Bitcoin Forum

But is there any reason why the current blockchain can't be downloaded outside the client for first-time setup?
Although I suppose in theory it could, but for some reason it seems to be unsupported and the client instead downloads it all from peers (at a very slow speed - it took hours for me when it could take minutes going over a simple HTTP download).

So, what have I missed?

If I give you a fake blockchain, I can give you a fake blockchain.

The blockchain to 120K blocks is on the sourceforge download page.

You can, but I can get the hash of the entire chain from peers and verify that what I just downloaded is correct while still getting the speed advantage.

But the client doesn't download that by default, right?

It's reasonable to choose security over speed in default settings.

The client ALWAYS downloads the block chain. It will download the block chain from its peers if one isn't already present.

I meant it doesn't download over HTTP - it downloads from peers, which can be slower

The client defaults to downloading from peers, which is probably more secure than an HTTP direct download.

It can always verify the HTTP download with other peers afterwards, so it wouldn't be any more insecure.

As I said:

I think the reason this is the default is simply for bandwidth reasons, but I could be wrong.

For whatever it's worth: the last time I walked someone through using the official Bitcoin client, I was paying attention to see what the least user-friendly part of the experience was. It seemed to be the long wait that the user had to go through while downloading the block chain. In addition to taking hours of time, there was no explanation of what was happening and no indication of when the process would be over.

Yeah, it would be nice if it could show the current block count so you knew how far you'd got, perhaps with a calculated ETA

Actually, the act of downloading the blockchain is quite fast. In fact, it's the verification part that takes the longest.

This will be easier once we have "light" clients. They can start in "light" mode and convert to a full node over time.

It takes 1.35 seconds to sha256sum the blockchain on my computer

The hard part is not doing the sha256 checksum - it's verifying whether the ECDSA signatures are valid, and the transaction inputs are spent yet. So it requires both computation (an ECDSA signature verification is close a millisecond of CPU time), and disk seeks (random access to the block chain database).