Would possessing a string with the SHA-256 hash of all zeros lead to an attack on SHA-256? Would possessing a string with the SHA-256 hash of all zero bits lead to an attack on SHA-256?

I could easily imagine that a specialized attack leaked knowledge of how to do it, but I could equally imagine that a bitcoin miner randomed onto one.
Would possessing a string with the sha-256 hash of all zero bits lead to an attack on sha-256?


No, it would not lead to any useful attack on SHA-256. You’ld only have an input string that hashes to 256 zero-bit output. It may be nice to know and look at, but it doesn’t buy you anything useful to attack SHA-256. All in all, it’s no different from any other hash value produced by hashing some value to another specific SHA-256 output. Just because your input renders an SHA-256 output of 256 zero-bits doesn’t mean, nor indicate that SHA-256 is broken and it certainly doesn’t open an attack vector on SHA-256.


  I could easily imagine that a specialized attack leaked knowledge of how to do it,


At the time of writing this, no such attack is known. Remember that SHA-256 has been around for a few years. The algorithm has been tested, analysed, attacked, and verified by governmental agencies as well as independant people worldwide. If there would be any weakness/shortcut in the algorithm which would enable someone to produce a 256 zero-bit hash, it would have been found. But no such findings have been made… which is why we all SHA-256 a “well vetted, cryptographically secure hash”.

Besides, being able to say “I have this set of bytes that, when I feed it to SHA-256, outputs an all-zero hash” doesn’t open any door to a cryptographical attack. It’s no different from knowing that “hello world” hashes to “b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9”. 

Having knowledge of something like that doesn’t point at a weakness or attack vector. It’s what SHA-256 does: hash a specific input into a specific output. If that output happens to be 000…000 then so be it. It may be nice for some people to know how to produce a 256 zero-bit output, but it doesn‘t have any impact whatsoever on the security claims of SHA-256.

Things would be different if some weakness in the algorithm was found which would enable us to produce any specific, chosen output we like… eg: 256 one-bits (FFF…FFF) or another series of 256 output bits (123…321). Even more interesting would be finding a way to produce collisions for SHA-256 (where two different inputs produce the same output), which would be disasterous. But all that isn’t the case. No such weaknesses have been found over the years. 


  but I could equally imagine that a bitcoin miner randomed onto one.


I honestly doubt that any bitcoin miner will produce anything that has any influence on SHA-256 just by hunting down the next Bitcoin block. Even if a Bitcoin miner would happen to find a 256 zero-bit output, it wouldn’t gain you anything as it’s still like the “hello world” hash example I mentioned above. Knowing which input bytes produce an SHA-256 output of 256 zero-bit hash doesn’t buy you anything which could be used as an attack… not even a coffee.



Some distantly related Q&amp;As which might (or might not) be interesting for further reading:


What are the time considerations with regard to security against birthday attack?
Computational requirements for breaking SHA-256?
Are there any well-known examples of SHA-256 collisions?
Cryptocurrency serving as hash breaking?