1 files changed, 111 insertions, 0 deletions
diff --git a/core/src/main/java/org/spongycastle/crypto/prng/ReversedWindowGenerator.java b/core/src/main/java/org/spongycastle/crypto/prng/ReversedWindowGenerator.java
new file mode 100644
index 00000000..0e12d41d
--- /dev/null
+++ b/core/src/main/java/org/spongycastle/crypto/prng/ReversedWindowGenerator.java
@@ -0,0 +1,111 @@
+package org.spongycastle.crypto.prng;
+
+/**
+ * Takes bytes generated by an underling RandomGenerator and reverses the order in
+ * each small window (of configurable size).
+ * <p>
+ * Access to internals is synchronized so a single one of these can be shared.
+ * </p>
+ */
+public class ReversedWindowGenerator
+    implements RandomGenerator
+{
+    private final RandomGenerator generator;
+
+    private byte[] window;
+    private int windowCount;
+
+    public ReversedWindowGenerator(
+        RandomGenerator generator,
+        int             windowSize)
+    {
+        if (generator == null)
+        {
+            throw new IllegalArgumentException("generator cannot be null");
+        }
+        if (windowSize < 2)
+        {
+            throw new IllegalArgumentException("windowSize must be at least 2");
+        }
+
+        this.generator = generator;
+        this.window = new byte[windowSize];
+    }
+
+    /**
+     * Add more seed material to the generator.
+     *
+     * @param seed a byte array to be mixed into the generator's state.
+     */
+    public void addSeedMaterial(
+        byte[] seed)
+    {
+        synchronized (this)
+        {
+            windowCount = 0;
+            generator.addSeedMaterial(seed);
+        }
+    }
+
+    /**
+     * Add more seed material to the generator.
+     *
+     * @param seed a long value to be mixed into the generator's state.
+     */
+    public void addSeedMaterial(
+        long seed)
+    {
+        synchronized (this)
+        {
+            windowCount = 0;
+            generator.addSeedMaterial(seed);
+        }
+    }
+
+    /**
+     * Fill bytes with random values.
+     *
+     * @param bytes byte array to be filled.
+     */
+    public void nextBytes(
+        byte[] bytes)
+    {
+        doNextBytes(bytes, 0, bytes.length);
+    }
+
+    /**
+     * Fill part of bytes with random values.
+     *
+     * @param bytes byte array to be filled.
+     * @param start index to start filling at.
+     * @param len length of segment to fill.
+     */
+    public void nextBytes(
+        byte[]  bytes,
+        int     start,
+        int     len)
+    {
+        doNextBytes(bytes, start, len);
+    }
+
+    private void doNextBytes(
+        byte[]  bytes,
+        int     start,
+        int     len)
+    {
+        synchronized (this)
+        {
+            int done = 0;
+            while (done < len)
+            {
+                if (windowCount < 1)
+                {
+                    generator.nextBytes(window, 0, window.length);
+                    windowCount = window.length;
+                }
+
+                bytes[start + done++] = window[--windowCount];
+            }
+        }
+    }
+}