1/*
2 * Copyright (C) 2004-2007  Internet Systems Consortium, Inc. ("ISC")
3 * Copyright (C) 1999-2001  Internet Software Consortium.
4 *
5 * Permission to use, copy, modify, and/or distribute this software for any
6 * purpose with or without fee is hereby granted, provided that the above
7 * copyright notice and this permission notice appear in all copies.
8 *
9 * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
10 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
11 * AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
12 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
13 * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
14 * OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
15 * PERFORMANCE OF THIS SOFTWARE.
16 */
17
18/* $Id: lfsr.h,v 1.17 2007/06/19 23:47:18 tbox Exp $ */
19
20#ifndef ISC_LFSR_H
21#define ISC_LFSR_H 1
22
23/*! \file isc/lfsr.h */
24
25#include <isc/lang.h>
26#include <isc/types.h>
27
28typedef struct isc_lfsr isc_lfsr_t;
29
30/*%
31 * This function is called when reseeding is needed.  It is allowed to
32 * modify any state in the LFSR in any way it sees fit OTHER THAN "bits".
33 *
34 * It MUST set "count" to a new value or the lfsr will never reseed again.
35 *
36 * Also, a reseed will never occur in the middle of an extraction.  This
37 * is purely an optimization, and is probably what one would want.
38 */
39typedef void (*isc_lfsrreseed_t)(isc_lfsr_t *, void *);
40
41/*%
42 * The members of this structure can be used by the application, but care
43 * needs to be taken to not change state once the lfsr is in operation.
44 */
45struct isc_lfsr {
46	isc_uint32_t		state;	/*%< previous state */
47	unsigned int		bits;	/*%< length */
48	isc_uint32_t		tap;	/*%< bit taps */
49	unsigned int		count;	/*%< reseed count (in BITS!) */
50	isc_lfsrreseed_t	reseed;	/*%< reseed function */
51	void		       *arg;	/*%< reseed function argument */
52};
53
54ISC_LANG_BEGINDECLS
55
56
57void
58isc_lfsr_init(isc_lfsr_t *lfsr, isc_uint32_t state, unsigned int bits,
59		   isc_uint32_t tap, unsigned int count,
60		   isc_lfsrreseed_t reseed, void *arg);
61/*%<
62 * Initialize an LFSR.
63 *
64 * Note:
65 *
66 *\li	Putting untrusted values into this function will cause the LFSR to
67 *	generate (perhaps) non-maximal length sequences.
68 *
69 * Requires:
70 *
71 *\li	lfsr != NULL
72 *
73 *\li	8 <= bits <= 32
74 *
75 *\li	tap != 0
76 */
77
78void
79isc_lfsr_generate(isc_lfsr_t *lfsr, void *data, unsigned int count);
80/*%<
81 * Returns "count" bytes of data from the LFSR.
82 *
83 * Requires:
84 *
85 *\li	lfsr be valid.
86 *
87 *\li	data != NULL.
88 *
89 *\li	count > 0.
90 */
91
92void
93isc_lfsr_skip(isc_lfsr_t *lfsr, unsigned int skip);
94/*%<
95 * Skip "skip" states.
96 *
97 * Requires:
98 *
99 *\li	lfsr be valid.
100 */
101
102isc_uint32_t
103isc_lfsr_generate32(isc_lfsr_t *lfsr1, isc_lfsr_t *lfsr2);
104/*%<
105 * Given two LFSRs, use the current state from each to skip entries in the
106 * other.  The next states are then xor'd together and returned.
107 *
108 * WARNING:
109 *
110 *\li	This function is used only for very, very low security data, such
111 *	as DNS message IDs where it is desired to have an unpredictable
112 *	stream of bytes that are harder to predict than a simple flooding
113 *	attack.
114 *
115 * Notes:
116 *
117 *\li	Since the current state from each of the LFSRs is used to skip
118 *	state in the other, it is important that no state be leaked
119 *	from either LFSR.
120 *
121 * Requires:
122 *
123 *\li	lfsr1 and lfsr2 be valid.
124 *
125 *\li	1 <= skipbits <= 31
126 */
127
128ISC_LANG_ENDDECLS
129
130#endif /* ISC_LFSR_H */
131