PEP 456: drop pluggable and go for compile time configuration of the hash algorithm.
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Christian Heimes
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pep-0456.txt
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pep-0456.txt
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PEP: 456
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Title: Pluggable and secure hash algorithm
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Title: Secure and interchangeable hash algorithm
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Version: $Revision$
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Last-Modified: $Date$
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Author: Christian Heimes <christian@python.org>
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@ -8,16 +8,16 @@ Type: Standards Track
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Content-Type: text/x-rst
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Created: 27-Sep-2013
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Python-Version: 3.4
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Post-History:
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Post-History: 06-Oct-2013
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Abstract
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========
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This PEP proposes SipHash as default string and bytes hash algorithm to properly
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fix hash randomization once and for all. It also proposes an addition to
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Python's C API in order to make the hash code pluggable. The new API allows to
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select the algorithm on startup as well as the addition of more hash algorithms.
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fix hash randomization once and for all. It also proposes modifications to
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Python's C code in order to unify the hash code and to make it easily
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interchangeable.
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Rationale
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@ -57,10 +57,8 @@ This PEP proposes three major changes to the hash code for strings and bytes:
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``Objects/object.c`` and ``Objects/unicodeobject.c``. The function takes a
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void pointer plus length and returns the hash for it.
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* The algorithm can be selected by the user with an environment variable,
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command line argument or with an API function (for embedders). FNV is
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guaranteed to exist on all platforms. SipHash is available on the majority
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of modern systems.
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* The algorithm can be selected at compile time. FNV is guaranteed to exist
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on all platforms. SipHash is available on the majority of modern systems.
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Requirements for a hash function
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@ -321,50 +319,25 @@ hash function table
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type definition::
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typedef struct {
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PyHash_Func hashfunc; /* function pointer */
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PyHash_Func hash; /* function pointer */
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char *name; /* name of the hash algorithm and variant */
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int hash_bits; /* internal size of hash value */
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int seed_bits; /* size of seed input */
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int precedence; /* ranking for auto-selection */
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} PyHash_FuncDef;
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} _PyHash_FuncDef;
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PyAPI_DATA(PyHash_FuncDef *) PyHash_FuncTable;
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PyAPI_DATA(_PyHash_FuncDef *) _PyHash_Func;
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Implementation::
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PyHash_FuncDef hash_func_table[] = {
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{fnv, "fnv", 64, 128, 10},
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#ifndef PY_HASH_FUNC
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#ifdef PY_UINT64_T
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{siphash24, "sip24", sizeof(Py_hash_t)*8, sizeof(Py_hash_t)*8, 20},
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_PyHash_Func = {siphash24, "sip24", 64, 128}
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#else
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_PyHash_Func = {fnv, "fnv", 8 * sizeof(Py_hash_t), 16 * sizeof(Py_hash_t)}
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#endif
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#endif
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{NULL, NULL},
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};
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PyHash_FuncDef *PyHash_FuncTable = hash_func_table;
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hash function API
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-----------------
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function proto types::
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PyAPI_FUNC(int) PyHash_SetHashAlgorithm(char *name);
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PyAPI_FUNC(PyHash_FuncDef *) PyHash_GetHashAlgorithm(void);
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PyAPI_DATA(PyHash_FuncDef *) _PyHash_Func;
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``PyHash_SetHashAlgorithm(NULL)`` selects the hash algorithm with the highest
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precedence. ``PyHash_SetHashAlgorithm("sip24")`` selects siphash24 as hash
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algorithm. The function returns ``0`` on success. In case the algorithm is
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not supported or a hash algorithm is already set it returns ``-1``.
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(XXX use enum?)
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``PyHash_GetHashAlgorithm()`` returns a pointer to current hash function
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definition or `NULL`.
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``_PyHash_Func`` holds the set hash function definition. It can't be modified
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or reset once a hash algorithm is set.
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TODO: select hash algorithm with autoconf variable
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Python API addition
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@ -379,9 +352,8 @@ algorithm as well as all available algorithms.
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::
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sys.hash_info(algorithm='siphash24',
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available_algorithms=('siphash24', 'fnv'),
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hash_bits=64,
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hash_output=64, # sizeof(Py_hash_t)*8
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hash_output=64, # 8 * sizeof(Py_hash_t)
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seed_bits=128)
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@ -439,7 +411,7 @@ multiplied with the size of the internal unicode kind::
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if (PyUnicode_READY(u) == -1)
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return -1;
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x = _PyHash_Func->hashfunc(PyUnicode_DATA(u),
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x = _PyHash_Func->hash(PyUnicode_DATA(u),
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PyUnicode_GET_LENGTH(u) * PyUnicode_KIND(u));
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@ -534,6 +506,19 @@ the past, but are not subject of this PEP.
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prefixes are stored within the tree structure.
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Discussion
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==========
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Pluggable
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---------
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The first draft of this PEP made the hash algorithm pluggable at runtime. It
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supported multiple hash algorithms in one binary to give the user the
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possibility to select a hash algorithm at startup. The approach was considered
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an unnecessary complication by several core committers [pluggable]_. Subsequent
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versions of the PEP aim for compile time configuration.
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Reference
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=========
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@ -567,6 +552,8 @@ Reference
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.. [aes-ni] http://en.wikipedia.org/wiki/AES_instruction_set
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.. [pluggable] https://mail.python.org/pipermail/python-dev/2013-October/129138.html
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Copyright
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=========
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