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| author | Devin Jeanpierre <jeanpierreda@google.com> | 2020-11-21 08:55:23 (GMT) |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2020-11-21 08:55:23 (GMT) |
| commit | 31729366e2bc09632e78f3896dbce0ae64914f28 (patch) | |
| tree | 0596a25a57302928a432ff81662d42141bfbc391 /Misc/NEWS.d/next/Security | |
| parent | aa3a3521cef3998d4f9e7f7ff721163b6e3e5f39 (diff) | |
| download | cpython-31729366e2bc09632e78f3896dbce0ae64914f28.zip cpython-31729366e2bc09632e78f3896dbce0ae64914f28.tar.gz cpython-31729366e2bc09632e78f3896dbce0ae64914f28.tar.bz2 | |
bpo-40791: Make compare_digest more constant-time. (GH-20444)
* bpo-40791: Make compare_digest more constant-time.
The existing volatile `left`/`right` pointers guarantee that the reads will all occur, but does not guarantee that they will be _used_. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between `result` and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization.
(This is change #1 from https://bugs.python.org/issue40791 .)
Diffstat (limited to 'Misc/NEWS.d/next/Security')
| -rw-r--r-- | Misc/NEWS.d/next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst | 1 |
1 files changed, 1 insertions, 0 deletions
diff --git a/Misc/NEWS.d/next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst b/Misc/NEWS.d/next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst new file mode 100644 index 0000000..69b9de1 --- /dev/null +++ b/Misc/NEWS.d/next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst @@ -0,0 +1 @@ +Add ``volatile`` to the accumulator variable in ``hmac.compare_digest``, making constant-time-defeating optimizations less likely.
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