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gh-141226: correctly support Py_HASH_EXTERNAL for Unix platforms #141245

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gh-141226: correctly support Py_HASH_EXTERNAL for Unix platforms #141245

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23 changes: 23 additions & 0 deletions Doc/c-api/hash.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -44,6 +44,29 @@ See also the :c:member:`PyTypeObject.tp_hash` member and :ref:`numeric-hash`.
Add :c:macro:`!Py_HASH_SIPHASH13`.


.. c:macro:: Py_HASH_EXTERNAL

If :c:macro:`Py_HASH_ALGORITHM` is set to that value, the hash function
definition ``PyHash_Func`` must be provided by embedders at compile time.
For instance, to use SipHash-4-8 for conservative security purposes

.. code-block:: c

static Py_hash_t
siphash48(const void *src, Py_ssize_t src_sz) { ... }

PyHash_FuncDef PyHash_Func = {
.hash = siphash48,
.name = "siphash48",
.hash_bits = 64,
.seed_bits = 128,
};

.. availability:: Unix

.. versionadded:: 3.4


.. c:macro:: Py_HASH_CUTOFF

Buffers of length in range ``[1, Py_HASH_CUTOFF)`` are hashed using DJBX33A
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5 changes: 3 additions & 2 deletions Doc/using/configure.rst
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Expand Up @@ -1024,13 +1024,14 @@ Libraries options
Security Options
----------------

.. option:: --with-hash-algorithm=[fnv|siphash13|siphash24]
.. option:: --with-hash-algorithm=[fnv|siphash13|siphash24|external]

Select hash algorithm for use in ``Python/pyhash.c``:

* ``siphash13`` (default);
* ``siphash24``;
* ``fnv``.
* ``fnv``;
* ``external``.

.. versionadded:: 3.4

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6 changes: 5 additions & 1 deletion configure
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5 changes: 4 additions & 1 deletion configure.ac
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Expand Up @@ -3966,12 +3966,15 @@ dnl quadrigraphs "@<:@" and "@:>@" produce "[" and "]" in the output
AC_ARG_WITH(
[hash_algorithm],
[AS_HELP_STRING(
[--with-hash-algorithm=@<:@fnv|siphash13|siphash24@:>@],
[--with-hash-algorithm=@<:@fnv|siphash13|siphash24|external@:>@],
[select hash algorithm for use in Python/pyhash.c (default is SipHash13)]
)],
[
AC_MSG_RESULT([$withval])
case "$withval" in
external)
AC_DEFINE([Py_HASH_ALGORITHM], [0])
;;
siphash13)
AC_DEFINE([Py_HASH_ALGORITHM], [3])
;;
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