I was in Class VI when I read Artemis Fowl, the first book of the series by Eoin Colfer. In it there is a secret code, a set of symbols — a human eye, an arrow, a hexagon with intersecting diagonals — underneath which the corresponding message in English is inscribed. I remember spending a free period correlating each symbol with its corresponding English alphabet so that I could discover the key to the code. It was only in college that I learnt this sort of code is called a substitution cipher, one of the two broad classes of classical cryptography. The other is the transposition cipher. In Dan Brown’s The Da Vinci Code, “O Draconian Devil! Oh lame saint!” is decoded to “Leonardo da Vinci, The Mona Lisa”. This is an application of anagrams, an elementary type of transposition cipher.

Cryptography, however, is not only about encrypting plain text into a secret message. It can also be used to ensure secure transmission of a message. Only permitted people can access it and no malicious third party can modify it. This highlights the three principles of information security — confidentiality, integrity and authenticity, or the CIA triad. Therefore, you can understand its importance in today’s networked world.

All kinds of software — from network security to image processing, online banking to college websites — use cryptography. When you download a new app on your phone, you are asked to verify your mobile number, which is done by sending across an OTP or one-time password. This too employs a principle of cryptography.

One of the biggest applications of cryptography in the real world is in financial transactions over the Internet. The transfer of money is done over a series of steps involving encryptions using public keys and private keys. The idea behind multiple levels of encryption is to diminish attacks from malicious parties, especially via a “brute force attack”, during which the attacker aims to guess ID and password, or forge digital signatures, through numerous consequent attempts. A lot of research has gone into building these systems that are “fraud-resistant”.

While talking cryptography and finance, the next logical topic is cryptocurrency — most popular of which is bitcoins. Krishanu Ranwan, who dabbled in cryptocurrency while studying at the University of Toronto in Canada, explains the founding principle behind it is blockchain technology. “The system is decentralised and distributed,” he says, “which means that people can send and receive money without a third party, like a bank, involved in between.” Krishanu has compiled a blockchain course for the online learning platform, Udemy.

Another facet of crypto-graphy deals with hiding messages rather than encrypting them. This is called steganography and has been in use since ancient times when secret messages were written on a person’s scalp. On reaching the desired recipient, the agent’s hair was shaved off to reveal the message underneath.

These days steganography is widely used in image processing. A machine interprets every shade of each colour using a series of binary codes in the hexadecimal number system. Each such code is said to be a bit pattern of that particular shade. An image is composed of pixels. Using steganography, we can hide an image within another by manipulating the bit patterns of the pixels.

As we can see, a cryptographic algorithm can work on anything — from bit codes on a computer to electrical pulses on the Enigma machine, which was developed by German engineer Arthur Scherbius and used extensively by the Nazi government during World War II to pass coded messages to troops. It is composed of rotors and varying electrical pathways. The rotors are composed of alphabet tyres — similar to the number tyres in suitcase combination locks — whose different positions, facilitated by rotating motions, created substitution ciphers. Polish cryptographer Marian Rejewski decrypted the Enigma machine by deducing the internal wiring and the process was further refined by British mathmatician Alan Turing.

The concept of encoding messages during war — using the Morse code, for example — has been one of the most pressing needs of cryptography, both in the past and in this age. I remember one of the first examples our professor used, to highlight something called an autokey cipher, was the message: “We attack at dawn”.

So, what makes a good cryptographer? A rudimentary skill expected in all budding cryptographers is mathematical ability. Cryptographic systems are built on strong mathematical concepts, coupled with knowledge of computer science. Many universities abroad also encourage students aiming for a degree in cryptography to take up language courses, so as to get them acquainted with different linguistic formats. English, for example, has an alphabet-based system, while Bengali has a syllable-based system. Cryptographic algorithms to execute operations in these two languages would need to be different.

If you find the subject interesting, a course you should look out for is the summer internship in cryptology organised by the R.C. Bose Centre for Cryptology and Security at the Indian Statistical Institute (ISI) in Calcutta. It is one of the most sought-after internships in the field, employing a screening process. Applicants must solve different problems in cryptology — from breaking codes to proving mathematical theorems.

Cryptologist Bimal Roy, who is also a former director of ISI, says, “The topics of more relevance today include cloud security, data integrity including blockchain, privacy preserving computation and data masking.” According to him, the need of the hour is quantum as well as post-quantum cryptology. Roy introduced the MTech programme in cryptology at ISI that covers the areas he mentions above.

With the explosion of data in the world, greater measures are being taken to effectively encrypt it. There is, however, still a long way to go for those of us trying to understand the extent to which cryptography can preserve the privacy of information.