b.uint32(h.CRC32)
		if h.isZip64() || h.offset >= uint32max {
			// the file needs a zip64 header. store maxint in both
			// 32 bit size fields (and offset later) to signal that the
			// zip64 extra header should be used.
			b.uint32(uint32max) // compressed size
			b.uint32(uint32max) // uncompressed size

			// append a zip64 extra block to Extra
			var buf [28]byte // 2x uint16 + 3x uint64
			eb := writeBuf(buf[:])