Date: Tue, 5 Nov 2013 15:51:24 -0500
From: colj@Safe-mail.net
To: Bitcoin-development@lists.sourceforge.net
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Subject: [Bitcoin-development] Possible Solution To SM Attack
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Preliminary:
Alice has the ability to hear of a block before all other miners do.

The Problem:
Say Alice built a block, A1, from previous block 0. She doesn't let other miners =
know about it. She then works on A2 with previous block A1. Bob on the other =
hand is still working on B1 with previous block 0. Bob now finds a block and =
he broadcasts it. The assumption here is Alice will be the first miner to hear =
of this block and she will send her previously mined block, A1, to all other =
miners. By the time Bobs block arrives to other miners majority of them will =
already have received Block A1 and Bobs block will most likely be orphaned. =
Alice revealed her block, A1, only when Bob broadcast his block. This means =
she has been mining on block A2 with previous block A1 for longer than any =
other miner thus gaining an advantage without increasing her hash rate.

What We Know:
Alice has gained an advantage with time. She mines longer on the valid block.
In order for this attack to work Alice must reveal her previously mined block =
as late as possible, gaining her the most time spent working on the valid block. =
Since she has such good view of the Bitcoin network she can wait until a miner =
finds a block to release her previously mined block.

The most obvious sign of this attack taking place is the timing. A miner will =
receive a block and very quickly hear of another block both built from the =
same previous block. 

The block that a miner hears first is the one which will be mined on.

Possible Solution:
If N amount of blocks built of the same previous block are received within =
a time frame of T mine on the block with the lowest hash.

Logic:
In order for Alice to pull of this attack she not only has to propagate her =
blocks first she must also ensure her blocks are of the smallest hash.

Alice would now have to decrease her target to pull of this attack. Since she =
has a lower target it will take her longer to find a valid block negating her =
time advantage.


colj