ssue #19183: Implement PEP 456 'secure and interchangeable hash algorithm'.
Python now uses SipHash24 on all major platforms.
diff --git a/Include/pyhash.h b/Include/pyhash.h
new file mode 100644
index 0000000..83c9281
--- /dev/null
+++ b/Include/pyhash.h
@@ -0,0 +1,147 @@
+#ifndef Py_HASH_H
+
+#define Py_HASH_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Helpers for hash functions */
+#ifndef Py_LIMITED_API
+PyAPI_FUNC(Py_hash_t) _Py_HashDouble(double);
+PyAPI_FUNC(Py_hash_t) _Py_HashPointer(void*);
+PyAPI_FUNC(Py_hash_t) _Py_HashBytes(const void*, Py_ssize_t);
+#endif
+
+/* Prime multiplier used in string and various other hashes. */
+#define _PyHASH_MULTIPLIER 1000003UL /* 0xf4243 */
+
+/* Parameters used for the numeric hash implementation. See notes for
+ _Py_HashDouble in Objects/object.c. Numeric hashes are based on
+ reduction modulo the prime 2**_PyHASH_BITS - 1. */
+
+#if SIZEOF_VOID_P >= 8
+# define _PyHASH_BITS 61
+#else
+# define _PyHASH_BITS 31
+#endif
+
+#define _PyHASH_MODULUS (((size_t)1 << _PyHASH_BITS) - 1)
+#define _PyHASH_INF 314159
+#define _PyHASH_NAN 0
+#define _PyHASH_IMAG _PyHASH_MULTIPLIER
+
+
+/* hash secret
+ *
+ * memory layout on 64 bit systems
+ * cccccccc cccccccc cccccccc uc -- unsigned char[24]
+ * pppppppp ssssssss ........ fnv -- two Py_hash_t
+ * k0k0k0k0 k1k1k1k1 ........ siphash -- two PY_UINT64_T
+ * ........ ........ ssssssss djbx33a -- 16 bytes padding + one Py_hash_t
+ * ........ ........ eeeeeeee pyexpat XML hash salt
+ *
+ * memory layout on 32 bit systems
+ * cccccccc cccccccc cccccccc uc
+ * ppppssss ........ ........ fnv -- two Py_hash_t
+ * k0k0k0k0 k1k1k1k1 ........ siphash -- two PY_UINT64_T (*)
+ * ........ ........ ssss.... djbx33a -- 16 bytes padding + one Py_hash_t
+ * ........ ........ eeee.... pyexpat XML hash salt
+ *
+ * (*) The siphash member may not be available on 32 bit platforms without
+ * an unsigned int64 data type.
+ */
+typedef union {
+ /* ensure 24 bytes */
+ unsigned char uc[24];
+ /* two Py_hash_t for FNV */
+ struct {
+ Py_hash_t prefix;
+ Py_hash_t suffix;
+ } fnv;
+#ifdef PY_UINT64_T
+ /* two uint64 for SipHash24 */
+ struct {
+ PY_UINT64_T k0;
+ PY_UINT64_T k1;
+ } siphash;
+#endif
+ /* a different (!) Py_hash_t for small string optimization */
+ struct {
+ unsigned char padding[16];
+ Py_hash_t suffix;
+ } djbx33a;
+ struct {
+ unsigned char padding[16];
+ Py_hash_t hashsalt;
+ } expat;
+} _Py_HashSecret_t;
+PyAPI_DATA(_Py_HashSecret_t) _Py_HashSecret;
+
+#ifdef Py_DEBUG
+PyAPI_DATA(int) _Py_HashSecret_Initialized;
+#endif
+
+
+/* hash function definition */
+#ifndef Py_LIMITED_API
+typedef struct {
+ Py_hash_t (*const hash)(const void *, Py_ssize_t);
+ const char *name;
+ const int hash_bits;
+ const int seed_bits;
+} PyHash_FuncDef;
+
+PyAPI_FUNC(PyHash_FuncDef*) PyHash_GetFuncDef(void);
+#endif
+
+
+/* cutoff for small string DJBX33A optimization in range [1, cutoff).
+ *
+ * About 50% of the strings in a typical Python application are smaller than
+ * 6 to 7 chars. However DJBX33A is vulnerable to hash collision attacks.
+ * NEVER use DJBX33A for long strings!
+ *
+ * A Py_HASH_CUTOFF of 0 disables small string optimization. 32 bit platforms
+ * should use a smaller cutoff because it is easier to create colliding
+ * strings. A cutoff of 7 on 64bit platforms and 5 on 32bit platforms should
+ * provide a decent safety margin.
+ */
+#ifndef Py_HASH_CUTOFF
+# define Py_HASH_CUTOFF 0
+#elif (Py_HASH_CUTOFF > 7 || Py_HASH_CUTOFF < 0)
+# error Py_HASH_CUTOFF must in range 0...7.
+#endif /* Py_HASH_CUTOFF */
+
+
+/* hash algorithm selection
+ *
+ * The values for Py_HASH_SIPHASH24 and Py_HASH_FNV are hard-coded in the
+ * configure script.
+ *
+ * - FNV is available on all platforms and architectures.
+ * - SIPHASH24 only works on plaforms that provide PY_UINT64_T and doesn't
+ * require aligned memory for integers.
+ * - With EXTERNAL embedders can provide an alternative implementation with::
+ *
+ * PyHash_FuncDef PyHash_Func = {...};
+ *
+ * XXX: Figure out __declspec() for extern PyHash_FuncDef.
+ */
+#define Py_HASH_EXTERNAL 0
+#define Py_HASH_SIPHASH24 1
+#define Py_HASH_FNV 2
+
+#ifndef Py_HASH_ALGORITHM
+# if (defined(PY_UINT64_T) && defined(PY_UINT32_T) \
+ && !defined(HAVE_ALIGNED_REQUIRED))
+# define Py_HASH_ALGORITHM Py_HASH_SIPHASH24
+# else
+# define Py_HASH_ALGORITHM Py_HASH_FNV
+# endif /* uint64_t && uint32_t && aligned */
+#endif /* Py_HASH_ALGORITHM */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* !Py_HASH_H */