December 7, 2021

I, Science

The science magazine of Imperial College

From morse code to quantum cryptography, Hilary Lamb explores a history of ciphers and codebreakers.

Cryptography, code breakers, and ciphers, oh my!

enigma machine looking like a typewriter
The Enigma Machine

The Kama Sutra instructs women who want to send naughty messages to use Mulavediya. This is an example of an early monoalphabetic cipher; each letter substituted for another. Aside from Pig Latin, perhaps, these ciphers offer the weakest concealment imaginable. They can be broken with frequency analysis, in which the codebreaker compares the frequency of letters in the message to the frequency of letters in the writer’s language, and matches letters. This takes a pen, paper and a few minutes of patience.

Polyalphabetic ciphers, in which each letter is substituted for multiple others, are far more resistant to attack by frequency analysis. The Vigenère cipher survived three centuries before being broken by Charles Babbage, who identified repeated groups of letters as the chink in its armour.

Twentieth Century military communications were encoded with polyalphabetic ciphers and transmitted by radio, making them open to interception. In 1918, French codebreakers unravelled the German ADFGVX cipher, providing enough intelligence to halt the Spring Offensive, and demonstrating the significance of cryptography in warfare.

World leaders wasted no time amassing armies of codebreakers at the outbreak of the Second World War. The development of the electromechanical Bombe by Polish and British codebreakers to combat Enigma, and the invention of Colossus (the first programmable, electronic, digital computer) to defeat the Lorentz Ciphers proved that codebreakers could deliver technological as well as military triumphs. Computers proved to be invaluable as tools for increasingly complicated encryption and as blunt instruments for decryption.

Today, it is no longer just wartime leaders who rely on cryptography; we all need to stay protected against interception online. Until recently, all our secure information was encoded using secret key systems, in which a single key (a set of instructions) is used to encrypt and decrypt. This key must be delivered securely from sender to recipient. Historical methods for exchanging secret information have varied from tattooing messages on slaves’ scalps to knitting Morse code into jumpers, but public key exchanges are far more complex, with millions transferring secure information every second.

The complication of secret key delivery, combined with the vulnerability of these systems to brute force attacks mean that a more refined technique is necessary for protecting our most sensitive information. Public key systems use a pair of keys: the public and the private. If a politician wants to send a suggestive message to his PA, he double-encrypts it using his private key and the PA’s public key. The PA decrypts it with her private key and the politician’s public key. The public keys can therefore be revealed and reused without compromising security. Public key systems work using one-way functions, which take minimal effort to carry out (e.g. multiplying primes), but a lot of effort to reverse (e.g. factorising); a tough defence against brute force attacks. Quantum computers, which use quantum mechanical principles to perform operations, could render every cryptographic system defunct, due to their enormous theoretical computational power. The only defence against a quantum brute force attack is similarly speculative: quantum cryptography.

quote blockImagine that our politician takes a photograph and encrypts the data as a series of polarised photons, which he delivers to his PA. She attempts to measure the polarisations, and the politician tells her which were correct. They compile the correct measurements into a one-time pad (which contains unique data to encode their message). Before sending the photograph, they check that their pads match. If not, somebody has tried to intercept their messages. The mere act of observation changes the state of a photon, so any attempt at eavesdropping is detected.

While scientists are yet to send quantum-encrypted messages over useful distances, quantum cryptography holds the promise of absolute secrecy, making interception impossible. The history of cryptography is a mathematical and technological arms race between those who want to conceal messages and those who want to read them. Quantum cryptography would signal the final defeat for codebreakers.

Hilary Lamb is studying for an MSc in Science Communication

Images: Combination Lock, shutterstock; The Enigma Machine, Bundesarchiv