Actually added digest implementation (and minor change to interface)

src/main/java/meerkat/crypto/Digest.java

@@ -1,7 +1,47 @@
 package meerkat.crypto;
 
+import com.google.protobuf.Message;
+
+import java.security.MessageDigest;
+
 /**
  * Created by talm on 11/9/15.
  */
 public interface Digest {
+
+    /**
+     * Marker between messages
+     */
+    public static final byte[] CONCAT_MARKER = {(byte) 0xde, (byte) 0xad, (byte) 0xbe, (byte) 0xef,
+            (byte) 0xba, (byte) 0x1d, (byte) 0xfa, (byte) 0xce};
+
+    /**
+     * Completes the hash computation by performing final operations such as padding.
+     * (copied from {@link MessageDigest#digest()})
+     * @return
+     */
+    byte[] digest();
+
+    /**
+     * Updates the digest using the specified message (in serialized wire form)
+     *
+     * Includes a special message concatenation marker (the 64 bit message {@link #CONCAT_MARKER}) in the digest (digesting a single message
+     * will give a different result than the same message split into two messages).
+     * Messages must not contain the {@link #CONCAT_MARKER}) marker.
+     * @param msg
+     * @return
+     */
+    void update(Message msg);
+
+    /**
+     * Resets the digest for further use.
+     */
+    void reset();
+
+    /**
+     * Clone the current digest state
+     * @return
+     */
+    public Digest clone() throws CloneNotSupportedException;
+
 }

src/main/java/meerkat/crypto/concrete/SHA256Digest.java

@@ -1,7 +1,72 @@
 package meerkat.crypto.concrete;
 
+import com.google.protobuf.Message;
+import meerkat.crypto.Digest;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+
 /**
  * Created by talm on 11/9/15.
  */
-public class SHA256Digest {
+public class SHA256Digest implements Digest {
+    final Logger logger = LoggerFactory.getLogger(getClass());
+    public static final String SHA256 = "SHA-256";
+
+    MessageDigest hash;
+
+    /**
+     * Instantiate with a specified algorithm.
+     * @param algorithm
+     * @throws NoSuchAlgorithmException
+     */
+    public SHA256Digest(String algorithm) throws NoSuchAlgorithmException {
+        hash = MessageDigest.getInstance(algorithm);
+    }
+
+    /**
+     * Instantiate with the default (SHA-256) algorithm
+     */
+    public SHA256Digest() { this(true); }
+
+
+    /**
+     * Instantiate with the default (SHA-256) algorithm,
+     * or create an empty class (for cloning)
+     */
+    private SHA256Digest(boolean initHash) {
+        if (initHash) {
+            try {
+                hash = MessageDigest.getInstance(SHA256);
+            } catch (NoSuchAlgorithmException e) {
+                // Should never happen!
+                logger.error("Couldn't find default {} algorhtm: {}", SHA256, e);
+                assert false;
+            }
+        }
+    }
+
+    @Override
+    public byte[] digest() {
+        return hash.digest();
+    }
+
+    @Override
+    public void update(Message msg) {
+        hash.update(msg.toByteString().asReadOnlyByteBuffer());
+    }
+
+    @Override
+    public void reset() {
+        hash.reset();
+    }
+
+    @Override
+    public SHA256Digest clone() throws CloneNotSupportedException {
+        SHA256Digest copy = new SHA256Digest(false);
+        copy.hash = (MessageDigest) hash.clone();
+        return copy;
+    }
 }