Public Key Encryption

Photo by Micah Williams

The story of this groundbreaking invention is a fascinating one — involving the British intelligence service “British Government Communications Headquarter” (GCHQ).

In 1969, a research scientist named John Ellis made one of the greatest breakthroughs in modern encryption and code breaking while working for GCHQ. His invention was so essential and far fetching to the future of information security, that the GCHG decided to keep it a secret for over 27 years — it was only declassified in 1997.

It’s an absolutely stunning piece of modern computer science history and if you’re enticed to read more about it, just click right here.

As an interesting twist of history, the same concept that John Ellis came up with originally, was replicated at a public institution in 1976 — a team of researchers at MIT and Stanford then had the ability to publish their findings and they were originally credited with the invention. It wasn’t until many years later, that John Ellis got the recognition for his idea.

So what exactly was so important that the British intelligence community decided to keep it a secret?

It’s a marvelous concept known as Public Key Encryption and the idea behind it is truly exceptional. While up to that point the responsibility of encrypting a message always laid on the sender of it, he thought out of the box and reversed the process — including the recipient in encrypting a message in a truly ingenious way.

It works as follows:

We start by taking a random string of numbers (e.g. 3860280357), from now on this will be called our private key) and mathematically derive another string of numbers from it — the resulting new string of numbers is called public key. A very important aspect of this process is, that it’s a so called trap door function, which means that it’s very easy to calculate it into one direction (ergo from private key to public key), but not the other way around (aka it’s almost impossible to derive the private key from the public key), without having some important information (aka the trapdoor).

This very abstract sounding concept enables us to do some marvelous things though, because we can now take a piece of data, use our public key and scramble it up (by using some encryption magic). For everybody out there this new data set is incomprehensible and unreadable. In order to unscramble it and to make it readable, one need to be in possession of the corresponding private key. Whoever has that key, can now use some mathematical magic and unscramble the data. Voila!

You can see this process illustrated in the picture below.

You can think of this abstractly like this — the public address is your bank account and the private key is your secret PIN. The address can be safely broadcasted to the public, while it is indispensable to keep your PIN safe and secret.

This amazing new way of encrypting information made it become an Important mathematical foundation for information and computer security, since it helps to ensure authenticity and integrity of a message when communication over an unreliable channel of communication (e.g. the internet).

“Strong, relatively cheap encryption became ‘democratised’ and enabled more secure communications on a global scale. Encryption went from being a tool of strategic advantage between super-power blocs, but to a key enabler of individual freedom and safety.” - Robert Hannigan, Director of GCHQ

The fact that the mathematical functions used in Public Key Cryptography have the unique characteristic that they are almost irreversible, meaning that they can only easily be calculated into one direction and not the opposing one, enabled something truly revolutionary — the creation of unforgeable digital signatures and digital secrets.