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lfsr.h
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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 http://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 <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 { isc_uint32_t state; /*%< previous state */ unsigned int bits; /*%< length */ isc_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, isc_uint32_t state, unsigned int bits, isc_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. */ isc_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 */