Bitcoin From Scratch – Part 1 • Hacker News Board

Re: Bitcoin From Scratch – Part 1

#11

Post by Jasper_ » Sat, Jul 04, 2020, 1:02 AM UTC

nemo1618 wrote: post #10

Jasper_ wrote: post #6

Earlier quoted context omitted.

It used to work this way, but got changed to a less-than check for performance reasons. The Bitcoin whitepaper [0] even has the original approach: > The proof-of-work involves scanning for a value that when hashed, such as with SHA-256, the hash begins with a number of zero bits. [0] https://bitcoin.org/bitcoin.pdf

> but got changed to a less-than check for performance reasons. Performance as in "it takes less time to verify the PoW," or something else? Because the "leading zeros" and "less than target" approaches would both take a negligible amount of time.

The story I was told from Bitcoin developers was that it was done for code performance reasons. However, since the less-than check goes back all the way to the code's "Initial commit" [0], I'm not sure if that's fully accurate -- it might have been based on correspondence from the original author. At the very least, the misconception comes from Bitcoin's own whitepaper.

[0] https://github.com/bitcoin/bitcoin/blob/e071a3f6c06f41068ad1...

Re: Bitcoin From Scratch – Part 1

#12

Post by nemo1618 » Sat, Jul 04, 2020, 1:26 AM UTC

Jasper_ wrote: post #11

nemo1618 wrote: post #10

Earlier quoted context omitted.

> but got changed to a less-than check for performance reasons. Performance as in "it takes less time to verify the PoW," or something else? Because the "leading zeros" and "less than target" approaches would both take a negligible amount of time.

The story I was told from Bitcoin developers was that it was done for code performance reasons. However, since the less-than check goes back all the way to the code's "Initial commit" [0], I'm not sure if that's fully accurate -- it might have been based on correspondence from the original author. At the very least, the misconception comes from Bitcoin's own whitepaper. [0] https://github.com/bitcoin/bitcoin/blob/e07…

Weird. That doesn't make any sense to me.

btw, the whitepaper probably used "number of leading zeros" because that's what Hashcash does.

Re: Bitcoin From Scratch – Part 1

#13

Post by monokh » Sat, Jul 04, 2020, 1:33 AM UTC

Jasper_ wrote: post #11

nemo1618 wrote: post #10

Earlier quoted context omitted.

> but got changed to a less-than check for performance reasons. Performance as in "it takes less time to verify the PoW," or something else? Because the "leading zeros" and "less than target" approaches would both take a negligible amount of time.

The story I was told from Bitcoin developers was that it was done for code performance reasons. However, since the less-than check goes back all the way to the code's "Initial commit" [0], I'm not sure if that's fully accurate -- it might have been based on correspondence from the original author. At the very least, the misconception comes from Bitcoin's own whitepaper. [0] https://github.com/bitcoin/bitcoin/blob/e07…

The article mentions the hash is required to begin with a certain number of 0s in hex digits (half byte) whereas the Bitcoin whitepaper refers to it as number of bits. I'm unsure about the comparison (== or >) but the whitepaper seems accurate with regards to the target.

Re: Bitcoin From Scratch – Part 1

#14

Post by nullc » Sat, Jul 04, 2020, 1:49 AM UTC

Jasper_ wrote: post #6

sodascripts wrote: post #3

Earlier quoted context omitted.

TIL. I wonder why that myth is so commonly perpetuated.

It used to work this way, but got changed to a less-than check for performance reasons. The Bitcoin whitepaper [0] even has the original approach: > The proof-of-work involves scanning for a value that when hashed, such as with SHA-256, the hash begins with a number of zero bits. [0] https://bitcoin.org/bitcoin.pdf

> It used to work this way, but got changed to a less-than check for performance reasons.

I don't know who told you that, but they were weirdly confused.

It not unlikely the case that sometime early in development (long before publication) that it was bits-based, not only is this how the standard hashcash code works-- but the Bitcoin code calls the relevant field that encods the difficulty "bits".

But Bitcoin itself never worked that way, not from the first instant. Changing it would have been a consensus incompatible change, and Bitcoin is still more or less consensus compatible all the way back. (There are outright bugs that make the old software get stuck, but if you fix those, it follows along).

Re: Bitcoin From Scratch – Part 1

#15

Post by garmaine » Sat, Jul 04, 2020, 1:50 AM UTC

Jasper_ wrote: post #6

sodascripts wrote: post #3

Earlier quoted context omitted.

TIL. I wonder why that myth is so commonly perpetuated.

It used to work this way, but got changed to a less-than check for performance reasons. The Bitcoin whitepaper [0] even has the original approach: > The proof-of-work involves scanning for a value that when hashed, such as with SHA-256, the hash begins with a number of zero bits. [0] https://bitcoin.org/bitcoin.pdf

Bitcoin never worked that way. However the original hashcash proposal was specified as leading zeroes. I presume that’s what you are mixing up with.

Re: Bitcoin From Scratch – Part 1

#16

Post by nullc » Sat, Jul 04, 2020, 1:52 AM UTC

sowbug wrote: post #9

monokh wrote: post #4

Earlier quoted context omitted.

(author here) Absolutely. Thanks for pointing this out. If difficulty adjusted in factors of two, the ability of the network to accurately maintain its 10 mins blocktime become impaired. I try to keep these concepts rather simplified to ease the reading and learning process. Perhaps there should be a note for these situations.

In defense of the explanation, sometimes it's asking for an unnecessary mental leap of the reader to think of a bit array as a number, especially if the reader is not accustomed to thinking in base-2 or base-16 number systems.

The hash is just a number regardless-- surely no one has any problem understanding any other number in the protocol as a number! :)

Re: Bitcoin From Scratch – Part 1

#17

Post by person_of_color » Sat, Jul 04, 2020, 2:06 AM UTC

ozankabak wrote: post #5

Clear, concise and to the point. Looking forward to reading the rest. If you finish the whole series and fix/improve the posts in this sequence using the feedback from here, I think it might serve as one of the go-to pages for technically inclined people who are interested to learn about Bitcoin and blockchains.

Except they’re polarising as hell. Expect someone to chime in and state how they are a complete waste of time and energy.

Re: Bitcoin From Scratch – Part 1

#18

Post by bouncycastle » Sat, Jul 04, 2020, 2:30 AM UTC

nullc wrote: post #16

sowbug wrote: post #9

Earlier quoted context omitted.

In defense of the explanation, sometimes it's asking for an unnecessary mental leap of the reader to think of a bit array as a number, especially if the reader is not accustomed to thinking in base-2 or base-16 number systems.

The hash is just a number regardless-- surely no one has any problem understanding any other number in the protocol as a number! :)

Even this sentence is just a number.

Re: Bitcoin From Scratch – Part 1

#19

Post by saurik » Sat, Jul 04, 2020, 2:39 AM UTC

nullc wrote: post #14

Jasper_ wrote: post #6

Earlier quoted context omitted.

It used to work this way, but got changed to a less-than check for performance reasons. The Bitcoin whitepaper [0] even has the original approach: > The proof-of-work involves scanning for a value that when hashed, such as with SHA-256, the hash begins with a number of zero bits. [0] https://bitcoin.org/bitcoin.pdf

> It used to work this way, but got changed to a less-than check for performance reasons. I don't know who told you that, but they were weirdly confused. It not unlikely the case that sometime early in development (long before publication) that it was bits-based, not only is this how the standard hashcash code works-- but the Bitcoin code calls the relevant field that encods the difficulty "bits". But Bitcoin itself…

I did not read the sentence you are responding to as implying it was changed after final network launch, only that it was changed.

Re: Bitcoin From Scratch – Part 1

#20

Post by cchance » Sat, Jul 04, 2020, 2:56 AM UTC

Please finish this series! Especially the networking and consensus models it's an interesting read, really like that it's in Rust as I'm also learning Rust so it's nice to see the concepts in that language :)