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	/*
	* Copyright (c) 2003, 2019, Oracle and/or its affiliates. All rights reserved.
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/
	package java.security.spec;

	import java.math.BigInteger;
	import java.util.Arrays;

	/**
	* This immutable class defines an elliptic curve (EC)
	* characteristic 2 finite field.
	*
	* @see ECField
	*
	* @author Valerie Peng
	*
	* @since 1.5
	*/
	public class ECFieldF2m implements ECField {

	private int m;
	private int[] ks;
	private BigInteger rp;

	/**
	* Creates an elliptic curve characteristic 2 finite
	* field which has 2^{@code m} elements with normal basis.
	* @param m with 2^{@code m} being the number of elements.
	* @throws IllegalArgumentException if {@code m}
	* is not positive.
	*/
	public ECFieldF2m(int m) {
	if (m <= 0) {
	throw new IllegalArgumentException("m is not positive");
	}
	this.m = m;
	this.ks = null;
	this.rp = null;
	}

	/**
	* Creates an elliptic curve characteristic 2 finite
	* field which has 2^{@code m} elements with
	* polynomial basis.
	* The reduction polynomial for this field is based
	* on {@code rp} whose i-th bit corresponds to
	* the i-th coefficient of the reduction polynomial.<p>
	* Note: A valid reduction polynomial is either a
	* trinomial (X^{@code m} + X^{@code k} + 1
	* with {@code m} > {@code k} >= 1) or a
	* pentanomial (X^{@code m} + X^{@code k3}
	* + X^{@code k2} + X^{@code k1} + 1 with
	* {@code m} > {@code k3} > {@code k2}
	* > {@code k1} >= 1).
	* @param m with 2^{@code m} being the number of elements.
	* @param rp the BigInteger whose i-th bit corresponds to
	* the i-th coefficient of the reduction polynomial.
	* @throws NullPointerException if {@code rp} is null.
	* @throws IllegalArgumentException if {@code m}
	* is not positive, or {@code rp} does not represent
	* a valid reduction polynomial.
	*/
	public ECFieldF2m(int m, BigInteger rp) {
	// check m and rp
	this.m = m;
	this.rp = rp;
	if (m <= 0) {
	throw new IllegalArgumentException("m is not positive");
	}
	int bitCount = this.rp.bitCount();
	if (!this.rp.testBit(0) \|\| !this.rp.testBit(m) \|\|
	((bitCount != 3) && (bitCount != 5))) {
	throw new IllegalArgumentException
	("rp does not represent a valid reduction polynomial");
	}
	// convert rp into ks
	BigInteger temp = this.rp.clearBit(0).clearBit(m);
	this.ks = new int[bitCount-2];
	for (int i = this.ks.length-1; i >= 0; i--) {
	int index = temp.getLowestSetBit();
	this.ks[i] = index;
	temp = temp.clearBit(index);
	}
	}

	/**
	* Creates an elliptic curve characteristic 2 finite
	* field which has 2^{@code m} elements with
	* polynomial basis. The reduction polynomial for this
	* field is based on {@code ks} whose content
	* contains the order of the middle term(s) of the
	* reduction polynomial.
	* Note: A valid reduction polynomial is either a
	* trinomial (X^{@code m} + X^{@code k} + 1
	* with {@code m} > {@code k} >= 1) or a
	* pentanomial (X^{@code m} + X^{@code k3}
	* + X^{@code k2} + X^{@code k1} + 1 with
	* {@code m} > {@code k3} > {@code k2}
	* > {@code k1} >= 1), so {@code ks} should
	* have length 1 or 3.
	* @param m with 2^{@code m} being the number of elements.
	* @param ks the order of the middle term(s) of the
	* reduction polynomial. Contents of this array are copied
	* to protect against subsequent modification.
	* @throws NullPointerException if {@code ks} is null.
	* @throws IllegalArgumentException if{@code m}
	* is not positive, or the length of {@code ks}
	* is neither 1 nor 3, or values in {@code ks}
	* are not between {@code m}-1 and 1 (inclusive)
	* and in descending order.
	*/
	public ECFieldF2m(int m, int[] ks) {
	// check m and ks
	this.m = m;
	this.ks = ks.clone();
	if (m <= 0) {
	throw new IllegalArgumentException("m is not positive");
	}
	if ((this.ks.length != 1) && (this.ks.length != 3)) {
	throw new IllegalArgumentException
	("length of ks is neither 1 nor 3");
	}
	for (int i = 0; i < this.ks.length; i++) {
	if ((this.ks[i] < 1) \|\| (this.ks[i] > m-1)) {
	throw new IllegalArgumentException
	("ks["+ i + "] is out of range");
	}
	if ((i != 0) && (this.ks[i] >= this.ks[i-1])) {
	throw new IllegalArgumentException
	("values in ks are not in descending order");
	}
	}
	// convert ks into rp
	this.rp = BigInteger.ONE;
	this.rp = rp.setBit(m);
	for (int j = 0; j < this.ks.length; j++) {
	rp = rp.setBit(this.ks[j]);
	}
	}

	/**
	* Returns the field size in bits which is {@code m}
	* for this characteristic 2 finite field.
	* @return the field size in bits.
	*/
	public int getFieldSize() {
	return m;
	}

	/**
	* Returns the value {@code m} of this characteristic
	* 2 finite field.
	* @return {@code m} with 2^{@code m} being the
	* number of elements.
	*/
	public int getM() {
	return m;
	}

	/**
	* Returns a BigInteger whose i-th bit corresponds to the
	* i-th coefficient of the reduction polynomial for polynomial
	* basis or null for normal basis.
	* @return a BigInteger whose i-th bit corresponds to the
	* i-th coefficient of the reduction polynomial for polynomial
	* basis or null for normal basis.
	*/
	public BigInteger getReductionPolynomial() {
	return rp;
	}

	/**
	* Returns an integer array which contains the order of the
	* middle term(s) of the reduction polynomial for polynomial
	* basis or null for normal basis.
	* @return an integer array which contains the order of the
	* middle term(s) of the reduction polynomial for polynomial
	* basis or null for normal basis. A new array is returned
	* each time this method is called.
	*/
	public int[] getMidTermsOfReductionPolynomial() {
	if (ks == null) {
	return null;
	} else {
	return ks.clone();
	}
	}

	/**
	* Compares this finite field for equality with the
	* specified object.
	* @param obj the object to be compared.
	* @return true if {@code obj} is an instance
	* of ECFieldF2m and both {@code m} and the reduction
	* polynomial match, false otherwise.
	*/
	public boolean equals(Object obj) {
	if (this == obj) return true;

	return obj instanceof ECFieldF2m other
	// no need to compare rp here since ks and rp
	// should be equivalent
	&& (m == other.m)
	&& (Arrays.equals(ks, other.ks));
	}

	/**
	* Returns a hash code value for this characteristic 2
	* finite field.
	* @return a hash code value.
	*/
	public int hashCode() {
	int value = m << 5;
	value += (rp==null? 0:rp.hashCode());
	// no need to involve ks here since ks and rp
	// should be equivalent.
	return value;
	}
	}

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