/*

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* Copyright (C) Internet Systems Consortium, Inc. ("ISC")

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*

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* This Source Code Form is subject to the terms of the Mozilla Public

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* License, v. 2.0. If a copy of the MPL was not distributed with this

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* file, you can obtain one at https://mozilla.org/MPL/2.0/.

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*

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* See the COPYRIGHT file distributed with this work for additional

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* information regarding copyright ownership.

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*/

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#ifndef ISC_LFSR_H

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#define ISC_LFSR_H 1

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/*! \file isc/lfsr.h */

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#include <inttypes.h>

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#include <isc/lang.h>

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#include <isc/types.h>

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typedef struct isc_lfsr isc_lfsr_t;

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/*%

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* This function is called when reseeding is needed. It is allowed to

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* modify any state in the LFSR in any way it sees fit OTHER THAN "bits".

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*

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* It MUST set "count" to a new value or the lfsr will never reseed again.

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*

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* Also, a reseed will never occur in the middle of an extraction. This

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* is purely an optimization, and is probably what one would want.

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*/

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typedef void (*isc_lfsrreseed_t)(isc_lfsr_t *, void *);

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/*%

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* The members of this structure can be used by the application, but care

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* needs to be taken to not change state once the lfsr is in operation.

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*/

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struct isc_lfsr {

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uint32_t state; /*%< previous state */

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unsigned int bits; /*%< length */

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uint32_t tap; /*%< bit taps */

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unsigned int count; /*%< reseed count (in BITS!) */

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isc_lfsrreseed_t reseed; /*%< reseed function */

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void *arg; /*%< reseed function argument */

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};

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ISC_LANG_BEGINDECLS

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void

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isc_lfsr_init(isc_lfsr_t *lfsr, uint32_t state, unsigned int bits,

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uint32_t tap, unsigned int count,

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isc_lfsrreseed_t reseed, void *arg);

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/*%<

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* Initialize an LFSR.

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*

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* Note:

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*

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*\li Putting untrusted values into this function will cause the LFSR to

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* generate (perhaps) non-maximal length sequences.

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*

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* Requires:

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*

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*\li lfsr != NULL

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*

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*\li 8 <= bits <= 32

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*

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*\li tap != 0

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*/

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void

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isc_lfsr_generate(isc_lfsr_t *lfsr, void *data, unsigned int count);

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/*%<

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* Returns "count" bytes of data from the LFSR.

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*

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* Requires:

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*

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*\li lfsr be valid.

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*

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*\li data != NULL.

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*

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*\li count > 0.

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*/

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void

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isc_lfsr_skip(isc_lfsr_t *lfsr, unsigned int skip);

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/*%<

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* Skip "skip" states.

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*

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* Requires:

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*

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*\li lfsr be valid.

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*/

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uint32_t

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isc_lfsr_generate32(isc_lfsr_t *lfsr1, isc_lfsr_t *lfsr2);

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/*%<

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* Given two LFSRs, use the current state from each to skip entries in the

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* other. The next states are then xor'd together and returned.

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*

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* WARNING:

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*

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*\li This function is used only for very, very low security data, such

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* as DNS message IDs where it is desired to have an unpredictable

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* stream of bytes that are harder to predict than a simple flooding

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* attack.

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*

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* Notes:

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*

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*\li Since the current state from each of the LFSRs is used to skip

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* state in the other, it is important that no state be leaked

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* from either LFSR.

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*

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* Requires:

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*

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*\li lfsr1 and lfsr2 be valid.

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*

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*\li 1 <= skipbits <= 31

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*/

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ISC_LANG_ENDDECLS

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#endif /* ISC_LFSR_H */

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