tribble.crypto
Class LFSR

java.lang.Object
  tribble.crypto.LFSR

public class LFSR

extends java.lang.Object

Logical Feedback Shift Register (LFSR) methods.

A Logical Feedback Shift Register, or LFSR, is a binary word of N bits in a Galois configuration that implements a shift operation. The shift operation examines the lowest (least significant) bit of the current value of the register and then exclusive-or's it with the mask value established for the LFSR; it then rotates the bits of the LFSR one bit, resulting in a new state for the LFSR. The output of the shift operation is the low bit.

By shifting an LFSR M times and concatenating the resulting output bits, an M-bit pseudo-random integer can be produced. An LFSR can thus be used as a pseudo-random number generator.

Since:

2002-11-10

Version:

$Revision: 1.1 $ $Date: 2002/11/14 02:57:18 $

Author:

David R. Tribble (david@tribble.com).
Copyright ©2002 by David R. Tribble, all rights reserved.
Permission is granted to freely use and distribute this source code provided that the original copyright and authorship notices remain intact.

Field Summary

static short	DFL_SIZE Default LFSR size (bits).
protected int[]	m_bits LFSR state (N bits, as N/32 32-bit words).
protected int[]	m_mask LFSR exclusive-or mask (N bits, as N/32 32-bit words).
protected short	m_size LFSR width (bits).
static int	SERIES Class version number.

Constructor Summary

LFSR()
Default constructor.

LFSR(int size)
Constructor.

Method Summary

byte[]	getBits() Retrieve the contents (value) of this LFSR.
byte[]	getMask() Retrieve the contents (value) of the mask for this LFSR.
static void	main(java.lang.String[] args) Test driver.
void	setBits(byte[] val) Set the contents (value) of this LFSR.
void	setMask(byte[] val) Set the contents (value) of the mask for this LFSR.
int	shift() Shift (rotate) this LFSR right one bit, returning the resulting output bit.
long	shift(int n) Shift (rotate) this LFSR multiple times, returning the concatenation of the resulting output bits.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

SERIES

public static final int SERIES

Class version number.

See Also:

Constant Field Values

DFL_SIZE

public static final short DFL_SIZE

Default LFSR size (bits).

See Also:

Constant Field Values

m_bits

protected int[] m_bits

LFSR state (N bits, as N/32 32-bit words). The words are stored in little-endian order.

m_mask

protected int[] m_mask

LFSR exclusive-or mask (N bits, as N/32 32-bit words). The words are stored in little-endian order.

m_size

protected short m_size

LFSR width (bits).

Constructor Detail

LFSR

public LFSR()

Default constructor. Constructs an LFSR with a size of 32 bits and an initial vector of 0x00000001.

Since:

1.0, 2002-11-07

LFSR

public LFSR(int size)

Constructor. Constructs an LFSR with a given size and an initial vector of 0x00000001.

Parameters:

size - The number of bits in the LFSR. This cannot be less than one or greater than 32,767.

Throws:

java.lang.IllegalArgumentException - Thrown if size is less than one.

Since:

1.0, 2002-11-07

Method Detail

main

public static void main(java.lang.String[] args)
                 throws java.lang.Exception

Test driver.

Parameters:

args - Command line arguments.

Throws:

java.lang.Exception

Since:

1.0, 2002-11-07

getBits

public byte[] getBits()

Retrieve the contents (value) of this LFSR.

Returns:

An array of bytes containing the bit value for this LFSR. The bits are in big-endian order, i.e., element [0] holds the most significant bits and element [val.length-1] holds the least significant bits.

Since:

1.0, 2002-11-07

setBits

public void setBits(byte[] val)

Set the contents (value) of this LFSR.

Note that setting all of the bits to zero results in an LFSR that generates a repeating sequence of nothing but zero bits.

Parameters:

val - An array of bytes containing the bit value for this LFSR. The bits are in big-endian order, i.e., element [0] holds the most significant bits and element [val.length-1] holds the least significant bits. Most significant bits beyond the size of this LFSR are ignored.

Since:

1.0, 2002-11-07

getMask

public byte[] getMask()

Retrieve the contents (value) of the mask for this LFSR.

Returns:

An array of bytes containing the bit value of the exclusive-or mask for this LFSR. The bits are in big-endian order, i.e., element [0] holds the most significant bits and element [val.length-1] holds the least significant bits.

Since:

1.0, 2002-11-07

setMask

public void setMask(byte[] val)

Set the contents (value) of the mask for this LFSR.

Parameters:

val - An array of bytes containing the bit value of the exclusive-or mask for this LFSR. The bits are in big-endian order, i.e., element [0] holds the most significant bits and element [val.length-1] holds the least significant bits. Most significant bits beyond the size of this LFSR are ignored.

Since:

1.0, 2002-11-07

shift

public int shift()

Shift (rotate) this LFSR right one bit, returning the resulting output bit.

Returns:

The low order output bit that results from changing the state of this LFSR. This is a one-bit value returned as an int.

Since:

1.0, 2002-11-07

shift

public long shift(int n)

Shift (rotate) this LFSR multiple times, returning the concatenation of the resulting output bits.

Parameters:

n - The number of times to shift (rotate) this LFSR, which is also the number of bits in the return value.

Returns:

The concatenation of the output bits of n shifts (rotations) of this LFSR. Since a long value is returned, only the last 64 bits of the shifts can be returned.

Since:

1.0, 2002-11-07