The word cryptography originates from two Greek words: kryptos (secret) and graphein (writing). Thus cryptography is the art of communicating a message in secret manner or in other words, it is the art of rendering a message unintelligible to any unauthorized party. Its history is very old and exciting. From the beginning of human civilization there was a need to hide information from unauthorized people. This need gave birth to cryptography. Initial cryptographic protocols were very simple. With time they have become much more sophisticated and secure. However, all the classical cryptographic protocols developed so far are secure under some assumptions. In contrast, the protocols of quantum cryptography are unconditionally secure. This particular aspect of quantum cryptography makes it very interesting. This chapter aims to provide a flavor of the intrinsic beauty of various aspects of secure quantum communication. Before we start a journey into the fascinating world of secure quantum communication, it would be apt to become familiar with the jargon used in this field and also with some interesting protocols of classical cryptography. Let us first do that.
The word cryptography originates from two Greek words: kryptos (secret) and graphein (writing). Thus cryptography is the art of communicating a message in secret manner or in other words, it is the art of rendering a message unintelligible to any unauthorized party. Its history is very old and exciting. From the beginning of human civilization there was a need to hide information from unauthorized people. This need gave birth to cryptography. Initial cryptographic protocols were very simple. With time they have become much more sophisticated and secure. However, all the classical cryptographic protocols developed so far are secure under some assumptions. In contrast, the protocols of quantum cryptography are unconditionally secure. This particular aspect of quantum cryptography makes it very interesting. This chapter aims to provide a flavor of the intrinsic beauty of various aspects of secure quantum communication. Before we start a journey into the fascinating world of secure quantum communication, it would be apt to become familiar with the jargon used in this field and also with some interesting protocols of classical cryptography. Let us first do that.
Quantum cryptography aims at solving the everlasting problem of unconditional security in private communication. Every time we send personal information over a telecom channel a sophisticate algorithm protect our privacy making our data unintelligible to unauthorized receivers. These protocols resulted from the long history of cryptography. The security of modern cryptographic systems is guaranteed by complexity: the computational power that would be needed for gaining info on the code key largely exceed available one. Security of actual crypto systems is not “by principle” but “practical”. On the contrary, quantum technology promises to make possible to realize provably secure protocols. Quantum cryptology exploits paradigmatic aspects of quantum mechanics, like superposition principle and uncertainty relations. In this contribution, after a brief historical introduction, we aim at giving a survey on the physical principles underlying the quantum approach to cryptography. Then, we analyze a possible continuous variable protocol.
Quantum cryptography uses physics to develop a cryptosystem completely secure against being compromised, without the sender or the receiver of messages being known. The word quantum itself refers to the most fundamental behavior of the smallest particles of matter and energy. Quantum cryptography is different from traditional cryptographic systems in that it relies more on physics, rather than mathematics, as a key aspect of its security model. Essentially, quantum cryptography is based on the usage of individual particles/waves of light (photon) and their intrinsic quantum properties to develop an unbreakable cryptosystem (because it is impossible to measure the quantum state of any system without disturbing that system). Regular, non-quantum encryption can work in a variety of ways but generally a message is scrambled and can only be unscrambled using a secret key. The trick is to make sure that whomever you’re trying to hide your communication from doesn’t get their hands on your secret key.
We dance round in a ring and suppose, But the Secret sits in the middle and knows. Robert Frost, The Secret Sits (1942) In this chapter we explore the merging of quantum computation and classical cryptography. This is a new and exciting field of pure and applied research known as quantum cryptography . We begin with the basics of classical cryptography in Section 9.1. Section 9.2 demonstrates a quantum cryptographic protocol that uses two different bases. We improve on this in Section 9.3, where a protocol with one basis is employed. Section 9.4 shows how to use entanglement to secretly send a message. We conclude with Section 9.5, in which teleportation is demonstrated. CLASSICAL CRYPTOGRAPHY Before delving into quantum cryptography, we need to familiarize ourselves with the core ideas of classical cryptography. A good place to start is the following definition. Definition 9.1.1 Cryptography is the art of concealing messages . Indeed, this is precisely what the etymology reveals: “Cryptography” is a compound of two Greek words, crypton and graphein , which mean, respectively, hidden and writing. Turning an ordinary message into an indecipherable one is called encryption . The opposite action, i.e., restoring the original message, is decryption . The original message is generally referred to as the plaintext , and the encrypted message is the ciphertext . A method for encryption is often referred to as an encryption protocol .
Users of communication channels wish to have their transactions secured. Therefore, it is necessary to develop techniques which can ensure that eavesdroppers cannot intercept the messages transmitted. The technology concerned with the secure communication of data is termed as cryptography [1-3]. It is the art of hiding information in the form of a string of bits to an unauthorized party. To achieve this goal, in a classical cryptography system the message bits are combined with random bits of equal length called a key. The combined data is known as cryptogram. This technique is called encryption. A key can be public or private. The public key is to encode plaintext, while the private key is for decoding the previously encoded plaintext or cryptogram. The cryptogram is generally sent through the communication channel. Because of the randomness of the key the encoded message is completely random. Consequently, the message is untraceable to an eavesdropper. The safety of the transmission completely depends on the safety of the key. Therefore, the key should be kept secret by the users.
The practical implementation of quantum information technologies requires, for the most part, highly advanced and currently experimental procedures. One exception is quantum cryptography, or quantum key distribution, which has been successfully demonstrated in many laboratories and has reached an advanced level of development. It will probably become the first commercial application of quantum information. In quantum key distribution, Alice and Bob exploit a quantum channel to create a secret shared key comprising a random string of binary digits. This key can then be used to protect a subsequent communication between them. The principal idea is that the secrecy of the key distribution is ensured by the laws of quantum physics. Proving security for practical communication systems is a challenging problem and requires techniques that are beyond the scope of this book. At a fundamental level, however, the ideas are simple and may readily be understood with the knowledge we have already acquired. Quantum cryptography is the latest idea in the long history of secure (and not so secure) communications and, if it is to develop, it will have to compete with existing technologies. For this reason we begin with a brief survey of the history and current state of the art in secure communications before turning to the possibilities offered by quantum communications. The history of cryptography is a long and fascinating one. As a consequence of the success or, more spectacularly, the failure of ciphers, wars have been fought, battles decided, kingdoms won, and heads lost. In the information age, ciphers and cryptosystems have become part of everyday life; we use them to protect our computers, to shop over the Internet, and to access our money via an ATM (automated teller machine). One of the oldest and simplest of all ciphers is the transposition or Caesarean cipher (attributed to Julius Caesar), in which the letters are shifted by a known (and secret) number of places in the alphabet. If the shift is 1, for example, then A is enciphered as B, B→C, · · ·, Y→Z, Z→A. A shift of five places leads us to make the replacements A→F, B→G, · · ·, Y→D, Z→E.
Classical cryptography is the process of hiding information and it manages the secret knowledge by encrypting the plain text message through the translation of it to an unintelligible message. Quantum cryptography is also hiding the plain text through encryption and it works based on the law of quantum physics for providing absolute security of data communication. It uses the idea of quantum mechanics to gadget a cryptographic system and the key problems are solved by intrusion detection including the Eavesdropping detection using quantum expertise. On the contrary, principles of secured communication protocol systems are demonstrated with classical quantum cryptography where the keys are distributed securely and are applied in quantum key distribution as well. In this paper, we provide a survey of works on classical cryptography and quantum cryptography and compare them with respect to time, security level and the classification of the data. Moreover, we perform a concise analysis of perspective classical cryptography and the conception of Quantum cryptography with various protocols and highlight the benefits of classical cryptography and Quantum cryptography in different applications. Finally, we provide a set of recommendations for selecting the suitable encryption model for securing the communication.