Cryptography is the practice and study of techniques for secure communication in the presence of third parties (called adversaries). More generally, it is about constructing and analyzing protocols that overcome the influence of adversaries and which are related to various aspects in information security.
Phil Zimmermann asserts "Cryptography used to be an obscure science, of little relevance to everyday life. Historically, it always had a special role in military and diplomatic communications. But in the Information Age, cryptography is about political power, and in particular, about the power relationship between a government and its people. It is about the right to privacy, freedom of speech, freedom of political association, freedom of the press, freedom from unreasonable search and seizure, freedom to be left alone."
A cryptographic algorithm, or cipher, is a mathematical function used in the encryption and decryption process. A cryptographic algorithm works in combination with a key — a word, number, or phrase — to encrypt the plaintext. The same plaintext encrypts to different ciphertext with different keys, and the security of encrypted data is entirely dependent on two things: the strength of the cryptographic algorithm and the secrecy of the key
When you substitute one word for another word or sentence, like using a foreign language dictionary, you are using a code. When you mix up or substitute existing letters, you are using a cipher. For a cipher to be useful, several things must be known at both the sending and receiving ends: the algorithm or method used to encipher the original message (known as the plaintext); the key used with the algorithm to allow the plaintext to be both enciphered and deciphered; the period or time during which the key is valid.
The development of cheap digital hardware has freed it from the design limitations of mechanical computing and brought the cost of high grade cryptographic devices down to where they can be used in such commercial applications as remote cash dispensers and computer terminals. In turn, such applications create a need for new types of cryptographic systems which minimize the necessity of secure key distribution channels and supply the equivalent of a written signature. At the same time, theoretical developments in information theory and computer science show promise of providing provably secure cryptosystems, changing this ancient art into a science.
Visual Cryptography is a special encryption technique, proposed by Naor and Shamir in 1994, to hide information in images in such a way that it can be decrypted by the human vision if the correct key image is used. Visual Cryptography uses two transparent images. One image contains random pixels and the other image contains the secret information. It is impossible to retrieve the secret information from one of the images. Both transparent images or layers are required to reveal the information.
Today, cryptography has moved beyond the realm of dilettantes and soldiers to become a sophisticated scientific art—combining mathematics, physics, computer science, and electrical engineering. From email to banking transactions, modern cryptography is used everywhere. Cryptography has a far-reaching influence throughout history (from Julius Caesar’s reign to Julian Assange’s WikiLeaks), and it—and our privacy—are constantly under assault today as threats lurk behind IP addresses, computational power increases, and our secrets move online.
Presently, cryptographic methods and tools are used not only by the state, but also in the private or corporate life, and classified information is not necessarily the main object of protection. The amount of information presented in the digital form and spread all over the world is now very large, and this information requires protection against nonfriendly intrusion, collection, substitution, falsification, and so on. Cryptography provides the most efficient tools for defending against these threats.
Prior to the 1970s, cryptography was a black art, understood and practised by only a few government and military personnel. It is now a well-established academic discipline that is taught in many universities. The most obvious forces behind this are the move towards automated business and the establishment of the Internet as a communications channel.
Cryptography in one form or another has probably been practiced ever since man has communicated his thoughts in speech or writing. References are made in the Bible, and more scientific cryptograms were devised by the early Greeks, who frequently used arithmetical figures. It has been widely used for the sake of economy, as indicated by the numerous existing commercial codes, and it has also identified itself in many ways with literature, and has even proved its value to kidnapers, gangster, and other modern racketeers.
Cryptography consists of many disciplines but is performed primarily for encryption and authentication. Encryption is the process of rendering information unreadable to anyone but the intended recipient(s). Authentication, as applied to persons or devices, seeks to ensure that a message has been sent by a party or device authorized to send it, or, a document has been created by the party represented as creating it.