lfsr.h File Reference

#include <isc/lang.h>
#include <isc/types.h>

Go to the source code of this file.

Data Structures
struct	isc_lfsr
	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. More...
Defines
#define	ISC_LFSR_H   1
Typedefs
typedef struct isc_lfsr	isc_lfsr_t
typedef void(*	isc_lfsrreseed_t )(isc_lfsr_t *, void *)
	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".
Functions
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.
void	isc_lfsr_generate (isc_lfsr_t *lfsr, void *data, unsigned int count)
	Returns "count" bytes of data from the LFSR.
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.h.

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Define Documentation

#define ISC_LFSR_H   1

Definition at line 21 of file lfsr.h.

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Typedef Documentation

typedef struct isc_lfsr isc_lfsr_t

Definition at line 28 of file lfsr.h.

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

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.

Definition at line 39 of file lfsr.h.

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Function Documentation

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:

• 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.

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.

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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