Source code: java/security/VMSecureRandom.java
1 /* VMSecureRandom.java -- random seed generator.
2 Copyright (C) 2006 Free Software Foundation, Inc.
3
4 This file is a part of GNU Classpath.
5
6 GNU Classpath is free software; you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 2 of the License, or (at
9 your option) any later version.
10
11 GNU Classpath is distributed in the hope that it will be useful, but
12 WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 General Public License for more details.
15
16 You should have received a copy of the GNU General Public License
17 along with GNU Classpath; if not, write to the Free Software
18 Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301
19 USA
20
21 Linking this library statically or dynamically with other modules is
22 making a combined work based on this library. Thus, the terms and
23 conditions of the GNU General Public License cover the whole
24 combination.
25
26 As a special exception, the copyright holders of this library give you
27 permission to link this library with independent modules to produce an
28 executable, regardless of the license terms of these independent
29 modules, and to copy and distribute the resulting executable under
30 terms of your choice, provided that you also meet, for each linked
31 independent module, the terms and conditions of the license of that
32 module. An independent module is a module which is not derived from
33 or based on this library. If you modify this library, you may extend
34 this exception to your version of the library, but you are not
35 obligated to do so. If you do not wish to do so, delete this
36 exception statement from your version. */
37
38
39 package java.security;
40
41 import gnu.classpath.SystemProperties;
42 import gnu.java.security.action.GetSecurityPropertyAction;
43
44 import java.net.URL;
45
46 /**
47 * VM-specific methods for generating real (or almost real) random
48 * seeds. VM implementors should write a version of this class that
49 * reads random bytes from some system source.
50 *
51 * <p>The default implementation of this class runs eight threads that
52 * increment counters in a tight loop, and XORs each counter to
53 * produce one byte of seed data. This is not very efficient, and is
54 * not guaranteed to be random (the thread scheduler is probably
55 * deterministic, after all). If possible, VM implementors should
56 * reimplement this class so it obtains a random seed from a system
57 * facility, such as a system entropy gathering device or hardware
58 * random number generator.
59 */
60 final class VMSecureRandom
61 {
62
63 /**
64 * Generate a random seed. Implementations are free to generate
65 * fewer random bytes than are requested, and leave the remaining
66 * bytes of the destination buffer as zeros. Implementations SHOULD,
67 * however, make a best-effort attempt to satisfy the request.
68 *
69 * @param buffer The destination buffer.
70 * @param offset The offset in the buffer to start putting bytes.
71 * @param length The number of random bytes to generate.
72 */
73 static int generateSeed(byte[] buffer, int offset, int length)
74 {
75 if (length < 0)
76 throw new IllegalArgumentException("length must be nonnegative");
77 if (offset < 0 || offset + length > buffer.length)
78 throw new IndexOutOfBoundsException();
79
80 Spinner[] spinners = new Spinner[8];
81 int n = 0x1;
82 for (int i = 0; i < spinners.length; i++)
83 {
84 spinners[i] = new Spinner((byte) n);
85 Thread t = new Thread(spinners[i]);
86 t.start();
87 n <<= 1;
88 }
89
90 // Wait until at least one spinner has started.
91 while (!(spinners[0].running || spinners[1].running || spinners[2].running
92 || spinners[3].running || spinners[4].running || spinners[5].running
93 || spinners[6].running || spinners[7].running))
94 {
95 Thread.yield();
96 }
97
98 for (int i = offset; i < length; i++)
99 {
100 buffer[i] = (byte) (spinners[0].value ^ spinners[1].value ^ spinners[2].value
101 ^ spinners[3].value ^ spinners[4].value ^ spinners[5].value
102 ^ spinners[6].value ^ spinners[7].value);
103 Thread.yield();
104 }
105
106 for (int i = 0; i < spinners.length; i++)
107 spinners[i].stop();
108
109 return length;
110 }
111
112 static class Spinner implements Runnable
113 {
114 volatile byte value;
115 volatile boolean running;
116
117 Spinner(final byte initial)
118 {
119 value = initial;
120 }
121
122 public void run()
123 {
124 running = true;
125 while (running)
126 value++;
127 }
128
129 private void stop()
130 {
131 running = false;
132 }
133 }
134 }