Interfacing with Python

[fjarri]

Necessary adjustments to reach binary compatibility with PyUmbral. See the related nucypher/pyUmbral#263

• stopped bundling Parameters with objects and decoupled u from g (as decided in Do we need to carry around UmbralParameters? #3)
• renamed UmbralDEM to DEM (we have namespaces for that)
• replaced ChaCha with XChaCha for DEM
• used domain separation tags for all instances of hashing
• exposed serialization via __bytes__() and from_bytes() in Python bindings (part of Expose serialization in bindings #11)
• added SecretKeyFactory for secret key derivation
• changed order of parameters in CapsuleFrag.verify() to match the rest of the library (and the Python version)
• various internal naming changes

Effectively fixes #27, although nothing really changed in the binary format on the Rust side (except for domain tags, missing Parameters and XChaCha); most of the work is done on the Python side

Note for reviewers:

• It may be more convenient to review commit by commit

Notes:

• In PyUmbral UmbralSecretKey is (de)serialized using nacl's SecretBox with the password expanded in nucypher. This behavior can be replicated by using xsalsa20poly1305 crate if necessary. But since we are breaking capsule compatibility anyway, users probably won't want to hold on to their UmbralPrivateKeys, so we can use the preferred chacha20poly1305 + password expansion with SHA256 (our hash of choice), which is already exposed via UmbralDEM (renamed to DEM in this PR)

[codecov-io]

Codecov Report

Merging #41 (3830188) into master (128eb0c) will increase coverage by 1.00%.
The diff coverage is 93.60%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   94.64%   95.65%   +1.00%     
==========================================
  Files          10       11       +1     
  Lines         766      759       -7     
==========================================
+ Hits          725      726       +1     
+ Misses         41       33       -8     

Impacted Files	Coverage Δ
umbral-pre/src/key_frag.rs	89.24% <85.18%> (+2.84%)	⬆️
umbral-pre/src/capsule_frag.rs	95.08% <85.71%> (+2.32%)	⬆️
umbral-pre/src/dem.rs	94.44% <91.66%> (ø)
umbral-pre/src/keys.rs	95.18% <95.18%> (ø)
umbral-pre/src/capsule.rs	100.00% <100.00%> (ø)
umbral-pre/src/curve.rs	95.55% <100.00%> (+0.04%)	⬆️
umbral-pre/src/hashing.rs	97.87% <100.00%> (-0.31%)	⬇️
umbral-pre/src/hashing_ds.rs	100.00% <100.00%> (ø)
umbral-pre/src/params.rs	100.00% <100.00%> (ø)
umbral-pre/src/pre.rs	100.00% <100.00%> (ø)
... and 1 more

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[piotr-roslaniec]

LGTM 👍
Looks very solid. I had no issues with running Python bindings locally.