EllipticCurve


1   /*
2    * @(#)EllipticCurve.java   1.3 03/12/19
3    *
4    * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
5    * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6    */
7   
8   package java.security.spec;
9   
10  import java.math.BigInteger  ;
11  import java.util.Arrays  ;
12  
13  /**
14   * This immutable class holds the necessary values needed to represent 
15   * an elliptic curve.
16   *
17   * @see ECField
18   * @see ECFieldFp
19   * @see ECFieldF2m
20   *
21   * @author Valerie Peng
22   * @version 1.3, 12/19/03
23   *
24   * @since 1.5
25   */
26  public class EllipticCurve {
27      
28      private final ECField   field;
29      private final BigInteger   a;
30      private final BigInteger   b;
31      private final byte[] seed;
32  
33      // Check coefficient c is a valid element in ECField field.
34      private static void checkValidity(ECField   field, BigInteger   c,
35      String   cName) { 
36      // can only perform check if field is ECFieldFp or ECFieldF2m. 
37      if (field instanceof ECFieldFp  ) {
38          BigInteger   p = ((ECFieldFp  )field).getP();
39          if (p.compareTo(c) != 1) {
40          throw new IllegalArgumentException  (cName + " is too large");
41          } else if (c.signum() != 1) {
42          throw new IllegalArgumentException  (cName + " is negative");
43          }
44      } else if (field instanceof ECFieldF2m  ) {
45          int m = ((ECFieldF2m  )field).getM();
46          if (c.bitLength() > m) {
47          throw new IllegalArgumentException  (cName + " is too large");
48          } 
49      }
50      }
51  
52      /**
53       * Creates an elliptic curve with the specified elliptic field
54       * <code>field</code> and the coefficients <code>a</code> and
55       * <code>b</code>.
56       * @param field the finite field that this elliptic curve is over.
57       * @param a the first coefficient of this elliptic curve.
58       * @param b the second coefficient of this elliptic curve.
59       * @exception NullPointerException if <code>field</code>,
60       * <code>a</code>, or <code>b</code> is null.
61       * @exception IllegalArgumentException if <code>a</code>
62       * or <code>b</code> is not null and not in <code>field</code>.
63       */
64      public EllipticCurve(ECField   field, BigInteger   a, 
65               BigInteger   b) {
66      this(field, a, b, null);
67      }    
68  
69      /**
70       * Creates an elliptic curve with the specified elliptic field
71       * <code>field</code>, the coefficients <code>a</code> and
72       * <code>b</code>, and the <code>seed</code> used for curve generation.
73       * @param field the finite field that this elliptic curve is over.
74       * @param a the first coefficient of this elliptic curve.
75       * @param b the second coefficient of this elliptic curve.
76       * @param seed the bytes used during curve generation for later
77       * validation. Contents of this array are copied to protect against
78       * subsequent modification.
79       * @exception NullPointerException if <code>field</code>,
80       * <code>a</code>, or <code>b</code> is null.
81       * @exception IllegalArgumentException if <code>a</code>
82       * or <code>b</code> is not null and not in <code>field</code>.
83       */
84      public EllipticCurve(ECField   field, BigInteger   a, 
85               BigInteger   b, byte[] seed) {
86          if (field == null) {
87              throw new NullPointerException  ("field is null");
88          }
89          if (a == null) {
90          throw new NullPointerException  ("first coefficient is null");
91      }
92          if (b == null) {
93              throw new NullPointerException  ("second coefficient is null");
94          }
95          checkValidity(field, a, "first coefficient");
96      checkValidity(field, b, "second coefficient");
97          this.field = field;
98          this.a = a;
99          this.b = b;
100     if (seed != null) {
101         this.seed = (byte[]) seed.clone();
102     } else {
103         this.seed = null;
104     }
105     }
106  
107     /**
108      * Returns the finite field <code>field</code> that this 
109      * elliptic curve is over.
110      * @return the field <code>field</code> that this curve 
111      * is over.
112      */    
113     public ECField   getField() {
114     return field;
115     }
116 
117     /**
118      * Returns the first coefficient <code>a</code> of the
119      * elliptic curve.
120      * @return the first coefficient <code>a</code>.
121      */
122     public BigInteger   getA() {
123     return a;
124     }
125 
126     /**
127      * Returns the second coefficient <code>b</code> of the
128      * elliptic curve.
129      * @return the second coefficient <code>b</code>.
130      */
131     public BigInteger   getB() {
132     return b;
133     }
134 
135     /**
136      * Returns the seeding bytes <code>seed</code> used 
137      * during curve generation. May be null if not specified.
138      * @return the seeding bytes <code>seed</code>. A new
139      * array is returned each time this method is called.
140      */
141     public byte[] getSeed() {
142     if (seed == null) return null;
143     else return (byte[]) seed.clone();
144     }
145 
146     /**
147      * Compares this elliptic curve for equality with the
148      * specified object. 
149      * @param obj the object to be compared.
150      * @return true if <code>obj</code> is an instance of
151      * EllipticCurve and the field, A, B, and seeding bytes 
152      * match, false otherwise.
153      */
154     public boolean equals(Object   obj) {
155     if (this == obj) return true;
156     if (obj instanceof EllipticCurve  ) {
157         EllipticCurve   curve = (EllipticCurve  ) obj;
158         if ((field.equals(curve.field)) &&
159         (a.equals(curve.a)) &&
160         (b.equals(curve.b)) &&
161         (Arrays.equals(seed, curve.seed))) {
162         return true;
163         } 
164     }
165     return false;
166     }
167 
168     /**
169      * Returns a hash code value for this elliptic curve.
170      * @return a hash code value.
171      */
172     public int hashCode() {
173     return (field.hashCode() << 6 + 
174         (a.hashCode() << 4) +
175         (b.hashCode() << 2) + 
176         (seed==null? 0:seed.length));
177     }
178 }
179
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