[Bitcoin-development] Lets discuss what to do if SHA256d is actually broken

Kevin kevinsisco61784 at gmail.com
Tue Jun 3 14:43:39 UTC 2014

On 6/3/2014 12:29 AM, xor wrote:
> Hash: SHA256
> Hi,
> I thought a lot about the worst case scenario of SHA256d being broken in a way
> which could be abused to
> A) reduce the work of mining a block by some significant amount
> B) reduce the work of mining a block to zero, i.e. allow instant mining.
> Bitcoin needs to be prepared for this as any hash function has a limited
> lifetime. Usually crypto stuff is not completely broken instantly by new
> attacks but gradually. For example first the attack difficulty is reduced from
> 2^128 to 2^100, then 2^64, etc.
> This would make scenario A more likely.
> Now while B sounds more dangerous, I think in fact A is:
> Consider how A would happen in real life: Someone publishes a paper of a
> theoretical reduction of SHA256d attacks to 2^96 bit. Mathematicians will
> consider this as a serious attack and create a lot of riot.
> If no plan is made early enough, as in now, the Bitcoin Core team might then
> probably want to just do the easiest approach of replacing the hash function
> after a certain block number, i.e. a hard fork.
> But what about the Bitcoin miners, those who need to actually accept a change
> of mining algorithm which renders their hardware which cost MILLIONS
> completely worthless?
> Over the years they have gotten used to exponential growth of the Bitcoin
> networks hashrate, and therefore exponential devaluation of their mining
> hardware. Even if the attack on SHA256d causes a significant growth of
> difficulty, the miners will not *believe* that it is an actual attack on SHA256d
> - - maybe it is just some new large mining operation?  They are used to this
> happening! Why should they believe this and switch to a new hash function
> which requires completely new hardware and therefore costs them millions?
> They will just keep mining SHA256d. Thats why this is more dangerous, because
> changing the hash funciton won't be accepted by the miners even though it is
> broken.
> Something smarter needs to be thought of.
> Now I must admit that I am not good at cryptography at all, but I had the
> following idea: Use the altcoin concept of having multiple hash functions in a
> chain. If SHA256d is broken, it is chained with a new hash function.
> Thereby, people who want to mine the new replacement hash function still will
> need ASICs which can solve the old SHA proof of work. So existing ASIC owners
> can amend their code to do SHA256d using the ASIC, and then the second hash
> function using a general purpose CPU.
> This would also allow a smooth migration of difficulty - I don't even know how
> difficulty would react with the naive approach of just replacing SHA with
> something else: It would probably be an unsolvable problem to define new rules
> to make it decrease enough so new blocks can actually be mined by the now
> several orders of magnitude slower CPU-only mining community but still be high
> enough to be able to deal with the fact that millions of people will try their
> luck with mining at the release date.
> While this sounds simple in theory, it might be a lot of work to implement, so
> you guys might want to take precautions for it soon :)
> Greetings,
> 	xor - A Freenet project developer
> Version: GnuPG v1.4.14 (GNU/Linux)
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> =dkuk
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It is good to start thinking about such things.  Let's face it, it could 
happen.  However, short of having bitcoin use another algorithm for 
encryption, I am not sure much could be done.  That's just me.


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