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

Ethan Heilman eth3rs at gmail.com
Tue Jun 3 11:51:45 UTC 2014

An attack on the mining difficulty algorithm does not imply violation of
the typical security properties of a cryptographic hash function*.

Assume someone discovers a method which makes it far easier to discover new
blocks, this method: may or may not be implementable by the current SHA256
ASIC hardware.

1. If it is usable by the mining hardware, then there will be brief period
of overproduction and then difficulty will adjust. If the attack is so bad
that difficulty can't scale and we run out of a leading zero's, then the
SHA256 collision resistance is broken and we have bigger problems. Under
this scenario, everyone would see the need to immediately switch to new
hardware as people could create cycles and irreconcilable forks in the
block chain

2. If the attack is not usable by the mining hardware, then the miners will
need to switch to new ASICs anyways and the hash function can be changed
without resistance.

But lets ignore all that and say, for some unspecified reason, the bitcoin
community wants to switch hash functions and has some lead time to do so.
One could require that miners find two blocks, one computed using SHA256
and one computed using the new hash function. We could then slowly shift
the difficulty from SHA256 to the new hash function. This would allow
miners a semi-predicable roadmap to switch their infrastructure away from

* It would be a distinguisher which would be bad, but collision resistance
could be merely weakened.

On Tue, Jun 3, 2014 at 12:52 AM, Luke Dashjr <luke at dashjr.org> wrote:

> On Tuesday, June 03, 2014 4:29:55 AM xor wrote:
> > 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.
> C) fabricate past blocks entirely.
> If SHA256d is broken, Bitcoin as it is fails entirely.
> Luke
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