by chance. 192 | Chapter 8: Mining and ConsensusPDF Image | Mastering Bitcoin
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