HD wallets: add a Wallet.getKeyChainSeed method.

core/src/main/java/com/google/bitcoin/core/Wallet.java

@@ -573,6 +573,22 @@ public class Wallet extends BaseTaggableObject implements Serializable, BlockCha
         return findKeyFromPubKey(pubkey) != null;
     }
 
+    /**
+     * Returns the immutable seed for the current active HD chain.
+     * @throws com.google.bitcoin.core.ECKey.MissingPrivateKeyException if the seed is unavailable (watching wallet)
+     */
+    public DeterministicSeed getKeyChainSeed() {
+        lock.lock();
+        try {
+            DeterministicSeed seed = keychain.getActiveKeyChain().getSeed();
+            if (seed == null)
+                throw new ECKey.MissingPrivateKeyException();
+            return seed;
+        } finally {
+            lock.unlock();
+        }
+    }
+
     ////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
     //
     // Serialization support

core/src/test/java/com/google/bitcoin/core/WalletTest.java

@@ -98,6 +98,12 @@ public class WalletTest extends TestWithWallet {
         super.tearDown();
     }
 
+    @Test
+    public void getSeedAsWords1() {
+        // Can't verify much here as the wallet is random each time. We could fix the RNG for the unit tests and solve.
+        assertEquals(12, wallet.getKeyChainSeed().toMnemonicCode().size());
+    }
+
     @Test
     public void basicSpending() throws Exception {
         basicSpendingCommon(wallet, myAddress, new ECKey().toAddress(params), false);