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/*
* 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_LFSR_H
#define ISC_LFSR_H 1
/*! \file isc/lfsr.h */
#include <inttypes.h>
#include <isc/lang.h>
#include <isc/types.h>
typedef struct isc_lfsr isc_lfsr_t;
/*%
* This function is called when reseeding is needed. It is allowed to
* modify any state in the LFSR in any way it sees fit OTHER THAN "bits".
*
* It MUST set "count" to a new value or the lfsr will never reseed again.
*
* Also, a reseed will never occur in the middle of an extraction. This
* is purely an optimization, and is probably what one would want.
*/
typedef void (*isc_lfsrreseed_t)(isc_lfsr_t *, void *);
/*%
* The members of this structure can be used by the application, but care
* needs to be taken to not change state once the lfsr is in operation.
*/
struct isc_lfsr {
uint32_t state; /*%< previous state */
unsigned int bits; /*%< length */
uint32_t tap; /*%< bit taps */
unsigned int count; /*%< reseed count (in BITS!) */
isc_lfsrreseed_t reseed; /*%< reseed function */
void *arg; /*%< reseed function argument */
};
ISC_LANG_BEGINDECLS
void
isc_lfsr_init(isc_lfsr_t *lfsr, uint32_t state, unsigned int bits,
uint32_t tap, unsigned int count,
isc_lfsrreseed_t reseed, void *arg);
/*%<
* Initialize an LFSR.
*
* Note:
*
*\li Putting untrusted values into this function will cause the LFSR to
* generate (perhaps) non-maximal length sequences.
*
* Requires:
*
*\li lfsr != NULL
*
*\li 8 <= bits <= 32
*
*\li tap != 0
*/
void
isc_lfsr_generate(isc_lfsr_t *lfsr, void *data, unsigned int count);
/*%<
* Returns "count" bytes of data from the LFSR.
*
* Requires:
*
*\li lfsr be valid.
*
*\li data != NULL.
*
*\li count > 0.
*/
void
isc_lfsr_skip(isc_lfsr_t *lfsr, unsigned int skip);
/*%<
* Skip "skip" states.
*
* Requires:
*
*\li lfsr be valid.
*/
uint32_t
isc_lfsr_generate32(isc_lfsr_t *lfsr1, isc_lfsr_t *lfsr2);
/*%<
* Given two LFSRs, use the current state from each to skip entries in the
* other. The next states are then xor'd together and returned.
*
* WARNING:
*
*\li This function is used only for very, very low security data, such
* as DNS message IDs where it is desired to have an unpredictable
* stream of bytes that are harder to predict than a simple flooding
* attack.
*
* Notes:
*
*\li Since the current state from each of the LFSRs is used to skip
* state in the other, it is important that no state be leaked
* from either LFSR.
*
* Requires:
*
*\li lfsr1 and lfsr2 be valid.
*
*\li 1 <= skipbits <= 31
*/
ISC_LANG_ENDDECLS
#endif /* ISC_LFSR_H */
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