Document current crypto and encoding in detail

[davidsarah]

Other than the code, the most comprehensive description of Tahoe's current crypto and erasure encoding that I'm aware of is the Storage Security and Survivability Workshop 2008 paper: ​http://allmydata.org/~zooko/lafs.pdf. However, that paper does not give the level of detail required for a spec or for a thorough security analysis (for example, it doesn't specify encryption modes or the encoding of inputs to crypto primitives).

This is an obstacle to designing the new crypto, since we don't want to lose features (unless we drop them deliberately) or make mistakes that were avoided in the original design.