code: public static String toBase58WithChecksum(byte[] b) {
          byte[] cs = Hash.hash(b);
          byte[] extended = new byte[b.length + 4];
          System.arraycopy(b, 0, extended, 0, b.length);
          System.arraycopy(cs, 0, extended, b.length, 4);
          return toBase58(extended);
      }
docs: Encode in base58 with an added checksum of four bytes.
  
  @param b
  byte[] data
  @return toBase58WithChecksum