buzhash64e: with rolling hash encryption

[ThomasWaldmann]

Rather simple and very slow encryption code, needs to be tuned later IF we find this worthwhile doing.

Note:

• the encryption of a buzhash64 current sum value must only depend on the sum value and the encryption key, NOT on the position in the current stream (thus: no AES-CTR or any other cipher that does not independently encrypt blocks, this is why ECB is used here) - otherwise it would cut the chunks not only based on the content but also based on position in the stream and that would cut differently and would destroy deduplication.
• instead of encrypting just 64bit using ECB, we could collect e.g. 1024 sums, encrypt them all on in one .encrypt_many() call and then do another loop over the esums to decide where to cut. would be much less overhead than now, but needs maintaining yet another buffer efficiently.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.11%. Comparing base (831e47d) to head (d95d503).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8920      +/-   ##
==========================================
- Coverage   85.17%   85.11%   -0.06%     
==========================================
  Files          93       93              
  Lines       15372    15389      +17     
  Branches     2318     2322       +4     
==========================================
+ Hits        13093    13099       +6     
- Misses       1583     1592       +9     
- Partials      696      698       +2     

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[darkk]

(As a sidenote on speed). I've found it quite interesting that fast PCG RNG (numpy's default prng) is predictable, but seed reconstruction out of 64 consecutive values is a non-trivial operation — 2¼ CPU-years of computation. However, it's still just a €108, if we assume a CPU core to cost 4 €/month.

[ThomasWaldmann]

rebased on current master, extended chunker comparison script to fastcdc, force pushed.