There is a relevant post from Satoshi on this:

https://bitcointalk.org/index.php?topic=191.msg1585#msg1585

Quote:

"If SHA-256 became completely broken, I think we could come to some agreement about what the honest block chain was before the trouble started, lock that in and continue from there with a new hash function.

If the hash breakdown came gradually, we could transition to a new hash in an orderly way.  The software would be programmed to start using a new hash after a certain block number.  Everyone would have to upgrade by that time.  The software could save the new hash of all the old blocks to make sure a different block with the same old hash can't be used."


On Tue, Jun 3, 2014 at 9:21 PM, Ethan Heilman <eth3rs@gmail.com> wrote:
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 SHA256.

* 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@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

------------------------------------------------------------------------------
Learn Graph Databases - Download FREE O'Reilly Book
"Graph Databases" is the definitive new guide to graph databases and their
applications. Written by three acclaimed leaders in the field,
this first edition is now available. Download your free book today!
http://p.sf.net/sfu/NeoTech
_______________________________________________
Bitcoin-development mailing list
Bitcoin-development@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/bitcoin-development


------------------------------------------------------------------------------
Learn Graph Databases - Download FREE O'Reilly Book
"Graph Databases" is the definitive new guide to graph databases and their
applications. Written by three acclaimed leaders in the field,
this first edition is now available. Download your free book today!
http://p.sf.net/sfu/NeoTech
_______________________________________________
Bitcoin-development mailing list
Bitcoin-development@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/bitcoin-development