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deterministic IV for writecaps for dir entries #751

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opened 2009-07-07 15:04:18 +00:00 by zooko · 1 comment
zooko commented 2009-07-07 15:04:18 +00:00
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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
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 added the
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zooko added this to the 1.5.0 milestone 2009-07-07 15:04:18 +00:00
zooko commented 2009-07-07 15:04:40 +00:00
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duplicate of #750

duplicate of #750
zooko added the
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zooko closed this issue 2009-07-07 15:04:40 +00:00
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