Quote from: Cryddit on May 16, 2014, 04:04:24 PM

Is that what the time warp has been all along? Just a particular type of 51% attack?

Yes the original time warp bug requires the attacker to have more computing power than the rest of the "legit" network. It is a potential attack for an attacker with a majority of the computing power. Double spending, timewarp exploit and transaction halting are all exploits which become possible when the attacker has a majority of the computing power.

As explained by ArtForz (whatever happened to him?) here:

Quote from: ArtForz on September 12, 2011, 10:10:58 PM

Quote from: ByteCoin on September 12, 2011, 10:06:23 PM

Quote from: ArtForz on September 12, 2011, 09:39:28 PM

Obviously this ignores the "problem" of the attackers chain having way lower sum-of-difficulty
but that's easy to fix - just keep driving diff up at maximum speed until you have the same total work as the real chain.

Here's where I get confused. I haven't looked at the calculation in detail but surely you can't get the same total work as the real chain without doing approximately the same amount of hashing as the real chain, no matter how many blocks is in your chain or how you've manipulated the difficulty? Please explain further...

No, obviously you need to do the same or more total hashes as the real chain (it is a 51% attack...)
The "bad" part is that you can make your chain have more blocks while having the same start and end nTime.
And yes, it *should* be using 3-7, 7-11, ... but it doesn't. (probably to avoid the issue of the first interval after genesis, as you'd need to know when hashing of genesis started = the timestamp of the block before genesis).
The code I'm currently playing with gets around this by special-casing that first retarget to have a nInterval-1 span instead of nInterval.

Now with so many developers of unknown skill combined with minimal peer review and often very rushed schedules (pump and dump) it is possible some altcoins may be attackable with less than a majority of the hashpower but the original attack was always limited to a 51% exploit. The sad thing is ArtForz outlined a fix back in 2011 yet many of the "innovative" altcoins not only didn't use the fix, they amplified the attack by not understanding the implications of changing the the adjustment period, the symmetrical max adjustments, and the block interval.

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But none of these difficulty adjustment bugs would EVER have allowed an attack chain formed with less than 50% of hashing power to displace the main chain, assuming the routine I quoted above remained in place. So the Time Warp attack as described to me is either not this attack, or the person who described it was simply wrong.

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a) has more blocks,
b) ends on or before the same timestamp as the main chain,
~~c) took less than the same amount of hash power to produce.~~

Baring evidence to the contrary, whoever informed you is wrong about c. In the case of Bitcoin it is definitively wrong. It may be true for some altcoins but as much as altcoins have messed up the Bitcoin security model in some cases I don't know of any which are so messed up that they make c true.