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+/* MDGenerator.java -- 
+   Copyright (C) 2001, 2002, 2006  Free Software Foundation, Inc.
+
+This file is a part of GNU Classpath.
+
+GNU Classpath is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or (at
+your option) any later version.
+
+GNU Classpath 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 for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Classpath; if not, write to the Free Software
+Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301
+USA
+
+Linking this library statically or dynamically with other modules is
+making a combined work based on this library.  Thus, the terms and
+conditions of the GNU General Public License cover the whole
+combination.
+
+As a special exception, the copyright holders of this library give you
+permission to link this library with independent modules to produce an
+executable, regardless of the license terms of these independent
+modules, and to copy and distribute the resulting executable under
+terms of your choice, provided that you also meet, for each linked
+independent module, the terms and conditions of the license of that
+module.  An independent module is a module which is not derived from
+or based on this library.  If you modify this library, you may extend
+this exception to your version of the library, but you are not
+obligated to do so.  If you do not wish to do so, delete this
+exception statement from your version.  */
+
+
+package gnu.java.security.prng;
+
+import gnu.java.security.Registry;
+import gnu.java.security.hash.HashFactory;
+import gnu.java.security.hash.IMessageDigest;
+
+import java.util.Map;
+
+/**
+ * <p>A simple pseudo-random number generator that relies on a hash algorithm,
+ * that (a) starts its operation by hashing a <code>seed</code>, and then (b)
+ * continuously re-hashing its output. If no hash algorithm name is specified
+ * in the {@link Map} of attributes used to initialise the instance then the
+ * SHA-160 algorithm is used as the underlying hash function. Also, if no
+ * <code>seed</code> is given, an empty octet sequence is used.</p>
+ */
+public class MDGenerator extends BasePRNG implements Cloneable
+{
+
+  // Constants and variables
+  // -------------------------------------------------------------------------
+
+  /** Property name of underlying hash algorithm for this generator. */
+  public static final String MD_NAME = "gnu.crypto.prng.md.hash.name";
+
+  /** Property name of seed material. */
+  public static final String SEEED = "gnu.crypto.prng.md.seed";
+
+  /** The underlying hash instance. */
+  private IMessageDigest md;
+
+  // Constructor(s)
+  // -------------------------------------------------------------------------
+
+  /** Trivial 0-arguments constructor. */
+  public MDGenerator()
+  {
+    super(Registry.MD_PRNG);
+  }
+
+  // Class methods
+  // -------------------------------------------------------------------------
+
+  // Instance methods
+  // -------------------------------------------------------------------------
+
+  // Implementation of abstract methods in BaseRandom ------------------------
+
+  public void setup(Map attributes)
+  {
+    // find out which hash to use
+    String underlyingMD = (String) attributes.get(MD_NAME);
+    if (underlyingMD == null)
+      {
+        if (md == null)
+          { // happy birthday
+            // ensure we have a reliable implementation of this hash
+            md = HashFactory.getInstance(Registry.SHA160_HASH);
+          }
+        else
+          { // a clone. reset it for reuse
+            md.reset();
+          }
+      }
+    else
+      { // ensure we have a reliable implementation of this hash
+        md = HashFactory.getInstance(underlyingMD);
+      }
+
+    // get the seeed
+    byte[] seed = (byte[]) attributes.get(SEEED);
+    if (seed == null)
+      {
+        seed = new byte[0];
+      }
+
+    md.update(seed, 0, seed.length);
+  }
+
+  public void fillBlock() throws LimitReachedException
+  {
+    IMessageDigest mdc = (IMessageDigest) md.clone();
+    buffer = mdc.digest();
+    md.update(buffer, 0, buffer.length);
+  }
+
+  // Cloneable interface implementation ---------------------------------------
+
+  public Object clone() throws CloneNotSupportedException
+  {
+    MDGenerator result = (MDGenerator) super.clone();
+    if (this.md != null)
+      result.md = (IMessageDigest) this.md.clone();
+
+    return result;
+  }
+}