1 files changed, 80 insertions, 0 deletions
diff --git a/core/src/main/java/org/spongycastle/crypto/digests/ShortenedDigest.java b/core/src/main/java/org/spongycastle/crypto/digests/ShortenedDigest.java
new file mode 100644
index 00000000..274959f5
--- /dev/null
+++ b/core/src/main/java/org/spongycastle/crypto/digests/ShortenedDigest.java
@@ -0,0 +1,80 @@
+package org.spongycastle.crypto.digests;
+
+import org.spongycastle.crypto.ExtendedDigest;
+
+/**
+ * Wrapper class that reduces the output length of a particular digest to
+ * only the first n bytes of the digest function.
+ */
+public class ShortenedDigest 
+    implements ExtendedDigest
+{
+    private ExtendedDigest baseDigest;
+    private int            length;
+    
+    /**
+     * Base constructor.
+     * 
+     * @param baseDigest underlying digest to use.
+     * @param length length in bytes of the output of doFinal.
+     * @exception IllegalArgumentException if baseDigest is null, or length is greater than baseDigest.getDigestSize().
+     */
+    public ShortenedDigest(
+        ExtendedDigest baseDigest,
+        int            length)
+    {
+        if (baseDigest == null)
+        {
+            throw new IllegalArgumentException("baseDigest must not be null");
+        }
+        
+        if (length > baseDigest.getDigestSize())
+        {
+            throw new IllegalArgumentException("baseDigest output not large enough to support length");
+        }
+        
+        this.baseDigest = baseDigest;
+        this.length = length;
+    }
+    
+    public String getAlgorithmName()
+    {
+        return baseDigest.getAlgorithmName() + "(" + length * 8 + ")";
+    }
+
+    public int getDigestSize()
+    {
+        return length;
+    }
+
+    public void update(byte in)
+    {
+        baseDigest.update(in);
+    }
+
+    public void update(byte[] in, int inOff, int len)
+    {
+        baseDigest.update(in, inOff, len);
+    }
+
+    public int doFinal(byte[] out, int outOff)
+    {
+        byte[] tmp = new byte[baseDigest.getDigestSize()];
+        
+        baseDigest.doFinal(tmp, 0);
+        
+        System.arraycopy(tmp, 0, out, outOff, length);
+        
+        return length;
+    }
+
+    public void reset()
+    {
+        baseDigest.reset();
+    }
+
+    public int getByteLength()
+    {
+        return baseDigest.getByteLength();
+    }
+}