One time pads: one more thing
"Conversely, the poor cipher used to try and hide the content of messages could have been rendered totally uncrackable, even at the time, by one-time pads, which just need dice, paper and pencil."
They need one more thing: a completely safe communication channel, even if it's later broken. If I make a set of pads and send them to you, and someone copies them in between, then our communications are wide open again. Unless I can hand them to you and both of us never have our pads copied, that system is not uncrackable. Cryptography isn't only dependent on the calculations. It is also, in fact it is more dependent on the pathways the data uses and the trustworthiness of anyone in the position to copy or read the communications. In the modern day, although one-time pads are used, it's usually only one of multiple algorithms used on the same piece of data.