What’s in a Block?

Blockchain Fundamentals Course Materials

What’s in a Block?

A block is a data structure that contains transactions as well as metadata about the block. All transactions must be included in a valid block in order to be considered final. All full nodes on the network will store a copy of the block in order to validate new transactions and share the data with other nodes.

Block Structure

$ bitcoin-cli -regtest getblockhash 0 
0f9188f13cb7b2c71f2a335e3a4fc328bf5beb436012afca590b1a11466e2206

$ bitcoin-cli -regtest getblock 0f9188f13cb7b2c71f2a335e3a4fc328bf5beb436012afca590b1a11466e2206 0

0100000000000000000000000000000000000000000000000000000000000000000000003ba3edfd7a7b12b27ac72c3e67768f617fc81bc3888a51323a9fb8aa4b1e5e4adae5494dffff7f20020000000101000000010000000000000000000000000000000000000000000000000000000000000000ffffffff4d04ffff001d0104455468652054696d65732030332f4a616e2f32303039204368616e63656c6c6f72206f6e206272696e6b206f66207365636f6e64206261696c6f757420666f722062616e6b73ffffffff0100f2052a01000000434104678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962e0ea1f61deb649f6bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d578a4c702b6bf11d5fac00000000

This is the genesis block for the regtest network, and as you can see, it is not really human readable. We are going to learn how the data is encoded from left to right.

{block header} {transaction counter} {transactions}

Block Header

{version 4B} {previous block hash 32B} {merkle root hash 32B} {time 4B} {bits 4B} {nonce 4B}

  • version (4 Bytes) - Block format version.
  • previous block hash (32 Bytes) - The hash of the preceding block. This is important to include in the header because the hash of the block is calculated from the header, and thus depends on the value of the previous block, linking each new block to the last. This is the link in the chain of the blockchain.
  • merkle root hash (32 Bytes) - The hash of the merkle tree root of all transactions in the block. If any transaction is changed, removed, or reordered, it will change the merkle root hash. This is what locks all of the transactions in the block.
  • time (4 Bytes) - Timestamp in Unix Time {TODO: provide link} (seconds). Since the clocks of each node around the world is not guaranteed to be synchronized, this is just required to be within {TODO} of the rest of the network.
  • bits (4 Bytes) - Target hash value in Compact Format. The block hash must be equal to or less than this value in order to be considered valid.
  • nonce (4 Bytes) - Can be any 4 Byte value, and is continuously changed while mining until a valid block hash is found.

Values::

  • version - 01000000 (1)
  • previous block hash - 0000000000000000000000000000000000000000000000000000000000000000
  • merkle root hash - 3ba3edfd7a7b12b27ac72c3e67768f617fc81bc3888a51323a9fb8aa4b1e5e4a
  • time - dae5494d (1296688602 Wednesday, February 2, 2011 11:16:42 PM GMT)
  • bits - ffff7f20 (7fffff0000000000000000000000000000000000000000000000000000000000)
  • nonce - 02000000 (2)

Note: Remember Bitcoin uses little endian format, which may seem backwards. You can use the provided tool $ reverse_endian {hex} to reverse the endianess of a hex string.

Transaction Counter

01 (1) Transaction in the block

  • Transaction Counter (Variable Length) - Number of transactions in the block represented as a Variable Length Integer.

Transactions

See 2.2-Transactions.md