"[T]hey devised a code that was undecipherable both then and now"

It is thought a “one-time pad” may have been used to encrypt the message.

“The advantage of this system is that, if used correctly, it is unbreakable as long as the key is kept secret. The disadvantage is that both the sending and receiving parties need to have access to the same key, which usually means producing and sharing a large keypad in advance,” GCHQ said.