Quote from: arjay45 on May 16, 2013, 04:21:27 AM

Please do not start at zero difficulty (0.00024414 to be precise, which is the default start difficulty of litecoin). I know you're hoping to get rich quick but here's why you should not do this: 51% attack.

All someone has to do is build a tool that will mine valid blocks using GPUs as quickly as possible without asking the network if the blocks are valid or not (except the first one). Then you periodically broadcast your attack chain to the network without caring at all about the longest chain seen by your client from the network. As long as the chain is valid this will work.

With the difficulty at the default of 0.00024414, all you need is 0.00024414 * 2^23 * 10 = 10.5 MHash/s to be able to mine 10 blocks per second (the equation is difficulty * 2^32 * blocks_per_second = hashrate_required). I have noticed usually the network is only able to go 2-5 blocks per second because of network latency, and nearly everyone is getting 95%+ orphans thus wasting their hashing power. So at 10 blocks per second an attacker would have absolutely no problem mining faster than a network that's having a laggy orphan-fest. And 10.5 MHash/s is cheap... quite a lot of people have that or more. You might even be able to pull off this attack with just 5 MHash/s as long as you can build your chain faster than the legit network can.

If your new or upcoming coin uses scrypt the above tool will be able to attack your coin! I don't even think there's any need for the tool to be modified for new coins in order to work, it just needs to grab your genesis block and then go nuts producing blocks as fast as it can, periodically broadcasting its chain to ensure if you use checkpoints there's a high chance you'll checkpoint the chain it's produced. It might need some tweaking knobs for commonly changed parameters like the difficulty retarget period, number of coins per block, etc. It's also quite possible this tool could be made by using a simple proxy wrapper around the *coind server to censor data coming in to ensure it's not aware of other chains but the attacker's chain. This ensures the attacker will be able to jump on your new coin an hijack the chain as soon as you launch.

If anyone can prove that the above tool cannot be created, I am all ears. Please quote source code or something as proof. But as far as I know I am correct and it's possible to perform this attack provided the difficulty is low enough.

TL;DR New altcoin authors, please stop using the default litecoin starting difficulty. Please use a sane value such as 0.25 or 1.0 as the starting difficulty. Otherwise a 51% attack can easily steal all your and your early adopter's coins! Checkpoints will not foil this attack!

Obviously this has been going on a LOT the last few days. Sorry people, those 'disappearing' coins are not orphans or invalid. They are just someone else's coins now!