Advances over the past decade in many aspects of digital technology – especially devices for image acquisition, data storage, and bitmapped printing and display – have brought about many applications of digital imaging. However, these applications tend to be specialized due to their relatively high cost. With the possible exception of facsimile, digital images are not commonplace in general-purpose computing systems the way text and geometric graphics are. The majority of modern business and consumer usage of photographs and other types of images takes place through more traditional analog means.
The key obstacle for many applications is the vast amount of data required to represent a digital image directly. A digitized version of a s ingle, color picture at TV resolution contains on the order of one million bytes; 35mm resolution requires ten
times that amount. Use of digital images often is not viable due to high storage or transmission costs, even when image capture and display devices are quite affordable.
Modern image compression technology offers a possible solution. State-of-the-art techniques can compress typical images from 1/10 to 1/50 their uncompressed size without visibly affecting image quality. But compression technology alone is not sufficient. For digital image applications involving storage or transmission to become widespread in today's marketplace, a standard image compression method is needed to enable interoperability of equipment from different manufacturers.
For the past few years, a standardization effort known by the acronym JPEG, for Joint Photographic Experts Group, has been working toward establishing the first international digital image compression standard for continuous-tone (multilevel) still images, both grayscale and color.
Photovideotex, desktop publishing, graphic arts, color facsimile, newspaper wirephoto transmission, medical imaging, and many other continuous-tone image applications require a compression standard in order to develop s ignificantly beyond their present state. JPEG has undertaken the ambitious task of developing a general-purpose compression standard to meet the needs of almost all continuous-tone still-image applications.
If this goal proves attainable, not only will individual applications flourish, but exchange of images across application boundaries will be facilitated. This latter feature will become increasingly important as more image applications are implemented on general-purpose computing systems, which are themselves becoming increasingly interoperable and internetworked. For applications which require specialized VLSI to meet their compression and decompression speed requirements, a common method will provide economies of scale not possible within a s ingle application.
This article gives an overview of JPEG's proposed image-compression standard.
Readers without prior knowledge of JPEG or compression based on the Discrete Cosine Transform (DCT) are encouraged to study first the detailed description of the Baseline sequential codec, which is the basis for all of the DCT-based decoders.
While this article provides many details, many more are necessarily omitted.
Some of the earliest industry attention to the JPEG proposal has been focused on the Baseline sequential codec as a motion image compression method – of the
«intraframe» class, where each frame is encoded as a separate image.
Background: Requirements and Selection Process
JPEG's goal has been to develop a method for continuous-tone image compression which meets the following requirements :
1) be at or near the state of the art with regard to compression rate and accompanying image fidelity, over a wide range of image quality ratings, and especially in the range where visual fidelity to parameterizable, so that the application (or user) can set the desired compression/quality tradeoff;
2) be applicable to practically any kind of continuous-tone digital source image (i.e. for most practical purposes not be restricted to images of certain
dimensions, color spaces, pixel aspect ratios, etc.) and not be limited to classes of imagery with restrictions on scene content, such as complexity, range of colors, or statistical properties;
3) have tractable computational complexity, to make feasible software implementations with viable performance on a range of CPU's, as well as hardware implementations with viable cost for applications requiring high performance;
4) have the following modes of operation:
• Sequential encoding: each image component is encoded in a single left-to-right, top-to-bottom scan;
• Progressive encoding: the image is encoded in multiple scans for applications in which transmission time is long, and the viewer prefers to watch the image build up in multiple coarse-to-clear passes;
• Lossless encoding: the image is encoded to guarantee exact recovery of every source image sample value (even though the result is low compression compared to the lossy modes);
• Hierarchical encoding: the image is encoded at multiple resolutions so that lower-resolution versions may be accessed without first having to decompress the image at its full resolution.
TEXT 11 CRYPTOGRAPHY
Cryptography is the science and art of secret writing – keeping information secret. When applied in a computing environment, cryptography can protect data against unauthorized disclosure; it can authenticate the identity of а user or program requesting service; and it can disclose unauthorized tampering.
Cryptanalysis is the related study of breaking ciphers. Cryptology is the combined study of cryptography and cryptanalysis.
Cryptography is an indispensable part of modern computer security.
A Brief History of Cryptography
Knowledge of cryptography can be traced back to ancient times. It's not difficult to understand why: as soon as three people had mastered the art of reading and writing, there was the possibility that two of them would want to send letters to each other that the third could not read.
In ancient Greece, the Spartan generals used a form of cryptography so that the generals could exchange secret messages: the messages were written on narrow ribbons of parchment that were wound spirally around a cylindrical staff called a scytale. After the ribbon was unwound, the writing on it could only be read by a person who had a matching cylinder of exactly the same s ize. This primitive system did a reasonably good job of protecting messages from interception and from the prying eyes of the message courier as well.
In modern times, cryptography's main role has been in securing electronic communications, moon after Samuel F. B, Morse publicly demonstrated the telegraph in 1845, users of the telegraph began worrying about the confidentiality of the messages that were being transmitted. What would happen if somebody tapped the telegraph line? What would prevent unscrupulous telegraph operators from keeping a copy of the messages that they relayed and then divulging them to others? The answer was to encode the messages with a secret code, so that nobody but the intended recipient could decrypt them.
Cryptography became even more important with the invention of radio, and its use in war. Without cryptography, messages transmitted to or from the front lines could easily be intercepted by the enemy.
Code Making and Code Breaking
As long as there have been code makers, there have been code breakers. Indeed, the two have been locked in a competition for centuries, with each advance on one side being matched by counter-advances on the other.
For people who use codes, the code-breaking efforts of cryptanalysts pose a danger that is potentially larger than the danger of not using cryptography in the first place. Without cryptography, you might be reluctant to send sensitive information through the mail, across a telex, or by radio. But if you think that you have a secure channel of communication, then you might use it to transmit secrets that should not be widely revealed.
For this reason, cryptographers and organizations that use cryptography routinely conduct their own code-breaking efforts to make sure that their codes are resistant to attack. The findings of these self-inflicted intrusions are not always pleasant. The following brief story from a 1943 book on cryptography demonstrates this point quite nicely.
The importance of the part played by cryptographers in military operations was demonstrated to us realistically in the First World War. One instructive incident occurred in September 1918, on the eve of the great offensive against Saint-Mihiel. A student cryptographer, fresh from Washington, arrived at United States Headquarters at the front. Promptly he threw the General Staff into a state of alarm by decrypting with comparative ease a secret radio message intercepted in the American sector.
The smashing of the German salient at Saint-Mihiel was one of the most gigantic tasks undertaken by the American forces during the war. For years that salient had stabbed into the Allied lines, cutting important railways and communication lines. Its lines of defense were thought to be virtually impregnable.
But for several months the Americans had been making secret preparations for attacking it and wiping it out. The state was set, the minutest details of strategy had been determined – when the young officer of the United States Military Intelligence spread consternation through our General Staff.
The dismay at Headquarters was not caused by any new information about the strength of the enemy forces, but by the realization that the Germans must know as much about our secret plans as we did ourselves – even the exact hour set for the
attack. The intercepted message had been from our own base. German cryptographers were as expert as any in the world, and what had been done by an American student cryptographer could surely have been done by German specialists.
The revelation was even more bitter because the cipher the young officer had broken, without any knowledge of the system, was considered absolutely safe and had long been used for most important and secret communications.
Cryptography and Digital Computers
Modern digital computers are, in some senses, the creations of cryptography.
Some of the first digital computers were built by the Allies to break messages that had been encrypted by the Germans with electromechanical encrypting machines.
Code breaking is usually a much harder problem than code making; after the Germans switched codes, the Allies often took several months to discover the new coding systems. Nevertheless, the codes were broken, and many historians say that World War II was shortened by at least a year as a result.
Things really picked up when computers were turned to the task of code making.
Before computers, all of cryptography was limited to two basic techniques:
transposition, or rearranging the order of letters in a message (such as the Spartan's scytale), and substitution, or replacing one letter with another one. The most sophisticated pre-computer cipher used five or six transposition or substitution operations, but rarely more.
With the coming of computers, ciphers could be built from dozens, hundreds, or thousands of complex operations, and yet could still encrypt and decrypt messages in a short amount of time. Computers have also opened up the possibility of using complex algebraic operations to encrypt messages. All of these advantages have had a profound impact on cryptography.
Modern Controversy
In recent years, encryption has gone from being an arcane science and the stuff of James Bond movies, to being the subject of debate in several nations (but we'll focus on the case in the U.S. in the next few paragraphs). In the U.S. that debate is playing itself out on the front pages of newspapers such as The New York Times and the San Jose Mercury News
On one side of the debate are a large number of computer professionals, civil libertarians, and perhaps a majority of the American public, who are rightly concerned about their privacy and the secrecy of their communications. These people want the right and the ability to protect their data with the most powerful encryption systems possible.
On the other side of the debate are the United States Government, members of the nation's law enforcement and intelligence communities, and (apparently) a small number of computer professionals, who argue that the use of cryptography should be limited because it can be used to hide illegal activities from authorized wiretaps and electronic searches.
MIT Professor Ronald Rivest has observed that the controversy over cryptography fundamentally boils down to one question: should the c itizens of a country have the right to create and store documents which their government cannot read?
What Is Encryption?
Encryption is a process by which a message (called plaintext) is transformed into another message (called ciphertext) using a mathematical function and a special encryption password, called the key.
Decryption is the reverse process: the ciphertext is transformed back into the original plaintext using a mathematical function and a key.
Indeed, the only way to decrypt the encrypted message and get printable text is by knowing the secret key nosmis. If you don't know the key, and you don't have access to a supercomputer, you can't decrypt the text. If you use a strong encryption system, even the supercomputer won't help you.
What You Can Do with Encryption
Encryption can play a very important role in your day-to-day computing and communicating:
• Encryption can protect information stored on your computer from unauthorized access – even from people who otherwise have access to your computer system.
• Encryption can protect information while it is in transit from one computer system to another.
• Encryption can be used to deter and detect accidental or intentional alterations in your data.
• Encryption can be used to verify whether or not the author of a document is really who you think it is.
Despite these advantages, encryption has its limits:
• Encryption can't prevent an attacker from deleting your data altogether.
• An attacker can compromise the encryption program itself. The attacker might modify the program to use a key different from the one you provide, or might record all of the encryption keys in a special file for later retrieval.
• An attacker might find a previous ly unknown and relatively easy way to decode messages encrypted with the algorithm you are using.
• An attacker could access your file before it is encrypted or after it is decrypted.
For all these reasons, encryption should be viewed as a part of your overall computer security strategy, but not as a substitute for other measures such as proper access controls.
The Elements of Encryption
There are many different ways that you can use a computer to encrypt or decrypt information. Nevertheless, each of these so-called encryption systems share common elements :
Encryption algorithm
The encryption algorithm is the function, usually with some mathematical foundations, which performs the task of encrypting and decrypting your data.
Encryption keys
Encryption keys are used by the encryption algorithm to determine how data is encrypted or decrypted. Keys are similar to computer passwords: when a piece of information is encrypted, you need to specify the correct key to access it again. But unlike a password program, an encryption program doesn't compare the key you provide with the key you originally used to encrypt the file, and grant you access if the two keys match. Instead, an encryption program uses your key to transform the ciphertext back into the plaintext. If you provide the correct key, you get back your original message.
Key length
As with passwords, encryption keys have a predetermined length. Longer keys are more difficult for an attacker to guess than shorter ones because there are more of them to try in a brute-force attack. Different encryption systems allow you to use keys of different lengths; some allow you to use variable-length keys.
Plaintext
The information which you wish to encrypt.
Ciphertext
The information after it is encrypted.
Cryptographic Strength
Different forms of cryptography are not equal. Some systems are easily circumvented, or broken. Others are quite resistant to even the most determined attack. The ability of a cryptographic system to protect information from attack is called its strength. Strength depends on many factors, including:
• The secrecy of the key.
• The difficulty of guessing the key or trying out all possible keys (a key search).
Longer keys are generally harder to guess or find.
• The difficulty of inverting the encryption algorithm without knowing the encryption key (breaking the encryption algorithm).
• The existence (or lack) of back doors, or additional ways by which an encrypted file can be decrypted more easily without knowing the key.
• The ability to decrypt an entire encrypted message if you know the way that a portion of it decrypts (called a known text attack).
• The properties of the plaintext and knowledge of those properties by an attacker. (For example, a cryptographic system may be vulnerable to attack if all messages encrypted with it begin or end with a known piece of plaintext.
These kinds of regularities were used by the Allies to crack the German Enigma cipher during the Second World War.)
The goal in cryptographic design is to develop an algorithm that is so difficult to reverse without the key that it is at least roughly equivalent to the effort required to guess the key by trying possible solutions one at a time. We would like this property to hold even when the attacker knows something about the contents of the messages encrypted with the cipher. Some very sophisticated mathematics are involved in such design.
APPENDIX
Что такое аннотация и peфepaт?
Аннотация (от лат. annotatio – замечание; англ.- summary, annotation, abstract) – краткая характеристика содержания произведения печати или рукописи. Аннотации по содержанию и целевому назначению могут быть справочные, раскрывающие тематику документов и сообщающие какие-либо сведения о нем, но не дающие его критической оценки, и рекомендательные, содержащие оценку документа с точки зрения его пригодности для оп-ределенной категории читателей. По охвату содержания аннотированного до-кумента и читательского назначения различают аннотации общие, характе-ризующие документ в целом и рассчитанные на широкий круг читателей и специализированные, раскрывающие документ лишь в определенных аспектах, интересующих узкого специалиста и описательные.
В аннотации указываются лишь существенные признаки содержания документа, т.е. те, которые позволяют выявить его научное и практическое значение и новизну, отличить его от других, близких к нему по тематике и целевому назначению.
При составлении аннотации не следует пересказывать содержание документов. Следует свести к минимуму использование сложных оборотов, употребление личных и указательных местоимений. Объем аннотации от 500 до 1000 печатных знаков.
Состав аннотации:
1. библиографическое описание;
2. данные об авторе: ученая с тепень, звание, принадлежнос ть к научной школе и др. Подробные данные об авторе не являются обязательным объектом аннотации;
3. конкретная форма аннотируемого документа: монография, учебник, учебное пособие и т.д.;
4. предмет изложения и его основные характерис тики: тема, основные понятия, процессы, место и время, в течение которого эти процессы происходят и т.д.;
5. отличительные черты документа по сравнению с родственными по тематике и целевому назначению: то новое в содержании, что несет в себе документ, например, постановка проблемы, решение час тного вопроса, новая методика, обобщение данных по различным источникам, новая оценка фактов, новая концепция или гипотеза и др.;
6. конкретный читательский адрес: кому адресуется книга, статья.
Аннотация служит только для осведомления о существовании документа определенного содержания и характера. В аннотировании основное заключа-ется в умении лаконично обобщить содержание документа.
Схема составления описательной аннотации.
1. Вводная часть.
Автор, название работы на инос транном языке и его перевод, название журнала на иностранном языке, его номер и год издания, название фирмы (на иностранном языке) для патента или каталога;
название издательства для книги; количество страниц, таблиц, рисунков;
на каком языке написана работа; библиография.
2. Описательная часть.
Сообщается о /+ сущ./...
Подробно описывается ...
Кратко говорится … 3. Заключительная часть.
Общий вывод, выделяется главное в работе. Может быть дана рекомендация, кому данная работа будет полезна.
Для составления аннотаций дос таточно просмотрового чтения работы. Это обычно работа без словаря.
Реферат (от лат. refero – сообщаю; англ.- essay, précis) – краткое изложение в письменном виде или в форме публичного доклада содержания научного труда (трудов), литературы по теме. Среди многочисленных видов рефератов следует выделить специализированные рефераты, в которых изложение ориентировано на специалистов определенной области или какой-нибудь деятельности и учитывает их запросы.
Целевое назначение реферата разнообразно. Его функции следующие:
1. реферат отвечает на вопросы, какая основная информация заключается в реферированном документе;
2. дает описание первичного документа;
3. оповещает о выходе в свет и о наличии соответс твующих первичных документов;
4. является источником для получения справочных данных.
Реферат является также одним из самостоятельных средств научной информации, может быть выполнен в форме устного доклада. При всем своем разнообразии рефераты обладают некоторыми общими чертами. В реферате не используются доказательства, рассуждения и ис торические экскурсы. Материал подается в форме консультации или описания фактов. Информация излагается точно, кратко, без искажений и субъективных оценок. Краткость достигается в основном за счет использования преимущественно терминологической лексики, а также средств лаконизации языка /таблиц, формул, иллюстраций/.
Реферат, как правило, включает следующие части:
1. библиографическое описание первичного документа;
2. собственно реферативная часть /текст реферата/;
3. справочный аппарат, т.е. дополнительные сведения и примечания.
Текст реферата следует строить по следующему плану:
а) цель и методика исследования/изучения/ или разработки;
б) конкретные данные о предмете исследования /изучения/ или разработки, его изучаемых свойствах;
в) временные и прос транс твенные характеристики исследования;
г) результаты и выводы.
Заглавие реферата не должно повторяться в тексте. Как и в аннотации, следует избегать лишних вводных фраз. Примерный объем реферата находится в пределах 10-15% объема реферируемой статьи. При необходимости объем может быть больше указанного. Реферирование предполагает владение мастерством сокращения текс та первичного документа.
Заглавие реферата может быть предс тавлено в двух вариантах:
а) заглавием реферата служит точный перевод на русский язык заголовка первичного документа, опубликованного на английском языке, например:
Национальная информационная система по физике. Koch H.W.A national information system for physics. «Phys. Today», 1998, №4 /англ./
б) заглавием реферата является смысловой перевод заголовка первичного документа, если этот заголовок неточно или недостаточно полно отражает основное содержание документа. В этом случае заглавие реферата выносится в квадратные скобки. Например: [o месте информации среди социальных наук и о причинах, препятствующих её развитию] Batten W.K. We know the enemу – do we know the friends? «Libr.J»,1998, №5 /англ./
Такое заглавие реферата рекомендуется составлять после того, как пол-ностью уяснена сущность первичного документа и составлен реферат.
Термины. В реферате должна быть использована научная терминология, принятая в научной литературе по данной отрасли науки и техники. Не следует употреблять инос транные термины, если имеются равнозначные русские.
Формулы. Формулы в тексте реферата следует приводить в следующих случаях:
а) когда без них невозможно составление текста реферата;
б) когда формулы выражают итоги работы, изложенной в первичном до-кументе;
в) когда формулы существенно облегчают понимание содержания первич-ного документа.
Единицы измерения переводятся в «Международную систему единиц СИ».
Иллюстрации: чертежи, карты, диаграммы, фотографии/ и таблицы могут быть включены в реферат полностью или частично, если они отражают основное содержание первичного документа и способствуют сокращению текста реферата.
Фамилии в тексте реферата, как правило, рекомендуется приводить на английском языке. Фамилии хорошо известных в России иностранных ученых
следует писать в русской транскрипции, например, закон Бойля-Мариотта.
Географические названия даются в русской транскрипции в соответствии с последним изданием «Атласа мира». Название страны следует давать с учетом установленных сокращений, например, США. Название фирм, учреждений.
организаций даются в оригинальном написании. После названия в круглых скобках указывается страна. Например: Lakheed (США).
Ссылки в тексте реферата делаются в следующих случаях:
а) когда в первичном документе обсуждается содержание другого документа;
б) когда первичный документ является продолжением ранее опубликован-ного документа. Ссылки в тексте документа ставятся в круглые скобки.
В заключение следует отметить, что аннотация, реферат и рецензия относятся к типам библиографического описания, причем наиболее распрост-раненным типом является аннотация.
Exercises
Exercise1.Употребите данную модель для компрессии высказываний:
1. Пример. Вместо: the parts of machine можно сказать: machine parts
the techniques of measurement, the change of speed, the production of computers, the design of engine, the changes of temperature, designing of computer keyboard.
2. Пример. Вместо: the components for circuit можно сказать: circuit components
parts for production, the equipment for tests, machine for demonstration, the arrangement for automatic checking, the equipment for automatic handling.
3. Пример. Вместо: the conference which took place in 2005 можно сказать: the 2005 conference
the conference which was conducted in 2000;
the computer which was produced in 1999;
the show which took place in 2001;
the car which was produced in 2002;
the exhibition which was in 2000.
4. Пример. Вместо: the equipment which is designed for automatic handling можно сказать: automatic handling equipment
the control system which is designed for measuring; the equipment which is used for radiographic examination; the equipment the function of which is measuring; the machines which are produced for industrial purposes.
5. Пример. Вместо: the chips made of silicon можно сказать: silicon chips
the models used in industry; the equipment used for the purpose of testing; the units used for the purpose of cooling.
6. Пример. Вместо: sections which have the length of 300 feet можно сказать: 300 feet long sections
holes which have the width of 20 feet; holes which have the depth of 4 metres;
the conveyer which has the length of 152 metres.
7. Пример. Вместо: the data which are not important можно сказать: unimportant data
the problems which are not solved (un-); the attemps which are not successful (un-); the element which is not active (in-); the work which is not effective (in-);
the information which is not known (un-); the production which is not economic (un-).
Exercise 2. Употребите причастие прошедшего времени вместо придаточного определительного предложении. По содержанию информации, они эквивалентны.
Пример: The power which is demanded is continually increased.
The power demanded is continually increased.
1. The research which is being carried out on this subject is important.
2. The manufacturing process which was adopted was a revolutionary one.
2. The manufacturing process which was adopted was a revolutionary one.