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Education college for girls Computer Science Dept. Subject: cryptography Year: 2011-2012 Lecture:1 Introduction to communications security The need to keep certain messages secret has been appreciated for thousands of years. Of course, people were not low to realize the advantages to be gained from intercepting secret information, and this has led to a continuous, fascinating battle between the 'codemakers' and the 'codebreakers'. The arena for this contest is the communications medium which has changed considerably over the years. Society is now highly dependent on our modern, fast and accurate means of transmitting messages. As well as the long-established forms, such as the post and courier services, we now have more technical and sophisticated media such as radio, television, telephone, telex and high speed data links. There are, however, a number of situations where the information is confidential, and where an interceptor might able to benefit immensely from the knowledge gained by monitoring the information circuit. In such situations, the communicants must take steps to conceal and protect the content of their message. Of course the amount of protection required will vary. On some occasion it is sufficient to prevent a casual listener from understanding the message, but there are other times when it is crucial that even the most determined interceptor must not be able to deduce it. If the communications were able to use a non- interceptible means of transmission then, obviously, all their messages would be secure. But the most common forms of communication don’t satisfy this requirement. The method which comes nearest to satisfying it is, probably, the use of courier. But this is likely to be very slow. Very expensive and, if the number of messages requiring transmission is large, might even be impossible. One alternative to a non- interceptible means of transmission is to conceal the content of each message by transforming it before transmission. This is the objective of a cipher system. The science of designing such systems is called cryptography. 1 2 Cryptography Cryptography (or cryptology; from Greek kryptos, "hidden, secret"; and gráph, "writing", is the practice and study of hiding information. Modern cryptography intersects the disciplines of mathematics, computer science, and electrical engineering. Cryptanalysis Cryptanalysis (from the Greek kryptós, "hidden", and analýein, "to loosen" or "to untie") is the study of methods for obtaining the meaning of encrypted information, without access to the secret information which is normally required to do so. Typically, this involves finding a secret key. Basic terms 1- Plaintext (P) & Ciphertext(C) In cryptography, plaintext is information a sender wishes to transmit to a receiver. Cleartext is, sometimes confusingly, often used as a synonym. Before the computer era, plaintext most commonly meant message text in the language of the communicating parties. Plaintext has reference to the operation of cryptographic algorithms, usually encryption algorithms, and is the input upon which they operate. Cleartext, by contrast, refers to data that is transmitted or stored unencrypted (that is, 'in the clear'). Ciphertext (or cyphertext) is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext that is unreadable by a human or computer without the proper cipher to decrypt it. Decryption, the inverse of encryption, is the process of turning ciphertext into readable plaintext. 2- Encryption and Decryption (E&D) In cryptography, encryption is the process of transforming information (referred to as plaintext) using an algorithm (called cipher) to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information (in cryptography, referred to as ciphertext). In many contexts, the word encryption also implicitly refers to the reverse process, decryption (e.g. “software for encryption” can typically also perform decryption), to make the encrypted information readable again (i.e. to make it unencrypted). 3 3- Key (K) In cryptography, a key is a piece of information (a parameter) that determines the functional output of a cryptographic algorithm or cipher. Without a key, the algorithm would produce no useful result. In encryption, a key specifies the particular transformation of plaintext into ciphertext, or vice versa during decryption. Keeping keys secret is one of the most difficult problems in practical cryptography. An attacker who obtains the key ( by, for example, theft, extortion ) can recover the original message from the encrypted data. Encryption algorithms which use the same key for both encryption and decryption are known as symmetric key algorithms. A newer class of " public key " cryptographic algorithms was invented in the 1970s which uses a pair of keys, one to encrypt and one to decrypt. These asymmetric key algorithms allow one key to be made public while training the private key is extremely difficult, even if the corresponding public key is known. A user of public key technology can publish their public key, while keeping their private key secret, allowing anyone to send them an encrypted message. Below the figure demonstrates the above basic terms of cryptography 4