If each block contains the hash of the previous block, then I think that it is a blockchain (regardless of if there is multiple authors or only a single author). A git repository is a blockchain, too.
> If each block contains the hash of the previous block, then I think that it is a blockchain […]
Or simply a 'hash chain':
> A hash chain is similar to a blockchain, as they both utilize a cryptographic hash function for creating a link between two nodes. However, a blockchain (as used by Bitcoin and related systems) is generally intended to support distributed agreement around a public ledger (data), and incorporates a set of rules for encapsulation of data and associated data permissions.
> Linked timestamping creates time-stamp tokens which are dependent on each other, entangled in some authenticated data structure. Later modification of the issued time-stamps would invalidate this structure. The temporal order of issued time-stamps is also protected by this data structure, making backdating of the issued time-stamps impossible, even by the issuing server itself.
This document describes a mechanism, called syslog-sign in this
document, that adds origin authentication, message integrity, replay
resistance, message sequencing, and detection of missing messages to
syslog. Essentially, this is accomplished by sending a special
syslog message. The content of this syslog message is called a
Signature Block. Each Signature Block contains, in effect, a
detached signature on some number of previously sent messages. It is
cryptographically signed and contains the hashes of previously sent
syslog messages. The originator of syslog-sign messages is simply
referred to as a "signer". The signer can be the same originator as
the originator whose messages it signs, or it can be a separate
originator.
I know of at least one: making electric heaters that actually contain obsolete mining hardware instead of heating elements. Obsolete to keep costs down and to have an excuse when someone complains "hey, at least we're recycling hardware!"
(/s also only a little)
Figure 6 is a good flowchart on helping a person decide whether it's a good solution for particular use cases. See "Distributed ledger need: blockchain, block matrix, or none?" at the bottom of:
"As Ethereum's cofounder Vitalik Buterin joked on Twitter, if someone wanted to compromise Surety's blockchain they could "make fake newspapers with a different chain of hashes and circulate them more widely." Given that the New York Times has an average daily print circulation of about 570,000 copies, this would probably be the stunt of the century."
What if the hash is published in multiple newspapers.
Circulating that many fake newspapers is not possible. If you printed up that many newspapers, who would you give them to? Anyone who wants to read the NYT likely has a source, or at least knows one; same for sellers. The NYT wishes that there were twice as many people who wanted to read their paper.
Wikipedia suggests that David Chaum first proposed what was basically a blockchain in 1982. He even had a crypto startup way before they were cool, with "eCash" in 1995.
People keep saying Merkle DAGs when someone calls a linear chain of recursively hashed data blocks a blockchain.
I don’t understand.
My understanding of the Merkle Tree is that it’s a recursive hash, but the leaf nodes are the data, each layer up the tree is the hash of the child nodes.
In a merkle tree, only the leaf nodes store (or reference) data, everything else is just a hash.
Is there another merkle structure I don’t know about?
Since posting this, I've discovered that IPFS has something it calls Merkle-DAGs.
A Block-Chain is a chain of blocks where there is one valid previous block and one valid next block.
A Block-Tree is a chain of blocks where there is one single valid previous block, and multiple valid next blocks.
A Block-DAG is a chain of blocks where there are multiple valid next blocks and multiple valid previous blocks, with the constraint that you can not form cycles.
They are analogues to linked-lists, trees, and directed-acyclic-graphs but with chained hashes.
From the Merkle-DAG article on the IPFS page:
> Merkle DAGs are similar to Merkle trees, but there are no balance requirements, and every node can carry a payload. In DAGs, several branches can re-converge or, in other words, a node can have several parents.
What's interesting here is that a Merkle Tree is a valid Merkle DAG, since a node can _optionally_ include a data payload. So a blockchain, a blocktree, and a blockdag are all also Merkle-DAGs. Merkle-DAG is a kind of unifying structure that can be used to model all of them.
Colloquially, it often refers to a consensus algorithm paired with a chain of blocks.
Bitcoin’s innovation wasn’t a blockchain, it was a proof-of-work backed consensus algorithm that allowed a group of adversarial peers to agree on the state of a shared blockchain datastructure.
According to the dictionary [1], a blockchain is "a digital database containing information (such as records of financial transactions) that can be simultaneously used and shared within a large decentralized, publicly accessible network"
The distinction here might be with a decentralized network.
"decentralized" isnt necessary to a block chain. However when people say "block chain" in everyday use, they're usually talking about that type. It's a case where the everyday use of a word is different to the actual technical meaning.
It's not a blockchain, but a single writer Merkle DAG. No consensus necessary. Much like a git repository with a single author.