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What actually prevents the substitution of old blocks?
I feel like this is a newbie question, but I've got a technical background and have been following Bitcoin for years. Perhaps I'm just forgetting something.
We ordinarily speak of each block as secured, or confirmed, by the subsequent blocks. We say this because each block contains the prior block's hash - which is why it's a "chain."
But the blocks themselves are found just by (effectively) brute-forcing SHA2. What prevents someone from doing that to an arbitrary old block to (say) remove a transaction and thus double spend? (Or just massively confuse the network.) In other words, shouldn't it be as easy, if we're currently at block 231375, for me to find a replacement for block 200000 as to find the next block? The replacement would, by design, have the same hash but different content - content that, for whatever reason, favors me as the attacker. Why couldn't it fit right into the "authentic" chain?
Satoshi's paper addresses a similar problem, but it doesn't seem to be the same. He shows that it becomes exponentially more difficult to dictate a new chain (whose blocks have different hashes from the "real" chain) against the "honest" hashing power. But does something other than the hash stored in block 200001 authenticate block 200000? And if not, how do we distinguish in any distributed way among any candidates for block 200000 that have the same hash and are otherwise valid blocks?
(Obviously, we could checkpoint, or go by which block a majority of nodes think came first, but that isn't really "distributed" in the way we say Bitcoin is. If a majority of hashing power is the only thing that decides which version of the old block to trust, that would make a 51% attack worse than people say it is, because it would allow the arbitrary rewriting even of ancient history.)
I'm sure I'm forgetting or misunderstanding something, but I don't know what it is. Thanks!
We ordinarily speak of each block as secured, or confirmed, by the subsequent blocks. We say this because each block contains the prior block's hash - which is why it's a "chain."
But the blocks themselves are found just by (effectively) brute-forcing SHA2. What prevents someone from doing that to an arbitrary old block to (say) remove a transaction and thus double spend? (Or just massively confuse the network.) In other words, shouldn't it be as easy, if we're currently at block 231375, for me to find a replacement for block 200000 as to find the next block? The replacement would, by design, have the same hash but different content - content that, for whatever reason, favors me as the attacker. Why couldn't it fit right into the "authentic" chain?
Satoshi's paper addresses a similar problem, but it doesn't seem to be the same. He shows that it becomes exponentially more difficult to dictate a new chain (whose blocks have different hashes from the "real" chain) against the "honest" hashing power. But does something other than the hash stored in block 200001 authenticate block 200000? And if not, how do we distinguish in any distributed way among any candidates for block 200000 that have the same hash and are otherwise valid blocks?
(Obviously, we could checkpoint, or go by which block a majority of nodes think came first, but that isn't really "distributed" in the way we say Bitcoin is. If a majority of hashing power is the only thing that decides which version of the old block to trust, that would make a 51% attack worse than people say it is, because it would allow the arbitrary rewriting even of ancient history.)
I'm sure I'm forgetting or misunderstanding something, but I don't know what it is. Thanks!
I don't think a bitcoin will be worth more then 250 ever again...
Why can't we all realize that we have no idea where the price will go? It's pure speculation. Everyone should admit they just don't know.
Received this through my temporal newsreader this morning:
Mining China
Published: December 8, 2041
Associated Press
BEIJING--The first human Bitcoin mining efforts began today in outlying
Chinese provinces. With the value of a Bitcoin approaching its high in
the 2030s of $7 million, the Weifan Mining Corporation has begun pooling
the mining efforts of people who used to work as peasant farmers.
"Given the price of silicon, hardware can't be produced to mine
effectively anymore," said Tian Dun, vice president of mining operations
for Weifan. "The next step is to teach farmers how to perform the
arithmetic necessary to mine for Bitcoins."
The yield from human mining is exceedingly low, but China's 6 billion
residents can, if they each perform a single floating-point operation per
second, complete a TeraFLOP in a little under half an hour. "Ideally we
could sign up the whole population to mine," said Mr. Tian. "A picocoin
can feed a farmer's family for a whole week."
Mark Halstrom, chairman of the Artforz Bitcoin Company, brushed off the
notion that Weifan posed any competitive thread to the company's mining
dominance but praised the idea of human mining. "If farmers can support
themselves through mining efforts, there's no reason to try to discourage
them."
Halstrom is, of course, known for various mining innovations throughout
the last decade. To power the company's Minnesota-based server farm, he
began burning US dollar bills after peak oil was confirmed. And he
encouraged nuclear-fission experiments to convert gold into hydrogen,
to provide fuel for the well-known farm.
Mining China
Published: December 8, 2041
Associated Press
BEIJING--The first human Bitcoin mining efforts began today in outlying
Chinese provinces. With the value of a Bitcoin approaching its high in
the 2030s of $7 million, the Weifan Mining Corporation has begun pooling
the mining efforts of people who used to work as peasant farmers.
"Given the price of silicon, hardware can't be produced to mine
effectively anymore," said Tian Dun, vice president of mining operations
for Weifan. "The next step is to teach farmers how to perform the
arithmetic necessary to mine for Bitcoins."
The yield from human mining is exceedingly low, but China's 6 billion
residents can, if they each perform a single floating-point operation per
second, complete a TeraFLOP in a little under half an hour. "Ideally we
could sign up the whole population to mine," said Mr. Tian. "A picocoin
can feed a farmer's family for a whole week."
Mark Halstrom, chairman of the Artforz Bitcoin Company, brushed off the
notion that Weifan posed any competitive thread to the company's mining
dominance but praised the idea of human mining. "If farmers can support
themselves through mining efforts, there's no reason to try to discourage
them."
Halstrom is, of course, known for various mining innovations throughout
the last decade. To power the company's Minnesota-based server farm, he
began burning US dollar bills after peak oil was confirmed. And he
encouraged nuclear-fission experiments to convert gold into hydrogen,
to provide fuel for the well-known farm.