The From the home of the co-breakers and the birthplace of modern computing, this is the Bletchley Park podcast.

Hello, this is M.C. Fontaine, welcoming you to another Bletchley Park podcast Extra.

This week again, we return to the Turing Education Day

for a talk from Whitfield Diffey on the historical turning point in cryptography.

When I originally was expecting to come here, I planned to speak on some dry points of the history of cryptomathematics. But this really isn't the best environment for that, so I decided to broaden the talk a little bit. I like to talk

about what I call turning points in cryptography. I'm going to discuss more than one, but I'm going

to focus in on some things around World War II, particularly having to do with Bletchley Park,

but not exclusively. But to set the stage for that, the turning point in cryptography,

although it occurs in two places and 700 years apart,

there is one distinct sort of set of discoveries,

first by Al-Kindi and presumably colleagues in Baghdad in the 9th century

AD, second century A.H. And then again, in Rome, by Alberti and Batista and some other people,

Visionaire, who's sort of the best known, he was the VP of Marketing, I think. Basically, before this,

you had what we now, roughly speaking, call code books. And there comes in two forms. I mean,

the small form, you have a cipher alphabet, and then because it's tedious to write everything

out in full length, you have a set of abbreviations called a nomenclator.

And it's tossable for that the nomenclator to prolapse up into a 100,000 main entry dictionary,

but you still don't have all the words in the world, and so you add what are called spelling groups.

And there's a certain body of cryptanalysis associated

with attacking these things. What was discovered, and I'll focus on the European path, because I know it

better, is what we now call polyalphabetic encryption and all of the systems they worked against.

No, not all the ones they worked against, but all the ones we were hearing about

...