Commits on Jul 24, 2019

1. yubihsm setup: use hkd32 crate to derive key hierarchy …

   `hkd32` is an implementation of the same hierarchical key derivation
   algorithm the KMS was previously using, which is an extracted subset of
   the symmetric parts of BIP32 derivation (to the point it could
   potentially be used to implement a full BIP32).
   
   The `hkd32` crate has the advantage of using a zeroize-on-drop type for
   all key material, as opposed to some of the manual zeroization this
   crate was previously using. In addition, it has some richer types for
   things like derivation paths, which may be potentially useful in the
   future.
   
   There is one case that deviated from the previous implementation, which
   is the behavior of calling derive with an empty derivation path.
   Before it would output the "chain code" derived after inputting the
   `DERIVATION_VERSION`, whereas when using `hkd32` it correctly outputs
   the other half of the derived key material, which is intended to be
   used as a secret key.
   
   Nothing presently calls the derivation function with an empty derivation
   path, except for a test I just added today in #299. While the output for
   this case differs, it has no practical impact, and if anything the
   function outputting the raw chain code for the first level of the
   hierarchy (which is the version number) is a sharp edge that could
   potentially leak what is the root key to the entire hierarchy if it were
   to be called with an empty derivation path.
   
   `hkd32` uses a fully uniform derivation algorithm which treats the
   `DERIVATION_VERSION` like any other part of the path, and therefore does
   not have this sharp edge.
   
   Test vectors for path lengths of 1, 2, and 3 all pass with the original
   vectors.

   

   tony-iqlusion committed Jul 24, 2019
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