Английский язык для пользователей ПК и программистов - Гольцова Е.В.
ISBN 5-7931-0086-5
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III. For example, suppose we choose "Pascal" as our keyword and we use the same plaintext as in our previous examples.
we merge This is a secret message with PascalPascalPasgalPascal
The kth character of the ciphertext is some function (such as exclusive or) of the kth character in the plaintext and the kth character in the repeating keyword string. The decryption algorithm appli es the repeating keyword to the ciphertext and reverses the letter function to recover the plaintext. If we map our alphabet onto the integers 0 to 52 and perform modulo 52 addition on a pair of characters, the ciphertext resulting from our example can be represented: hIaVAUHATCTQrSXWAYtTkDHQ
IV. Frequencies of letters in the first four chapters of this book were counted. Mapping lower onto upper case letters, the ranks were:
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The number of possible keys is now larger (N! rather than N, where N is the size of the alphabet) . However, the method is easily attacked because every occurrence of a particular character in the plaintext is mapped onto a particular character in the encrypted text. We saw in the previous chapter that natural language is redundant — letters and letter groups do not occur with equal frequencies. In English, the rank ordering of the most frequent half-dozen or so letters is fairly constant from text to text. In the second section an example is shown.[ ]
If the text is large enough, the frequency distribution of the characters in the ciphertext gives clues as to their identity in the plaintext. Even in the short example above we might deduce that a space is represented by R because of the frequency and spacing of that character. It is an interesting project to write an interactive program that helps discover the key. 5.2.3 Repeated keywords
We can avoid the one-to-one correspondence between characters in the plaintext and characters in the encrypted text by merging the plaintext in some way with (repetitions of a keyword. See Kernighan and Plauger [4 sec. 2.6] for a discussion of this method. [ ]
Note that if we further encrypt the result with a (repeating) keyword of, for example, length 7, we get something of the same effect as if we encrypted once with a keyword of length 42 (LCM(6,7)) . However, repeating patterns make the ciphertext more vulnerable than if an arbitrary 42-character keyword had been used.
Chapterl ETAIORNS Chapter3 ETAIRNSO
Chapter2 ETASOINR Chapter4 ETAOSINR
For comparison, the ranking in the million word Brown Corpus [3] is ETAOINSR.
Because it is frequently used at the beginning of a sentence, T is typically the most common upper case letter in a tort.
V. Simple substitution code ABCDEFGHIJKUINOPQRSTOTWXYZ abcdefghij klmnnopgrstuvwxyz PEtelsOSTICYpg
xBrWNfкj FuoRJXcmUwAZaMGzvKndbLVDQhHygi
The same encryption and decryption techniques are used and now we get plaintext:
This is a secret message ciphertext: NZaVRaVRJRVUcLUDRvUWJAU Английский язык для пользователей ПК и программистов
Упражнение 16. Пробуем говорить. Расскажите схему по-английски, каждый пункти каждый процесс. Используйте слова: is called, readable, to protect, to produce, we term, to recover, и слова из Текста.
Plaintext —» Encryption —» Ciphertext —» Decription —» Plaintext Algorithm Algorithm
Key Key
Упражнение 17. Chose the correct variant of translation of word-combinations. Mind the rules of translation. These are data security terms. If you do so you'll get a small dictionary on data security.
Выберите правильный вариант перевода словосочетаний. Помните правила перевода многокомпонентных словосочетаний. Это термины по безопасности компьютерных систем. Если вы сделаете выбор правильно, получите небольшой словарик.
TEST
А
1) access control list
а) список доступа
б) лист контрольного доступа
2) accountability
а) наблюдаемость
б) возможность подсчёта
3) admissible rate point
а)' точка допустимой скорости
б) допустимая точка скорости
4) authentication exchange
а) подтверждение подлинности
б) подтверждение авторства
5) availability
а) возможность
б) доступность
С
6) central keying authority
а) центр распределения ключей
б) автор центрального ключа
7) compatibility
а) соревновательность
б) совместимость
8) constraint length
а) длина кодового ограничения свернутого кода
б) длина отрезка
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9) convolutional code
а) сверточный код
б) код конвульсии
10) corruptible store method
а) метод записи с частичным разрушением памяти
б) метод сохранения разрушений
11) cryptographic information protection facility
а) средство защиты зашифрованной информации
б) средство криптографической защиты информации
12) cyclic redundancy check (CRC)
а) контроль на основе циклического избыточного кода
б) контроль избыточной цикличности
D
13) data origin authentication
а) установление оригинальности данных
б) аутентификация источника данных
