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Bitcoin could work in principle with pen and paper and carrier pigeons bringing the result of computations-by-hand back and forth to a public square: the ledger would work, but its intended use case as money likely would not.

Source - Only the strong survive on page 7 of 48

Isn't one of the main properties of bitcoin its immutability? If so, how is it possible for it to work with pen and paper, since then it's not immutable? Can anyone explain this quote in different terms? What would mining even look like? Solving sudoku puzzles by hand?

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Isn't one of the main properties of bitcoin its immutability?

Yes. It is probabilistically immutable. In theory there could be a re-org right back to the genesis block but the probability of this happening is effectively zero (but not exactly zero).

If so, how is it possible for it to work with pen and paper, since then it's not immutable?

In a similar way this would be probabilistically immutable. To do a x block re-org you would need to solve x proof of work challenges or sudoku puzzles. It does rely on the public square though which brings some centralization into it. With Bitcoin, transactions, blocks and proof of work solutions are broadcast globally with no single central point of failure. If someone dropped a nuclear bomb on the public square in this example that might kill this pen and paper solution.

What would mining even look like? Solving sudoku puzzles by hand?

Effectively. You wouldn't want to calculate outputs from hash function by hand so solving a sufficiently hard sudoku puzzle (with the dimensions of the puzzle adjusting depending on the number of sudoku puzzle solvers) to give you the ability to present a newly mined block could be the way to do it.

So in theory this scheme could work but it already relies on a central public square so in practice you would ditch this scheme and use more efficient centralized schemes.

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  • As @RedGrittyBrick says you wouldn't be able to calculate digital signatures by hand so you would need to fall back to handwritten signatures. Oct 21 at 16:49
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The immutability being talked of is that of the list of transactions which we call the blockchain.

This immutability does not arise from the medium on which the list of transactions is recorded. It is recorded on media whose contents can easily be altered.

The immutability is an engineered property that I would say is based mainly on two features of Bitcoin:

  • The use of digital signatures.
  • The "proof of work" concept.

Since these are mathematical in nature, their properties depends on mathematics, not on any specific storage medium.


In practice, Bitcoin could not be run on pencil, paper and human brainpower because the mathematical operations are too intensive for human convenience and desired timescales.

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