Cracking Codes

Katie Eagleton, curator of modern money at the British Museum

Codes, security and deciphering

As long as you have a code, somebody is going to want to break it. We caught up with Katie Eagleton, curator of modern money at the British Museum (and a keen code-smith), to help us work through some of the conundrums.

In their simplest form, codes essentially work by having a structured method of scrambling, or encrypting, information. This can be understood by both the sender and the intended receiver of the message because both of them have a key, or cipher, to the code.

The substitution code

One of the simplest, and most common, cryptographic tools is the substitution code. Try this example. The coded message is:

9 12 15 22 5 3 1 11 5

The key to the code is:

In this example, each letter of the alphabet has been given a numerical substitute. Therefore, ‘9 12 15 22 5 3 1 11 5’ means: ‘I love cake.’

The only problem with a simple substitution code like this is that it is very easy to break. Some elements, such as vowels, occur regularly in English: for example, 'e' appears twice in the decrypted message 'I love cake'. By identifying regular occurrences of encrypted characters – for instance, our code features two 5s – code breakers will try to marry up the most common numbers or characters with the most common letters. And if code breakers know the true meaning of just a few letters used in a code, they can start to fill in the gaps – just like the competitors on Codex do.

'It's like working out cryptic crossword clues – a mixture of lateral thinking and crunching through the possibilities,' says Katie. Although codes – and, in many cases, unknown languages – can be a lot more complicated, essentially the same approach is required for all types of decryption.

The dark side

It's not all just fun and games. The ability to encrypt information safely is big business, both commercially and for personal and national security.

While most of us may have dreamed of getting our home PCs to magically talk to our banks and add a few 0s to the balance in our current accounts, the reality is that there is a criminal element of society that often tries to do just that or to move money from other people’s accounts into their own. Encryption plays a big part in protecting our finances and our own identities when we use credit cards, e-mail, websites and text messages.

Layer by layer

The answer to increased security is to add layers of encryption to the code and to advance the technology of the delivery system. Imagine a substitution code that changes the substitute at every occurrence of the original letter. So 'e' could be a 5 the first time it's used, then a 7, then a 19 and so on. The chances of spotting the regular occurrences by examining the scrambled message become ever more remote. The code breaker needs to work out not only the original key, but also the formula of the encryption system. A full explanation of this 'Caesar system' is covered on the Science of Secrecy microsite by the code historian Simon Singh.

Delivery is another matter. 'The developments in quantum cryptography are exciting,' says Katie, 'You can tell if someone has tried to listen in to a message you're receiving by measuring changes in the signal.' Quantum cryptography essentially employs the mind-boggling complexity of quantum physics to transmit encrypted messages securely using light. Just as you can tell if somebody has tampered with an envelope, you can detect if somebody has tried to crack your quantum code.

The ancients keep their secrets

If computers can do the really tough sums, and the new technology can transmit data in a way that nobody can intercept without detection, surely we now have the power to decipher any ancient code?

'Not exactly,' says Katie, 'Modern cryptography mostly uses completely different encryption techniques to ancient codes. No matter how powerful they are, computers aren't as good as people at making the kind of historically and culturally informed decisions about the content of a centuries-old encrypted message, possibly in an unknown language.'

For more from Katie Eagleton, see Becoming a curator.

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