Tuesday, June 21, 2011

CryptoFace Digest Design Oops

Designing an interface for managing a library of cryptographic digests seems so easy, right?

Select a digest from a list, process data, get a hash... all there is to it, right?


While pulling in another digest provider, Botan, I found some items that did not fit into the simple model. Namely the configurability of some of the uncommon and new digest algorithms:

  • Customizable output size of the 3 Skein internal storage variants
  • Customizable "personalization" value of Skein
  • Custom number of rounds and output size for Tiger
  • ...
This is even without the notions of composing digests in various fashions, such as in parallel or in a Feistel scheme.

In light of this, I anticipate changing my mechanism for obtaining and enumerating digest implementations. Changes will likely include moving the enumeration of digests to more of a secondary feature, making the move to a set of 'well-defined' digest identifiers to be mapped from strings, and making way for parameterized construction of digests to accommodate more complex notions, including hash-based MACs/etc.

The change will not be without complication, however in light of analyzing the problem and the Botan library, I think I may be able to make some elegant structures possible for dealing with complex algorithms... at least with the Lua engine. An example set of structures could be:

-- Simple sized sha2 filter
x = Filter(SHA2(256))
x = Filter(HMAC(SHA2(256),"KEY")
-- Complex chain of hashes
x = Filter(Parallel(SHA2(512), Skein(512,1024,"Personalization")))
-- Take the filter and stream file-to-file using ltn12
  ltn12.sink.chain(x, ltn12.sink.file("SOURCEFILE.hash")))

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