[bitcoindev] RE BIP 300: 26% censoring attack

[Ori Newman <orinewman1@gmail.com>]

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It seems like a 26% attacker can censor BIP 300 bundles indefinitely:

Assume some drivechain publishes a valid bundle, the attacker can send 
ALARM messages in all of her blocks to reduce its score by 1 in each block. 
Since the expected score increase per block is 0.74 - 0.26 = 0.48, its 
expected score in the 26300 block window is 0.48 * 26300 = 12624, which is 
less than the required score of 13150, so the bundle will almost certainly 
fail (this is only an approximation, since hitting the 13150 wall at any 
point is enough for the bundle to get accepted).

I think the fix to it should be quite simple: once we forbid ALARM messages 
or any kind of downvote, we can revert to the honest majority security 
assumption - a proposal with 51% support will almost certainly succeed to 
pass the threshold, and a proposal with 49% support will almost certainly 
fail to pass it.

If this is intentional, I suggest changing the BIP to reflect the 75% 
honest majority assumption, since this is a strong deviation from Bitcoin's 
security model (one argument for it would be to make it extra difficult for 
an attacker with less than 75% of the hashrate to steal funds). If this is 
not intentional, I suggest to forbid ALARM messages or any kind of 
downvote, so we revert to the 50% honest majority security assumption - a 
proposal with 51% support will almost certainly succeed to pass the 
threshold, and a proposal with 49% support will almost certainly fail to 
pass it.

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