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File hash.h

/*
 * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, you can obtain one at https://mozilla.org/MPL/2.0/.
 *
 * See the COPYRIGHT file distributed with this work for additional
 * information regarding copyright ownership.
 */

#ifndef ISC_HASH_H
#define ISC_HASH_H 1

#include <stdbool.h>

#include <isc/deprecated.h>
#include <isc/types.h>

/*****
 ***** Module Info
 *****/

/*! \file isc/hash.h
 *
 * \brief The hash API
 *	provides an unpredictable hash value for variable length data.
 *	A hash object contains a random vector (which is hidden from clients
 *	of this API) to make the actual hash value unpredictable.
 *
 *	The algorithm used in the API guarantees the probability of hash
 *	collision; in the current implementation, as long as the values stored
 *	in the random vector are unpredictable, the probability of hash
 *	collision between arbitrary two different values is at most 1/2^16.
 *
 *	Although the API is generic about the hash keys, it mainly expects
 *	DNS names (and sometimes IPv4/v6 addresses) as inputs.  It has an
 *	upper limit of the input length, and may run slow to calculate the
 *	hash values for large inputs.
 *
 *	This API is designed to be general so that it can provide multiple
 *	different hash contexts that have different random vectors.  However,
 *	it should be typical to have a single context for an entire system.
 *	To support such cases, the API also provides a single-context mode.
 *
 * \li MP:
 *	The hash object is almost read-only.  Once the internal random vector
 *	is initialized, no write operation will occur, and there will be no
 *	need to lock the object to calculate actual hash values.
 *
 * \li Reliability:
 *	In some cases this module uses low-level data copy to initialize the
 *	random vector.  Errors in this part are likely to crash the server or
 *	corrupt memory.
 *
 * \li Resources:
 *	A buffer, used as a random vector for calculating hash values.
 *
 * \li Security:
 *	This module intends to provide unpredictable hash values in
 *	adversarial environments in order to avoid denial of service attacks
 *	to hash buckets.
 *	Its unpredictability relies on the quality of entropy to build the
 *	random vector.
 *
 * \li Standards:
 *	None.
 */

/***
 *** Imports
 ***/

#include <isc/types.h>

/***
 *** Functions
 ***/
ISC_LANG_BEGINDECLS

LIBISC_EXTERNAL_DATA extern isc_hash_t *isc_hashctx;

isc_result_t
isc_hash_ctxcreate(isc_mem_t *mctx, isc_entropy_t *entropy, size_t limit,
		   isc_hash_t **hctx);
isc_result_t
isc_hash_create(isc_mem_t *mctx, isc_entropy_t *entropy, size_t limit);
/*!<
 * \brief Create a new hash object.
 *
 * isc_hash_ctxcreate() creates a different object.
 *
 * isc_hash_create() creates a module-internal object to support the
 * single-context mode.  It should be called only once.
 *
 * 'entropy' must be NULL or a valid entropy object.  If 'entropy' is NULL,
 * pseudo random values will be used to build the random vector, which may
 * weaken security.
 *
 * 'limit' specifies the maximum number of hash keys.  If it is too large,
 * these functions may fail.
 */

void
isc_hash_ctxattach(isc_hash_t *hctx, isc_hash_t **hctxp)
	ISC_DEPRECATED;
/*!<
 * \brief Attach to a hash object.
 *
 * This function is only necessary for the multiple-context mode.
 */

void
isc_hash_ctxdetach(isc_hash_t **hctxp)
	ISC_DEPRECATED;
/*!<
 * \brief Detach from a hash object.
 *
 * This function  is for the multiple-context mode, and takes a valid
 * hash object as an argument.
 */

void
isc_hash_destroy(void);
/*!<
 * \brief This function is for the single-context mode, and is expected to be used
 * as a counterpart of isc_hash_create().
 *
 * A valid module-internal hash object must have been created, and this
 * function should be called only once.
 */

/*@{*/
void
isc_hash_ctxinit(isc_hash_t *hctx);
void
isc_hash_init(void);
/*!<
 * \brief Initialize a hash object.
 *
 * It fills in the random vector with a proper
 * source of entropy, which is typically from the entropy object specified
 * at the creation.  Thus, it is desirable to call these functions after
 * initializing the entropy object with some good entropy sources.
 *
 * These functions should be called before the first hash calculation.
 *
 * isc_hash_ctxinit() is for the multiple-context mode, and takes a valid hash
 * object as an argument.
 *
 * isc_hash_init() is for the single-context mode.  A valid module-internal
 * hash object must have been created, and this function should be called only
 * once.
 */
/*@}*/

/*@{*/
unsigned int
isc_hash_ctxcalc(isc_hash_t *hctx, const unsigned char *key,
		 unsigned int keylen, bool case_sensitive)
	ISC_DEPRECATED;
unsigned int
isc_hash_calc(const unsigned char *key, unsigned int keylen,
	      bool case_sensitive)
	ISC_DEPRECATED;
/*!<
 * \brief Calculate a hash value.
 *
 * isc_hash_ctxinit() is for the multiple-context mode, and takes a valid hash
 * object as an argument.
 *
 * isc_hash_init() is for the single-context mode.  A valid module-internal
 * hash object must have been created.
 *
 * 'key' is the hash key, which is a variable length buffer.
 *
 * 'keylen' specifies the key length, which must not be larger than the limit
 * specified for the corresponding hash object.
 *
 * 'case_sensitive' specifies whether the hash key should be treated as
 * case_sensitive values.  It should typically be false if the hash key
 * is a DNS name.
 */
/*@}*/

void
isc__hash_setvec(const uint16_t *vec)
	ISC_DEPRECATED;

/*!<
 * \brief Set the contents of the random vector used in hashing.
 *
 * WARNING: This function is meant to be used only in testing code. It
 * must not be used anywhere in normally running code.
 *
 * The hash context must have been created beforehand, otherwise this
 * function is a nop.
 *
 * 'vec' is not documented here on purpose. You should know what you are
 * doing before using this function.
 */

const void *
isc_hash_get_initializer(void);

void
isc_hash_set_initializer(const void *initializer);

uint32_t
isc_hash_function(const void *data, size_t length,
		  bool case_sensitive,
		  const uint32_t *previous_hashp);
uint32_t
isc_hash_function_reverse(const void *data, size_t length,
			  bool case_sensitive,
			  const uint32_t *previous_hashp);
/*!<
 * \brief Calculate a hash over data.
 *
 * This hash function is useful for hashtables. The hash function is
 * opaque and not important to the caller. The returned hash values are
 * non-deterministic and will have different mapping every time a
 * process using this library is run, but will have uniform
 * distribution.
 *
 * isc_hash_function() calculates the hash from start to end over the
 * input data. isc_hash_function_reverse() calculates the hash from the
 * end to the start over the input data. The difference in order is
 * useful in incremental hashing; for example, a previously hashed
 * value for 'com' can be used as input when hashing 'example.com'.
 *
 * This is a new variant of isc_hash_calc() and will supersede
 * isc_hash_calc() eventually.
 *
 * 'data' is the data to be hashed.
 *
 * 'length' is the size of the data to be hashed.
 *
 * 'case_sensitive' specifies whether the hash key should be treated as
 * case_sensitive values.  It should typically be false if the hash key
 * is a DNS name.
 *
 * 'previous_hashp' is a pointer to a previous hash value returned by
 * this function. It can be used to perform incremental hashing. NULL
 * must be passed during first calls.
 */

ISC_LANG_ENDDECLS

#endif /* ISC_HASH_H */

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