1 /*
2 * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25 package java.security.spec;
26
27 import java.math.BigInteger;
28 import java.util.Arrays;
29
30 /**
31 * This immutable class defines an elliptic curve (EC)
32 * characteristic 2 finite field.
33 *
34 * @see ECField
35 *
36 * @author Valerie Peng
37 *
38 * @since 1.5
39 */
40 public class ECFieldF2m implements ECField {
41
42 private int m;
43 private int[] ks;
44 private BigInteger rp;
45
46 /**
47 * Creates an elliptic curve characteristic 2 finite
48 * field which has 2^{@code m} elements with normal basis.
49 * @param m with 2^{@code m} being the number of elements.
50 * @exception IllegalArgumentException if {@code m}
51 * is not positive.
52 */
53 public ECFieldF2m(int m) {
54 if (m <= 0) {
55 throw new IllegalArgumentException("m is not positive");
56 }
57 this.m = m;
58 this.ks = null;
59 this.rp = null;
60 }
61
62 /**
63 * Creates an elliptic curve characteristic 2 finite
64 * field which has 2^{@code m} elements with
65 * polynomial basis.
66 * The reduction polynomial for this field is based
67 * on {@code rp} whose i-th bit corresponds to
68 * the i-th coefficient of the reduction polynomial.<p>
69 * Note: A valid reduction polynomial is either a
70 * trinomial (X^{@code m} + X^{@code k} + 1
71 * with {@code m} > {@code k} >= 1) or a
72 * pentanomial (X^{@code m} + X^{@code k3}
73 * + X^{@code k2} + X^{@code k1} + 1 with
74 * {@code m} > {@code k3} > {@code k2}
75 * > {@code k1} >= 1).
76 * @param m with 2^{@code m} being the number of elements.
77 * @param rp the BigInteger whose i-th bit corresponds to
78 * the i-th coefficient of the reduction polynomial.
79 * @exception NullPointerException if {@code rp} is null.
80 * @exception IllegalArgumentException if {@code m}
81 * is not positive, or {@code rp} does not represent
82 * a valid reduction polynomial.
83 */
84 public ECFieldF2m(int m, BigInteger rp) {
85 // check m and rp
86 this.m = m;
87 this.rp = rp;
88 if (m <= 0) {
89 throw new IllegalArgumentException("m is not positive");
90 }
91 int bitCount = this.rp.bitCount();
92 if (!this.rp.testBit(0) || !this.rp.testBit(m) ||
93 ((bitCount != 3) && (bitCount != 5))) {
94 throw new IllegalArgumentException
95 ("rp does not represent a valid reduction polynomial");
96 }
97 // convert rp into ks
98 BigInteger temp = this.rp.clearBit(0).clearBit(m);
99 this.ks = new int[bitCount-2];
100 for (int i = this.ks.length-1; i >= 0; i--) {
101 int index = temp.getLowestSetBit();
102 this.ks[i] = index;
103 temp = temp.clearBit(index);
104 }
105 }
106
107 /**
108 * Creates an elliptic curve characteristic 2 finite
109 * field which has 2^{@code m} elements with
110 * polynomial basis. The reduction polynomial for this
111 * field is based on {@code ks} whose content
112 * contains the order of the middle term(s) of the
113 * reduction polynomial.
114 * Note: A valid reduction polynomial is either a
115 * trinomial (X^{@code m} + X^{@code k} + 1
116 * with {@code m} > {@code k} >= 1) or a
117 * pentanomial (X^{@code m} + X^{@code k3}
118 * + X^{@code k2} + X^{@code k1} + 1 with
119 * {@code m} > {@code k3} > {@code k2}
120 * > {@code k1} >= 1), so {@code ks} should
121 * have length 1 or 3.
122 * @param m with 2^{@code m} being the number of elements.
123 * @param ks the order of the middle term(s) of the
124 * reduction polynomial. Contents of this array are copied
125 * to protect against subsequent modification.
126 * @exception NullPointerException if {@code ks} is null.
127 * @exception IllegalArgumentException if{@code m}
128 * is not positive, or the length of {@code ks}
129 * is neither 1 nor 3, or values in {@code ks}
130 * are not between {@code m}-1 and 1 (inclusive)
131 * and in descending order.
132 */
133 public ECFieldF2m(int m, int[] ks) {
134 // check m and ks
135 this.m = m;
136 this.ks = ks.clone();
137 if (m <= 0) {
138 throw new IllegalArgumentException("m is not positive");
139 }
140 if ((this.ks.length != 1) && (this.ks.length != 3)) {
141 throw new IllegalArgumentException
142 ("length of ks is neither 1 nor 3");
143 }
144 for (int i = 0; i < this.ks.length; i++) {
145 if ((this.ks[i] < 1) || (this.ks[i] > m-1)) {
146 throw new IllegalArgumentException
147 ("ks["+ i + "] is out of range");
148 }
149 if ((i != 0) && (this.ks[i] >= this.ks[i-1])) {
150 throw new IllegalArgumentException
151 ("values in ks are not in descending order");
152 }
153 }
154 // convert ks into rp
155 this.rp = BigInteger.ONE;
156 this.rp = rp.setBit(m);
157 for (int j = 0; j < this.ks.length; j++) {
158 rp = rp.setBit(this.ks[j]);
159 }
160 }
161
162 /**
163 * Returns the field size in bits which is {@code m}
164 * for this characteristic 2 finite field.
165 * @return the field size in bits.
166 */
167 public int getFieldSize() {
168 return m;
169 }
170
171 /**
172 * Returns the value {@code m} of this characteristic
173 * 2 finite field.
174 * @return {@code m} with 2^{@code m} being the
175 * number of elements.
176 */
177 public int getM() {
178 return m;
179 }
180
181 /**
182 * Returns a BigInteger whose i-th bit corresponds to the
183 * i-th coefficient of the reduction polynomial for polynomial
184 * basis or null for normal basis.
185 * @return a BigInteger whose i-th bit corresponds to the
186 * i-th coefficient of the reduction polynomial for polynomial
187 * basis or null for normal basis.
188 */
189 public BigInteger getReductionPolynomial() {
190 return rp;
191 }
192
193 /**
194 * Returns an integer array which contains the order of the
195 * middle term(s) of the reduction polynomial for polynomial
196 * basis or null for normal basis.
197 * @return an integer array which contains the order of the
198 * middle term(s) of the reduction polynomial for polynomial
199 * basis or null for normal basis. A new array is returned
200 * each time this method is called.
201 */
202 public int[] getMidTermsOfReductionPolynomial() {
203 if (ks == null) {
204 return null;
205 } else {
206 return ks.clone();
207 }
208 }
209
210 /**
211 * Compares this finite field for equality with the
212 * specified object.
213 * @param obj the object to be compared.
214 * @return true if {@code obj} is an instance
215 * of ECFieldF2m and both {@code m} and the reduction
216 * polynomial match, false otherwise.
217 */
218 public boolean equals(Object obj) {
219 if (this == obj) return true;
220 if (obj instanceof ECFieldF2m) {
221 // no need to compare rp here since ks and rp
222 // should be equivalent
223 return ((m == ((ECFieldF2m)obj).m) &&
224 (Arrays.equals(ks, ((ECFieldF2m) obj).ks)));
225 }
226 return false;
227 }
228
229 /**
230 * Returns a hash code value for this characteristic 2
231 * finite field.
232 * @return a hash code value.
233 */
234 public int hashCode() {
235 int value = m << 5;
236 value += (rp==null? 0:rp.hashCode());
237 // no need to involve ks here since ks and rp
238 // should be equivalent.
239 return value;
240 }
241 }