1 /* VMSecureRandom.java -- random seed generator. 2 Copyright (C) 2006, 2010 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 gnu.java.security.jce.prng; 40 41 /** 42 * VM-specific methods for generating real (or almost real) random 43 * seeds. VM implementors should write a version of this class that 44 * reads random bytes from some system source. 45 * 46 * <p>The default implementation of this class runs eight threads that 47 * increment counters in a tight loop, and XORs each counter to 48 * produce one byte of seed data. This is not very efficient, and is 49 * not guaranteed to be random (the thread scheduler is probably 50 * deterministic, after all). If possible, VM implementors should 51 * reimplement this class so it obtains a random seed from a system 52 * facility, such as a system entropy gathering device or hardware 53 * random number generator. 54 */ 55 final class VMSecureRandom 56 { 57 VMSecureRandom()58 private VMSecureRandom() {} // Prohibits instantiation. 59 60 /** 61 * Generate a random seed. Implementations are free to generate 62 * fewer random bytes than are requested, and leave the remaining 63 * bytes of the destination buffer as zeros. Implementations SHOULD, 64 * however, make a best-effort attempt to satisfy the request. 65 * 66 * @param buffer The destination buffer. 67 * @param offset The offset in the buffer to start putting bytes. 68 * @param length The number of random bytes to generate. 69 */ generateSeed(byte[] buffer, int offset, int length)70 static int generateSeed(byte[] buffer, int offset, int length) 71 { 72 if (length < 0) 73 throw new IllegalArgumentException("length must be nonnegative"); 74 if (offset < 0 || offset + length > buffer.length) 75 throw new IndexOutOfBoundsException(); 76 77 Spinner[] spinners = new Spinner[8]; 78 int n = 0x1; 79 for (int i = 0; i < spinners.length; i++) 80 { 81 spinners[i] = new Spinner((byte) n); 82 Thread t = new Thread(spinners[i]); 83 t.start(); 84 n <<= 1; 85 } 86 87 // Wait until at least one spinner has started. 88 while (!(spinners[0].running || spinners[1].running || spinners[2].running 89 || spinners[3].running || spinners[4].running || spinners[5].running 90 || spinners[6].running || spinners[7].running)) 91 { 92 Thread.yield(); 93 } 94 95 for (int i = offset; i < length; i++) 96 { 97 buffer[i] = (byte) (spinners[0].value ^ spinners[1].value ^ spinners[2].value 98 ^ spinners[3].value ^ spinners[4].value ^ spinners[5].value 99 ^ spinners[6].value ^ spinners[7].value); 100 Thread.yield(); 101 } 102 103 for (int i = 0; i < spinners.length; i++) 104 spinners[i].stop(); 105 106 return length; 107 } 108 109 static class Spinner 110 implements Runnable 111 { 112 volatile byte value; 113 volatile boolean running; 114 Spinner(final byte initial)115 Spinner(final byte initial) 116 { 117 value = initial; 118 } 119 run()120 public void run() 121 { 122 running = true; 123 while (running) 124 value++; 125 } 126 stop()127 void stop() 128 { 129 running = false; 130 } 131 } 132 } 133