The Bitcoin blockchain is a permissionless, decentralized and immutable public ledger that contains the history of every bitcoin transaction. Transactions are grouped into blocks and linked together in a chronological chain, hence the name "blockchain."

Bitcoin is open-source, and thus, permissionless by design, meaning that there are no restrictions on who can join, manage or modify the Bitcoin protocol.

Decentralization is achieved through the implementation of a globally distributed network of nodes, whereby each individual network participant maintains a copy of the blockchain, thereby distributing the control over a global network of nodes, preventing any single entity from unilaterally changing the rules or tampering with the blockchain. The network participants hold the sole decision-making power, which enables them to collectively agree on an authoritative record of the blockchain- ensuring that all nodes operate on the same transaction history-, as well as to verify, validate and agree on which blocks and transactions are to be included in their copy of the blockchain.

The Bitcoin blockchain is rendered immutable through integration of the block hash of a preceding block into the following block; the block hash of said block is derived from the contents of the block in question, as well as the previous block hash of the preceding block. The block hash of the block which follows after that block, is also derived from the contents of the block in question, as well as the previous block hash of its preceding block, in other words:

The block hash of block “2” consists of the contents of block “2” + the block hash of the preceding block “1”. The SHA-256(bit) hashing algorithm is applied twice to this data in order to create the block hash for block “2”: the first hash being the result of the contents of block “2” + the previous block hash of block “1”, the second hash being the result of applying the SHA-256(bit) hashing algorithm on the first hash again, resulting in the block hash of block “2”.

The block hash of block “3” consists of the contents of block “3” + the block hash of the preceding block “2”. The SHA-256(bit) hashing algorithm is applied twice to this data, too, in order to create the block hash for block “3”: the first hash being the result of the contents of block “3” + the previous block hash of block “2”, the second hash being the result of applying the SHA-256(bit) hashing algorithm on the first hash again, resulting in the block hash of block “3”.

● A “block hash” refers to the unique identifier or “fingerprint” of a block.

● A “hash” in the context of bitcoin refers to the results of applying the SHA-256(bit) hashing algorithm to a specific piece or set of data, which results in a fixed-size and unique alphanumeric string, or “hash”.

Including the block hash of a preceding block into the block hash of a following block achieves, that both blocks are cryptographically linked together, which means that a change in the contents of a preceding block will alter not only the block hash of the altered block in question, but also the block hash of any block thereafter; the contents of block “2” + the block hash of block “1” result in the block hash of block “2”, and the contents of block “3” + the block hash of block “2” result in the block hash of block “3”.

If someone were to modify transactions in block “2”, he would alter the composition of the block’s contents, which in turn would alter the block’s block hash. One could rework the altered block and create a valid block hash with the block’s altered contents, but since the altered block hash would collide with the block hash used in the creation of the block hash of block “3”, the block would be rejected and discarded.

A malicious actor striving to successfully implement changes to the contents of one or more blocks, would be forced to create a new longest chain in order to force the distributed network of nodes to abandon the honest chain, and switch to the malicious chain. In order to create a new “longest chain”, the malicious actor would be required to rework not only the block hash of the altered block(s) in question, but also the block hash of all the following blocks, while outpacing any newly added blocks from honest actors.

A malicious actor would need to operate more than half of the currently deployed hash rate long enough to not only build a new “longest chain” which includes the altered blocks(s), but also add new blocks to his chain long enough for the network to switch to- and adopt the malicious chain, thereby validating and activating the altered block(s).

A venture which in today’s day and age is either computationally infeasible, economically unviable, or both.

● Participants of the Bitcoin network follow the principle of the “longest chain rule”, which refers to the blockchain featuring the most cumulative work (hash rate), rendering it the authoritative record of Bitcoin’s transaction history, and thus, the transaction history all nodes operate on.

Once a block is added to the blockchain and confirmed by the network, it is practically immutable and irreversible.

The pain of explaining Bitcoin lies within the fact that whenever you try to explain one thing, you automatically touch on two other topics one should be able to understand for the bigger picture to make sense.

I've tried improving a big part of the text I've posted yesterday, and I think that it turned out quite well- albeit that I did add a level of complexity to it.

I'm looking forward to corrections / improvements in regards to the above, It's still a work in progress though, and things that woulda, shoulda, coulda been mentioned already, are reserved for future additions, as it would've absolutely bloated the above, just so you know. 😶‍🌫️