Support for importing/exporting private keys in sipa format. Resolves issue 48.

src/com/google/bitcoin/core/DumpedPrivateKey.java

@@ -23,16 +23,12 @@ import java.math.BigInteger;
  * bytes with a header byte and 4 checksum bytes at the end.
  */
 public class DumpedPrivateKey extends VersionedChecksummedBytes {
-    /**
-     * Allows the output of a private key in versioned, checksummed form.
-     *
-     * @param params The network parameters of this key, needed for the version byte.
-     * @param keyBytes The 256-bit private key.
-     */
-    public DumpedPrivateKey(NetworkParameters params, byte[] keyBytes) {
+    // Used by ECKey.getPrivateKeyEncoded()
+    DumpedPrivateKey(NetworkParameters params, byte[] keyBytes) {
         super(params.dumpedPrivateKeyHeader, keyBytes);
         if (keyBytes.length != 32)  // 256 bit keys
-            throw new RuntimeException("Keys are 256 bits, so you must provide 32 bytes.");
+            throw new RuntimeException("Keys are 256 bits, so you must provide 32 bytes, got " +
+                    keyBytes.length + " bytes");
     }
 
     /**

src/com/google/bitcoin/core/ECKey.java

@@ -236,4 +236,24 @@ public class ECKey implements Serializable {
             throw new RuntimeException(e);  // Cannot happen, reading from memory stream.
         }
     }
+
+    /** Returns a 32 byte array containing the private key. */
+    public byte[] getPrivKeyBytes() {
+        // Getting the bytes out of a BigInteger gives us an extra null byte on the end (for signedness), so snip
+        // it off here.
+        byte[] bytes = new byte[32];
+        System.arraycopy(priv.toByteArray(), 1, bytes, 0, 32);
+        return bytes;
+    }
+
+    /**
+     * Exports the private key in the form used by the Satoshi client "dumpprivkey" and "importprivkey" commands. Use
+     * the {@link com.google.bitcoin.core.DumpedPrivateKey#toString()} method to get the string.
+     *
+     * @param params The network this key is intended for use on.
+     * @return Private key bytes as a {@link DumpedPrivateKey}.
+     */
+    public DumpedPrivateKey getPrivateKeyEncoded(NetworkParameters params) {
+        return new DumpedPrivateKey(params, getPrivKeyBytes());
+    }
 }

src/com/google/bitcoin/core/VersionedChecksummedBytes.java

@@ -45,13 +45,13 @@ public class VersionedChecksummedBytes {
 
     @Override
     public String toString() {
-        // A stringified address is:
-        //   1 byte version + 20 bytes hash + 4 bytes check code (itself a truncated hash)
-        byte[] addressBytes = new byte[1 + 20 + 4];
+        // A stringified buffer is:
+        //   1 byte version + data bytes + 4 bytes check code (a truncated hash)
+        byte[] addressBytes = new byte[1 + bytes.length + 4];
         addressBytes[0] = (byte)version;
-        System.arraycopy(bytes, 0, addressBytes, 1, 20);
-        byte[] check = Utils.doubleDigest(addressBytes, 0, 21);
-        System.arraycopy(check, 0, addressBytes, 21, 4);
+        System.arraycopy(bytes, 0, addressBytes, 1, bytes.length);
+        byte[] check = Utils.doubleDigest(addressBytes, 0, bytes.length + 1);
+        System.arraycopy(check, 0, addressBytes, bytes.length + 1, 4);
         return Base58.encode(addressBytes);
     }
 

tests/com/google/bitcoin/core/ECKeyTest.java

@@ -20,8 +20,10 @@ import org.bouncycastle.util.encoders.Hex;
 import org.junit.Test;
 
 import java.math.BigInteger;
+import java.security.PrivateKey;
 
 import static com.google.bitcoin.core.Utils.*;
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
 
 public class ECKeyTest {
@@ -68,4 +70,13 @@ public class ECKeyTest {
         assertTrue(roundtripKey.verify(message, decodedKey.sign(message)));
         assertTrue(decodedKey.verify(message, roundtripKey.sign(message)));
     }
+
+    @Test
+    public void base58Encoding() throws Exception {
+        String addr = "mqAJmaxMcG5pPHHc3H3NtyXzY7kGbJLuMF";
+        String privkey = "92shANodC6Y4evT5kFzjNFQAdjqTtHAnDTLzqBBq4BbKUPyx6CD";
+        ECKey key = new DumpedPrivateKey(NetworkParameters.testNet(), privkey).getKey();
+        assertEquals(privkey, key.getPrivateKeyEncoded(NetworkParameters.testNet()).toString());
+        assertEquals(addr, key.toAddress(NetworkParameters.testNet()).toString());
+    }
 }