# Zero knowledge proof for value of a hash function

Is there a zero knowledge proof which demonstrates that Peggy knows a value v whose hash-function is w?

In my understanding of the general theorems on zero-k there EXISTS such a function if the has-function has polynomial time complexity. However, I would like to know how such the protocol would look like for the case of SHA-256. My problem is, that the usual proof requires me to reduce the fact of having a certain SHA-256 to Graph 3 Coloring, for example, and I do not see how this could be done practically.

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