[tahoe-dev] Recombinant Hashes

David-Sarah Hopwood david-sarah at jacaranda.org
Thu Feb 11 19:21:28 UTC 2010

Chris Dew wrote:
> Hi, I just read about this project on Reddit, and wondered whether you
> use an algorithm similar to that which I've developed.
> It lets you make an unlimited amount of hashes from some data.  So
> long as the total length of the hashes you possess are greater than
> the length of the original data, you can recreate the original data -
> regardless of which combination of hashes you possess.

Yes, that's a form of Forward Error Correction code (a block code,
more specifically).

> My use case for this algorithm is very similar to your own project, so
> I thought I'd get in touch.
> http://www.finalcog.com/recombinant-hashes-demo

In the demo, 5 of 11 hashes are needed to recover the data. But the
maximum input size was 16 bytes, and the total size of the hashes was
11*4 = 44 bytes. That's an expansion of 44/16 = 2.75, when an expansion
of 11/5 = 2.2 should be possible.

The Reed-Solomon FEC code that Tahoe uses also has the property that
the first few shares are just slices of the data, so less computation
is needed to either generate or recombine them.

David-Sarah Hopwood  ⚥  http://davidsarah.livejournal.com

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