A block is a proposed ledger page: it groups transactions, links to an earlier block, and carries evidence that the network can use to order or certify it. In Bitcoin, the block includes the previous block’s hash; in DiemBFT, it can also include signatures over that hash. section 12.2.2 section 12.4.2
A block’s role changes slightly with the consensus system:
- Bitcoin miners propose a block after finding a digest below the required threshold, commonly described as a hash beginning with a required number of zeros. section 12.2.2
- Each Bitcoin block points to its predecessor, forming a chain back to the genesis block. Changing an earlier block changes its digest and breaks later references. section 12.2.2
- Bitcoin resolves competing valid blocks by adjusting proof-of-work difficulty and following the chain containing the most work. section 12.2.3
- DiemBFT’s designated leader collects transactions, signs a block, and sends it to validators; their votes can form a quorum certificate for a later proposal. section 12.4.2
- A certified DiemBFT block is not automatically final: a chain of three certified blocks proposed in contiguous rounds triggers commitment for the first block and its ancestors. section 12.4.5
A tampered toy chain
Imagine a three-block chain in which every block records its parent’s digest. Modify block 2, and its digest changes; block 3 still points to the old digest, so the later chain no longer authenticates the modification.
