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Update notes.adoc

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Peter Dimov
2023-03-02 19:45:18 +02:00
parent 7c49f0bfb1
commit 5638134081
1 changed files with 3 additions and 10 deletions
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doc/hash/notes.adoc
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@@ -200,16 +200,9 @@ In Boost 1.81, `hash_range` was changed to process elements of type `char`,
A `uint32_t` is composed from `first[0]` to `first[3]`, and that `uint32_t`
is fed to `hash_combine`.
In principle, when `size_t` is 64 bit, we could have used `uint64_t` instead.
We do not, because this allows producing an arbitrary hash value by choosing
the input bytes appropriately (because `hash_combine` is reversible.)
Allowing control only over 32 bits of the full 64 bit `size_t` value makes
these "chosen plaintext attacks" harder.
This is not as harmful to performance as it first appears, because the
input to `hash<string>` (e.g. the key in an unordered container) is often
short (9 to 13 bytes in some typical scenarios.)
In Boost 1.82, `hash_range` for these types was changed to use
https://github.com/pdimov/mulxp_hash[`mulxp1_hash`]. This improves both
quality and speed of string hashing.
Note that `hash_range` has also traditionally guaranteed that the same element
sequence yields the same hash value regardless of the iterator type. This