code: public static String encodeWithColons(byte[] in, int off, int len) {
          return inst.encode(in, off, len, ":", 2).toString();
      }
docs: Encodes the given bytes into a base-16 string, inserting colons after
  every 2 characters of output.
  
  @param in
  the bytes to encode.
  @param off
  the start offset of the data within {@code in}.
  @param len
  the number of bytes to encode.
  
  @return The formatted base-16 string.