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I've been doing some research into Bitcoin and the blockchain in general, and I've come upon a question that I can't find much of an answer to.

With my current understanding of the blockchain, I wonder if it would possible to create a "summary" block, and start the chain over? reducing its size while still maintaining continuity.

  1. The summary would have the hash of the last block in the old chain.
  2. For length calculation, it would be considered an extension of the old chain, and therefore its length calculated by adding its length to that of the old chain.
  3. I'm thinking that it would need to contain a summary of all accounts at the time of the summarization; essentially it would be the net of the old chain.

I found this question from five years ago, which helped a little bit.

This article states that the current size of the blockchain is over 100GB, and thats from 6 months back.

I'm just trying to learn, and this question has been bugging me for a little while. Is something like this possible?

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  • To explore this idea, you might want to estimate how big the "summary of all accounts" data might be.
    – Greg Hewgill
    Sep 6, 2017 at 3:13
  • Just using some estimation, according to this there are approximately 20,000,000 wallets with positive balance. A balance and address can be stored with 288 bytes [35 characters + 8 byte balance (assuming a double, could be larger)], lets be conservative and round that to 300 bytes if we include other information or need more decimal precision. That leads to 20,000,000 * 300 = 6GB, which is large, but a significant reduction.
    – Adam Schiavone
    Sep 6, 2017 at 4:47

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it is not possible, because there is no consensus rule who, how and when should do it and how sould others accept "new genesis" block

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  • It seems to me like that could be orchestrated. Just like BTC halves every 210000 blocks, its an agreed upon event. Once the "new genesis" block as you called it is made, its just like everything else in that if you see the same block coming from a majority of the community, its probably safe to trust.
    – Adam Schiavone
    Sep 6, 2017 at 5:35
  • how shall new clients verify the blockchain is they have only genesis by satoshi? of course, hard-fork/alt-coin is possible, but it will be not a Bitcoin
    – amaclin
    Sep 6, 2017 at 7:52
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It is a thing that can be done, but it requires trust. Any user may, at any time, confirm the chain they have is a legitimate chain by starting from the genesis block. They merely need to crank through a few gigabytes of data and hash it all. Okay, maybe more than a few.

There are algorithms to create a "truncated" chain. In this, we observe that most transactions that have been created (coins sent to it) have also been used to create other transactions (coins sent out of it). Once this happens, a transaction can no longer be used, so could be culled. Much of the block chain can be culled in this way.

This sort of approach can be very useful for a pragmatic bitcoiner, who just wants to make sure the people who are paying them are paying with legit coins. The blockchain they need is quite a lot shorter. However, someone had to shorten it. Do you trust them? The only way to verify a truncated chain is to go verify it against the bitcoin blockchain all the way from the genesis block, which is the step you were trying to avoid.

As for everyone agreeing, this is trickier. Remember that a newly minted block doesn't magically become the tip of the block chain. There's a consensus algorithm which makes sure it most likely eventually becomes part of the chain. Orchestrating this could be quite a feat.

If you got people to agree to do this, it would be handled as a "soft fork," in that your new shortened chain would be compatible with bitcoin. You would merely add a layer of trust in the person that built the chain. I'd argue this would be a minimal layer of trust. It would be easy for the miners to publish a substantial consensus saying "we have verified chain XYZ is consistent with Bitcoin."

Verifying off of the genesis block is a clever way around this trust issue. When you look at bitcoin from the genesis block on, it's not that "my bitcoins only have value as long as I can trust that the genesis block was indeed the start of the bitcoin chain," its "my bitcoins value is intrinsically tied into the genesis block."

Would it be bitcoin if you forked it? Amaclin, in their comments argues no, but I'd say that topic gets really murky. Read up on the Ship of Theseus and then make your own decision on that.

As for shortening block chains, there is a great interest these days in "off chain" transactions to shorten the chains. However, as far as I am aware, Bitcoin does not currently have support for these.

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