lfsr.c File Reference

#include <config.h>
#include <stddef.h>
#include <stdlib.h>
#include <isc/assertions.h>
#include <isc/lfsr.h>
#include <isc/util.h>

Go to the source code of this file.

Defines
#define	VALID_LFSR(x)   (x != NULL)
Functions
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.
static isc_uint32_t	lfsr_generate (isc_lfsr_t *lfsr)
void	isc_lfsr_generate (isc_lfsr_t *lfsr, void *data, unsigned int count)
	Returns "count" bytes of data from the LFSR.
static isc_uint32_t	lfsr_skipgenerate (isc_lfsr_t *lfsr, unsigned int skip)
void	isc_lfsr_skip (isc_lfsr_t *lfsr, unsigned int skip)
	Skip "skip" states.
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.

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Detailed Description

Definition in file lfsr.c.

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Define Documentation

#define VALID_LFSR

(

x

)

(x != NULL)

Definition at line 31 of file lfsr.c.

Referenced by isc_lfsr_generate(), isc_lfsr_generate32(), isc_lfsr_init(), and isc_lfsr_skip().

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Function Documentation

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:

• Putting untrusted values into this function will cause the LFSR to generate (perhaps) non-maximal length sequences.

Requires:

• lfsr != NULL

• 8 <= bits <= 32

• tap != 0

Definition at line 34 of file lfsr.c.

References isc_lfsr::arg, isc_lfsr::bits, isc_lfsr::count, REQUIRE, reseed(), isc_lfsr::reseed, isc_lfsr::state, isc_lfsr::tap, and VALID_LFSR.

static isc_uint32_t lfsr_generate

(

isc_lfsr_t *

lfsr

)

[inline, static]

Return the next state of the lfsr.

Definition at line 59 of file lfsr.c.

References isc_lfsr::arg, isc_lfsr::bits, isc_lfsr::reseed, return(), isc_lfsr::state, and isc_lfsr::tap.

Referenced by isc_lfsr_generate(), isc_lfsr_skip(), and lfsr_skipgenerate().

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

Returns "count" bytes of data from the LFSR.

Requires:

• lfsr be valid.

• data != NULL.

• count > 0.

Definition at line 86 of file lfsr.c.

References isc_lfsr::arg, isc_lfsr::count, lfsr_generate(), REQUIRE, isc_lfsr::reseed, and VALID_LFSR.

static isc_uint32_t lfsr_skipgenerate	(	isc_lfsr_t *	lfsr,
		unsigned int	skip
	)			[inline, static]

Definition at line 118 of file lfsr.c.

References lfsr_generate(), and isc_lfsr::state.

Referenced by isc_lfsr_generate32().

void isc_lfsr_skip	(	isc_lfsr_t *	lfsr,
		unsigned int	skip
	)

Skip "skip" states.

Requires:

• lfsr be valid.

Definition at line 132 of file lfsr.c.

References lfsr_generate(), REQUIRE, and VALID_LFSR.

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:

• 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:

• 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:

• lfsr1 and lfsr2 be valid.

• 1 <= skipbits <= 31

Definition at line 145 of file lfsr.c.

References lfsr_skipgenerate(), REQUIRE, isc_lfsr::state, and VALID_LFSR.

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