8_05_14

home assignment deadline: 6.05.14. - 20.00

In the text below think of the possible ways of translation for the words and word combinations in **bold**. Write your variants and ideas under the text.


 * Chapter 2**

2.1. Terminology and Background Consider the steps involved in sending messages from a **sender**, S, to a **recipient**, R. If S **entrusts the message** to T, who then delivers it to R , T then becomes the **transmission medium**. If an outsider, O, wants to access the message (to read, change, or even destroy it), we call O an **interceptor** or **intruder**. Any time after S transmits it via T, the message is vulnerable to exploitation, and O might try to access the message in any of the following ways: **E**** ncryption ** is the process of encoding a message so that its meaning is not obvious; ** decryption ** is the reverse process, transforming an encrypted message back into its normal, original form. Alternatively, the terms ** encode ** and ** decode ** or ** encipher ** and ** decipher ** are used instead of encrypt and decrypt.[1] That is, we say that we encode, encrypt, or encipher the original message to hide its meaning. Then, we decode, decrypt, or decipher it to reveal the original message. A system for encryption and decryption is called a ** cryptosystem **. > [1] There are slight differences in the meanings of these three pairs of words, although they are not significant in this context. Strictly speaking, encoding is the process of translating entire words or phrases to other words or phrases, whereas enciphering is translating letters or symbols individually; encryption is the group term that covers both encoding and enciphering. The original form of a message is known as **plaintext**, and the encrypted form is called **ciphertext**.
 * Block it, by preventing its reaching R, thereby affecting the availability of the message.
 * Intercept it, by reading or listening to the message, thereby affecting the confidentiality of the message.
 * Modify it, by seizing the message and changing it in some way, affecting the message's integrity.
 * Fabricate an authentic-looking message, arranging for it to be delivered as if it came from S, thereby also affecting the **integrity of the message**.
 * ====Terminology====

Encryption Algorithms
The **cryptosystem** involves a set of rules for how to encrypt the plaintext and how to decrypt the ciphertext. The encryption and decryption rules, called algorithms, often use a device called a **key**, denoted by K, so that the resulting ciphertext depends on the **original plaintext message**, the algorithm, and the **key value**. We write this dependence as C = E ( K, P ). Essentially, E is **a set of encryption algorithms**, and the key K selects one specific algorithm from the set. We see later in this chapter that a cryptosystem, such as the Caesar cipher, is keyless but that keyed encryptions are more difficult to break.

Sometimes the **encryption and decryption keys** are the same, so P = D ( K, E ( K,P )). This form is called **symmetric** **encryption** because D and E are mirror-image processes. At other times, encryption and decryption keys come in pairs. Then, a decryption key, KD, inverts the encryption of key KE so that P = D ( KD, E ( KE,P )). Encryption algorithms of this form are called **asymmetric** because converting C back to P involves a series of steps and a key that are different from the steps and key of E.

A key gives us flexibility in using an **encryption scheme**. We can create different encryptions of one plaintext message just by changing the key. Moreover, using a key provides additional security. If the encryption algorithm should fall into the interceptor's hands, future messages can still be kept secret because the interceptor will not know the key value. An encryption scheme that does not require the use of a key is called a **keyless cipher**. The word "cryptography" means hidden writing, and it refers to the practice of using encryption to conceal text. A **cryptanalyst** studies encryption and encrypted messages, hoping to find the hidden meanings. Both a ** cryptographer ** and a cryptanalyst attempt to translate coded material back to its original form. Normally, a cryptographer works on behalf of a legitimate sender or receiver, whereas a cryptanalyst works on behalf of an unauthorized interceptor. Finally, **cryptology** is the research into and study of encryption and decryption; it includes both cryptography and cryptanalysis. 

Cryptanalysis
A cryptanalyst's chore is to **break an encryption**. That is, the cryptanalyst attempts to deduce the original meaning of a ciphertext message. Better yet, he or she hopes to determine which decrypting algorithm matches the encrypting algorithm so that other messages encoded in the same way can be broken.[ ...] Thus, a cryptanalyst can attempt to do any or all of six different things:
 * break **a single message**
 * **recognize patterns in encrypted messages**, to be able to break **subsequent ones** by applying a **straightforward** **decryption algorithm**
 * **infer some meaning** without even breaking the encryption, such as noticing **an unusual frequency of communication** or determining something by whether the communication was short or long
 * deduce the key, to break subsequent messages easily
 * find weaknesses in **the implementation or environment of use of encryption**
 * find general weaknesses in an encryption algorithm, without necessarily having intercepted any messages

An analyst works with a variety of **pieces of information**: encrypted messages, known encryption algorithms, intercepted plaintext, **data items** known or suspected to be in a ciphertext message, mathematical or statistical tools and techniques, properties of languages, computers, and plenty of ingenuity and luck. 

**Breakable Encryption**
An encryption algorithm is called breakable when, given enough time and data, an analyst can determine the algorithm. However, an algorithm that is theoretically breakable may in fact be impractical to try to break. To see why, consider a 25-character message that is expressed in just uppercase letters. A given cipher scheme may have 26^25 (approximately 10^35) possible **decipherments**, so the task is to select the right one out of the 26^25. If your computer could perform on the order of 10^10 operations per second, finding this decipherment would require on the order of 10^16 seconds, or roughly 10^11 years. In this case, although we know that theoretically we could generate the solution, determining the deciphering algorithm by examining all possibilities can be ignored as **infeasible** with current technology. Two other important issues must be addressed when considering the breakability of encryption algorithms. First, the cryptanalyst cannot be expected to try only the hard, long way. In the example just presented, the obvious decryption might require 26^25 machine operations, but a more ingenious approach might require only 10^15 operations. At the speed of 10^10 operations per second, 10^15 operations take slightly more than one day. The **ingenious approach** is certainly feasible. As we see later in this chapter, some of the algorithms we study in this book are based on known "hard" problems that take an unreasonably long time to solve.  Second, **estimates of breakability** are based on current technology. An enormous advance in computing technology has occurred since 1950. Things that were infeasible in 1940 became possible by the 1950s, and every succeeding decade has brought greater improvements. A **conjecture** known as "Moore's Law" asserts that the speed of processors doubles every 1.5 years, and this conjecture has been true for over two decades. It is risky to pronounce an algorithm secure just because it cannot be broken with current technology, or worse, that it has not been broken yet. 

Representing Characters
We want to study ways of encrypting any computer material, whether it is written as ASCII characters, binary data, object code, or a control stream. However, to simplify the explanations, we begin with the encryption of messages written in the standard 26-letter English[2] alphabet, A through Z. > [2] Because this book is written in English, the explanations refer to English. However, with slight variations, the techniques are applicable to most other written languages as well. Throughout the book, we use the **convention** that plaintext is written in UPPERCASE letters, and ciphertext is in lowercase letters. Because most encryption algorithms are based on **mathematical transformations**, they can be explained or studied more easily in mathematical form. There are many types of encryption. In the next two sections we look at two simple forms of encryption: **substitutions**, in which one letter is exchanged for another, and **transpositions**, in which the order of the letters is rearranged. ||

-отправитель(Рябухин) сообщения -адресант(Порошин); -получатель сообщения, адресат(Порошин) -получатель(Рябухин); -вверять(Рябухин) - передаёт сообщение(Порошин); -средство связи (передачи)(Порошин); -промежуточное звено(Степанов) -передающая среда(Рябухин) -посредник1399905378 - перехватчик(Порошин+Рябухин); - злоумышленник(Порошин); -захватчик(Рябухин) - неприкосновенность(сохранность) сообщения(Порошин); - шифрование(Порошин); -шифрование(Рябухин) -кодировка(Рябухин) -зашифровывание(Рябухин) - расшифровка(Порошин); -дешифрование(Рябухин) -декодирование(Рябухин) -дешифровывание(Рябухин) - система шифрования, криптографическая система(Порошин); -криптографическая система(Рябухин) -криптосистема1399905378 - не зашифрованный(открытый) текст(Порошин+Рябухин); - зашифрованный текст (криптограмма)(Береснева+Рябухин); - ключ(Порошин); -ключ шифрования(Рябухин) - открытый текст сообщения (не зашифрованное сообщение)(Береснева); -исходный текст(Рябухин) - значение ключа(Рябухин); -функция шифрования(Рябухин) -алгоритм шифрования1399905378 - набор алгоритмов шифрования; - ключи шифровки и дешифровки(ключи шифрования и дешифрования)(Рябухин); - симметричное/асимметричное шифрование(Порошин); -одноключевое/ двухключевое шифрование(Рябухин) - схема(способ) шифрования(Степанов+Назипов) - бесключевой шифр(Степанов) - без ключа шифрования(Назипов) - криптология - наука о шифровании и дешифровании(Степанов) - взломать шифр(Степанов+Петухов) - взломать шифр всего сообщения(Степанов) -расшифровать одно сообщение1399905378 - одиночное сообщение(Петухов) - разобрать(определить) части сообщений(Степанов) -находить части сообщений в расшифрованном(Петухов) -распознать шаблоны в зашифрованном сообщении - последующие(Степанов) - следующие(Петухов) - прямой(пронизывающий, проходящий через все сообщения) алгоритм дешифровки(Степанов) - предугадать смысл(Степанов) -сделать некие выводы1399905378 - слишком частое взаимодействие(Степанов) -необычная частота связи1399905378 - в реализации или месте (среде) использования шифрования(Береснева) - легко взламываемое шифрование(Петухов) - варианты расшифровки(Петухов) - невыполнимый/невозможный(Петухов+Назипов) -- гениальный подход(Назипов) - обдуманный подход(Петухов) - договоренность(Петухов) -соглашение1399905378 - математические преобразования(Петухов) - подстановки (замена)(Береснева) -замены(Назипов) - перемещения (перестановка) (Береснева) -перестановки(Назипов) - гипотеза(Назипов) - показатель раскрываемости(Назипов) -оценки взломостойкости1399905378 - часть информации(Назипов) -сведения1399905378 - элемент данных, информационный элемент(Береснева) - раздел с данными 1399919937 - аналитик по криптографии(Назипов)
 * a sender**
 * a recipient**
 * entrusts**
 * entrusts the message**
 * transmission medium**
 * interceptor**
 * intruder**
 * the integrity of the message**
 * Encryption, encode, encipher**
 * Encryption**
 * Encode**
 * Encipher**
 * decryption, decode, decipher**
 * Decryption**
 * Decode**
 * Decipher**
 * a cryptosystem**
 * plaintext**
 * ciphertext**
 * a key**
 * original plaintext message**
 * the key value**
 * Encryption algorithms**
 * a set of encryption algorithms**
 * the encryption and decryption keys**
 * symmetric/asymmetric encryption**
 * encryption scheme**
 * keyless cipher**
 * cryptology**
 * break an encryption**
 * break a single message**
 * single message**
 * recognize patterns in enctypted messages**
 * subsequent ones**
 * a straightforward decryption algorithm**
 * infer some meaning**
 * an unusual frequency of communication**
 * in the implementation or environment of use of encryption**
 * Breakable Encryption**
 * decipherments**
 * infeasible**
 * ingenious approach**
 * convention**
 * mathematical transformations**
 * substitutions**
 * transpositions**
 * conjecture**
 * estimates of breakability**
 * pieces of information**
 * data items**
 * cryptanalyst**

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