Bitcoin Whitepaper Exercises – Proof of Work

In this exercise, we’re going back to the simplicity of the first exercise; we’re just adding a piece of text (a line from the poem) to a block, no transactions or signatures or inputs/outputs or any of that complication.

Instead, this exercise focuses on a simple implementation of the “Proof of Work” consensus algorithm — the goal of which being to make it “difficult” / “not worth it” for a malicious person to recompute all the hashes in an entire chain just so they can rewrite a bit of history.

To implement Proof of Work, we’re going to keep trying to compute a hash for a block until the hash is “lower” than a certain threshold, as defined by an incrementing difficulty integer. This difficulty value represents the required number of leading (on the left) 0‘s in the binary representation of the hash — not the normal hexadecimal represtation where each character represents 4 bits. The more leading 0‘s there are, the lower that hash value is.

To get a new hash each time you compute, you need to change the data in the block. For this reason, blocks need a nonce field added, which is simply a random number. Each time you generate a new nonce, recompute the hash and compare (with hashIsLowEnough(..)) to see if it’s low enough to be accepted for the current difficulty.

There are different ways of comparing a hash value’s binary bit representation to see if it’s low enough. Here are some hints to get you started:

  • You don’t need to compare the whole hash, only the first X hexadecimal digits (characters) of it from the left, depending on how many bits the difficulty value implies. Remember, 4 bits is one hexadecimal digit character.

  • You can create a number value from a string representing its binary bits like this: Number("0b001011011"), which produces the number value 91.

  • difficulty means how many leading 0‘s must be present when representing a hash’s left-most characters in binary. You may do this comparison based on string characters or numeric values (of either binary or base-10 form), whichever seems best to you. But, make sure you compare values in the same kind of representation/base.

  • JavaScript now supports a padStart(..) utility for strings, which may be useful here.

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