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It looks like the difficulty is deterministic, but only if the timestamps are trusted.

What's to prevent a large miner from starting back much earlier in the chain (when difficultly was miniscule) and quickly making their own chain with forged timestamps? Wouldn't it be possible to quickly "catch up" and surpass the block height of the real chain, and thus convince clients to switch to a chain built with a non-competitive difficulty?

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I think this would be considered a time warp attack.

What prevents this strategy from working is actually quite simple, as explained in https://bitcoin.stackexchange.com/a/37960/26673:

Bitcoin nodes do not compare chains by their height. (Although they used to do this.) They calculate the total work in each chain (nChainWork), and compare by that. This is calculated by looking at the number of attempts it would take, on average, to find each block, and summing it.

My question assumed that blockchains were compared by height — but they are actually compared by the amount of work encapsulated. So a chain mined with competitive difficulty would clearly represent more work than a forged one "solo mined" with a much lower difficulty reached per block.

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  • I assume the other nodes would reject the malicious chain because it's blocks would have a nonce that does not actually create a hash lower than the difficulty. Is this assumption true?
    – Cedric Martens
    Dec 21, 2017 at 20:41

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