deterministic IV for writecaps for dir entries

[zooko]

Okay, here is a patch which replaces os.urandom(16) with a (tagged) secure hash of the write cap. This protects the directory entries against the "VM rollback problem", which otherwise might expose the writecaps of entries to someone who has only the readcap to the dir in the case that the writer of the dir suffered a vm rollback.

In addition, this makes packing deterministic, so that pack(unpack(packeddir)) == packeddir. (Kevan's original unit tests for this ticket assumed that would be the case.)

Is there any cryptographic problem that making this change would raise? It means that encryption of the writecaps doesn't have "semantic security" (which I think corresponds to IND-CPA?), but they actually didn't anyway since each one is accompanied by its readcap.

Perhaps surprisingly, this patch appears to reduce the CPU usage a little bit for packing directories (at least on Mac OS X).

Please examine this patch, all cryptographers and security experts!

Before this patch:

benchmarking <function unpack at 0x3282370>
N:      64, time: best:    0.07,   2th-best:    0.07, ave:    0.08,   2th-worst:    0.07, worst:    0.09 (of      5), reps/s:     13, ave rate:      837
N:     256, time: best:    0.29,   2th-best:    0.29, ave:    0.30,   2th-worst:    0.30, worst:    0.32 (of      5), reps/s:      3, ave rate:      851
N:    1024, time: best:    1.15,   2th-best:    1.15, ave:    1.18,   2th-worst:    1.15, worst:    1.31 (of      5), reps/s:      0, ave rate:      866
N:    4096, time: best:    4.37,   2th-best:    4.37, ave:    4.49,   2th-worst:    4.41, worst:    4.93 (of      5), reps/s:      0, ave rate:      912
benchmarking <function pack at 0x3282330>
N:      64, time: best:    0.07,   2th-best:    0.07, ave:    0.07,   2th-worst:    0.07, worst:    0.08 (of      5), reps/s:     13, ave rate:      862
N:     256, time: best:    0.29,   2th-best:    0.29, ave:    0.29,   2th-worst:    0.29, worst:    0.29 (of      5), reps/s:      3, ave rate:      872
N:    1024, time: best:    1.15,   2th-best:    1.15, ave:    1.15,   2th-worst:    1.15, worst:    1.15 (of      5), reps/s:      0, ave rate:      889
N:    4096, time: best:    4.36,   2th-best:    4.38, ave:    4.39,   2th-worst:    4.38, worst:    4.45 (of      5), reps/s:      0, ave rate:      933
benchmarking <function unpack_and_repack at 0x32823b0>
N:      64, time: best:    0.07,   2th-best:    0.07, ave:    0.07,   2th-worst:    0.07, worst:    0.08 (of      5), reps/s:     13, ave rate:      857
N:     256, time: best:    0.29,   2th-best:    0.29, ave:    0.29,   2th-worst:    0.29, worst:    0.30 (of      5), reps/s:      3, ave rate:      870
N:    1024, time: best:    1.15,   2th-best:    1.15, ave:    1.15,   2th-worst:    1.15, worst:    1.15 (of      5), reps/s:      0, ave rate:      888
N:    4096, time: best:    4.37,   2th-best:    4.38, ave:    4.38,   2th-worst:    4.38, worst:    4.39 (of      5), reps/s:      0, ave rate:      935

after this patch:

benchmarking <function unpack at 0x32823f0>
N:      64, time: best:    0.07,   2th-best:    0.07, ave:    0.08,   2th-worst:    0.07, worst:    0.08 (of      5), reps/s:     13, ave rate:      844
N:     256, time: best:    0.29,   2th-best:    0.29, ave:    0.30,   2th-worst:    0.29, worst:    0.32 (of      5), reps/s:      3, ave rate:      858
N:    1024, time: best:    1.11,   2th-best:    1.11, ave:    1.15,   2th-worst:    1.11, worst:    1.27 (of      5), reps/s:      0, ave rate:      894
N:    4096, time: best:    4.25,   2th-best:    4.26, ave:    4.37,   2th-worst:    4.27, worst:    4.79 (of      5), reps/s:      0, ave rate:      938
benchmarking <function pack at 0x32823b0>
N:      64, time: best:    0.07,   2th-best:    0.07, ave:    0.07,   2th-worst:    0.07, worst:    0.08 (of      5), reps/s:     13, ave rate:      859
N:     256, time: best:    0.29,   2th-best:    0.29, ave:    0.29,   2th-worst:    0.29, worst:    0.29 (of      5), reps/s:      3, ave rate:      874
N:    1024, time: best:    1.11,   2th-best:    1.12, ave:    1.12,   2th-worst:    1.12, worst:    1.12 (of      5), reps/s:      0, ave rate:      917
N:    4096, time: best:    4.26,   2th-best:    4.26, ave:    4.27,   2th-worst:    4.28, worst:    4.28 (of      5), reps/s:      0, ave rate:      959
benchmarking <function unpack_and_repack at 0x3282430>
N:      64, time: best:    0.07,   2th-best:    0.07, ave:    0.07,   2th-worst:    0.07, worst:    0.08 (of      5), reps/s:     13, ave rate:      856
N:     256, time: best:    0.29,   2th-best:    0.29, ave:    0.29,   2th-worst:    0.29, worst:    0.29 (of      5), reps/s:      3, ave rate:      871
N:    1024, time: best:    1.11,   2th-best:    1.11, ave:    1.13,   2th-worst:    1.12, worst:    1.18 (of      5), reps/s:      0, ave rate:      908
N:    4096, time: best:    4.25,   2th-best:    4.25, ave:    4.25,   2th-worst:    4.26, worst:    4.26 (of      5), reps/s:      0, ave rate:      963

[zooko]

duplicate of #750