Zero knowledge proof for value of a hash function
In the rump session of Crypto 98, Hal Finney provided a talk which might be of your interest. The title is "A zero-knowledge proof of possession of a pre-image of a SHA-1 hash." I have not found any transcripts., but the video is available on both YouTube and Google Videos. (the videos are no longer available; see the comments below.) The video is now available from my Google Drive.