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1、單位代碼10學(xué) 號分 類 號TP393密 級文獻(xiàn)翻譯計(jì)算機(jī)安全概述院(系)名 稱 專 業(yè) 名 稱 學(xué) 生 姓 名 指 導(dǎo) 教 師 2012年2月28日英文譯文計(jì)算機(jī)安全概述摘 要計(jì)算機(jī)安全主要包括用于檢測和組織的防御方法組織任何可能的入侵者。計(jì)算機(jī)安全的原則因此在各種有威脅的入侵者的壓力下產(chǎn)生。首先給出現(xiàn)有的已知存在于系統(tǒng)中的安全威脅的例子。第二部分介紹了安全威脅的分類，最后一節(jié)提出來一些保護(hù)機(jī)制和技術(shù)來確保一個(gè)計(jì)算機(jī)系統(tǒng)的安全。本文沒有涉及硬件安全，通信安全的問題，也沒有涉及到一些敏感的信息。也許最公開的威脅是一個(gè)入侵者猜測用戶的密碼。隨著個(gè)人電腦，撥號調(diào)制解調(diào)器和代理服務(wù)器的出現(xiàn)，這成了一2、個(gè)大問題。穿甲彈通常有一個(gè)常用的密碼列表，然后他們可以嘗試在個(gè)人電腦的幫助下使用他們解密。另外，如果密碼太短他們很容易找到一份詳細(xì)的搜索。還有標(biāo)準(zhǔn)的賬戶，默認(rèn)的密碼總是系統(tǒng)分配的，而且可能沒有更改。另一種威脅是所謂的欺騙。這是通過通過與用戶交談進(jìn)行欺騙，導(dǎo)致信息的外泄。例如，欺騙程序可以讓一個(gè)毫無戒心的用戶訪問該網(wǎng)站，通過漏斗欺騙程序的機(jī)器，讓欺騙程序，檢查受害者的所有活動，包括任何密碼或受害人的存款數(shù)目。欺騙程序也可能導(dǎo)致虛假或誤導(dǎo)數(shù)據(jù)以受害人的名義進(jìn)入網(wǎng)站服務(wù)器，反之以網(wǎng)站服務(wù)器的名義對受害人。總之，攻擊者觀察和控制所有工作在網(wǎng)站服務(wù)器上的受害者。另外一個(gè)例子也可以用來舉例，欺騙程序可能會3、顯示在終端的登陸界面，這使得終端的現(xiàn)實(shí)為空閑。然后，當(dāng)一毫無戒心的用戶開始通信終端，欺騙程序錄入登錄名并要求用戶的密碼。在獲得信息之后，欺騙程序顯示一個(gè)再試一次的消息或者其他東西，并返回他先前獲得的所有權(quán)。另一個(gè)威脅是用戶對敏感信息的瀏覽，這發(fā)生在一個(gè)合法的可用的所有文件和手機(jī)的點(diǎn)點(diǎn)滴滴的有用的信息。例如，一個(gè)瀏覽器可能會在不經(jīng)意間找到一個(gè)公共可讀的文件留下的密碼。一個(gè)更復(fù)雜的威脅，通常被認(rèn)為是已知的特洛伊木馬，是一個(gè)比打算做的更多或者是一個(gè)以為用來做好事的程序，而實(shí)際上它是在后臺做一些討厭的。例如，一個(gè)背景程序或者一些軟件可能會被公開。然后，當(dāng)沒有檢測到這個(gè)程序時(shí)，特洛伊木馬，以用戶的訪問權(quán)4、利和自己的執(zhí)行文件暗中讀取用戶的文件，甚至還郵寄給游戲的創(chuàng)造者，如果用戶用戶自己登錄到了網(wǎng)絡(luò)。最近，我們看到了被譽(yù)為互聯(lián)網(wǎng)網(wǎng)站榮譽(yù)的分布式拒絕服務(wù)攻擊。黑客利用這些木馬策劃攻擊。另一個(gè)威脅是一個(gè)狡猾疲憊的共享資源用戶，這樣的結(jié)果使得合法的用戶無法完成工作。例如，一個(gè)網(wǎng)絡(luò)前端的用戶可以利用現(xiàn)有的信息所有的緩沖區(qū)使合法用戶不可能完成任何有用的工作。故意造成崩潰的所有工作提高到一個(gè)停止系統(tǒng)是這種類型的威脅一個(gè)更深的例子。另一類威脅是一個(gè)能夠從由數(shù)據(jù)庫返回的非敏感信息和敏感數(shù)據(jù)推斷統(tǒng)計(jì)數(shù)據(jù)庫用戶的結(jié)果。例如，如果RAM是只有在一個(gè)特定班級心理學(xué)一個(gè)主要可以推斷出平均成績的等級課程和在班級中的所有非心理5、學(xué)專業(yè)學(xué)生的平均成績。讀者可能會問，“如果有這么多的計(jì)算機(jī)犯罪，為什么我沒有聽說過呢？”統(tǒng)計(jì)顯示，所有的計(jì)算機(jī)犯罪中大約1%被發(fā)現(xiàn)，7%被檢測出來的犯罪被報(bào)道過。33個(gè)報(bào)道的罪犯中有一個(gè)被定罪，22000個(gè)人中有一個(gè)蹲監(jiān)獄。犯罪不被報(bào)道的原因之一是因?yàn)橐淮纬晒Φ墓艚?jīng)常顯示出一些如電可以被其他潛在的黑客攻擊。此外，他們的犯罪也經(jīng)常被視作惡作劇，當(dāng)人們發(fā)現(xiàn)了之后也不會引起足夠重視來報(bào)警。本節(jié)嘗試進(jìn)行給中威脅的分類，這種分類等級曾經(jīng)被鄧寧使用過。瀏覽介紹了主要和次要存儲器通過搜索方法搜索殘余信息。瀏覽器通常是不找什么特別的，但警惕可能有用的信息。瀏覽器可能會發(fā)現(xiàn)包含敏感信息的文件或包含有助于訪問其6、他敏感信息的信息。最有用的威懾是組織瀏覽時(shí)控制、限制用戶只能使用中獲取信息但也阻礙了瀏覽使用。泄露是信息一個(gè)未經(jīng)授權(quán)的用戶在用戶傳輸?shù)倪^程中訪問。大眾步步高游戲就是這種類型的威脅。根據(jù)推論威脅存在可能一個(gè)用戶從非敏感數(shù)據(jù)中推斷出敏感信息。這種通常是關(guān)于個(gè)人群體相關(guān)信息，來獲取有關(guān)個(gè)人信息。推論控制在下一節(jié)介紹時(shí)用來對付這種類型的威脅。篡改是指未經(jīng)過允許對那些存儲在計(jì)算機(jī)中有價(jià)值的信息作出改變的過程。一個(gè)例子是一個(gè)學(xué)生在篡改他的分?jǐn)?shù)在年級文件中。篡改可以避免如果只允許用戶修改它們自己的文件。加密檢查總結(jié)，可用于檢測篡改。這種方法使用密碼塊鏈接，如加密技術(shù)，以生成每個(gè)文件的檢驗(yàn)和。這種方法使用檢測7、技術(shù)，比如加密技術(shù)，以生成每個(gè)文件的檢驗(yàn)和。這種技術(shù)只能檢測出來改變，但不能阻止它們。數(shù)據(jù)的意外破壞雖然是無辜的，但是代價(jià)卻很昂貴。意外破壞可能是軟件和硬件缺陷。例如，有缺陷的軟件可以允許程序?qū)懭氤銎鋽?shù)據(jù)空間，并覆蓋其他用戶的數(shù)據(jù)。訪問控制技術(shù)可以用來限制覆蓋到用戶的空間，自己的數(shù)據(jù)。加密檢查總結(jié)，也可以用來檢測銷毀意外的數(shù)據(jù)。瀏覽，泄露和推理是數(shù)據(jù)安全的主要威脅，并且篡改和意外銷毀是影響數(shù)據(jù)完整性的威脅。這兩種威脅的分類既不符合保密或完整性類別的偽裝和拒絕服務(wù)。偽裝是過程，其中一個(gè)入侵者能夠訪問在另一個(gè)用戶的賬戶添加到系統(tǒng)的應(yīng)用。欺騙和猜測密碼是偽裝的威脅。首先入侵者冒充系統(tǒng)，然后入侵者冒8、充為合法用戶。拒絕服務(wù)的威脅限制合法用戶做有用的工作。這就是狡猾的用戶耗盡所有可用的資源威脅的例子。本節(jié)介紹保護(hù)機(jī)制用來提高計(jì)算機(jī)安全保護(hù)。該機(jī)制介紹分為認(rèn)證機(jī)制，訪問控制，推力控制。另外，這種滲透分析的方法，形式化驗(yàn)證技術(shù)，并將其轉(zhuǎn)換渠道分析方法進(jìn)行了介紹。認(rèn)證機(jī)制-認(rèn)證機(jī)制主要解決的是偽裝的威脅。第一個(gè)機(jī)制討論的是安全注意的關(guān)鍵。關(guān)鍵之處在于，當(dāng)通過在終端的用戶的打擊，殺死任何真實(shí)系統(tǒng)監(jiān)聽器之外運(yùn)行的終端，從而保證了可信路徑到系統(tǒng)。這將欺騙毫無戒心的用戶的嘗試。然后，重要的是使用戶集中安全注意鍵，養(yǎng)成開始與系統(tǒng)之間對話的習(xí)慣。確保這種系統(tǒng)的一個(gè)方式是，只顯示登陸提示后鍵才被按下。防止密碼猜9、測可以使用簡單的指引。每個(gè)人都應(yīng)該選擇長密碼（至少8個(gè)字符），它并不明顯，而不應(yīng)使用容易喜歡配偶的姓名或名稱猜測的密碼。此外，密碼不應(yīng)該寫在明顯的位置。另外，用戶應(yīng)該培養(yǎng)成輸入密碼時(shí)應(yīng)有合適的時(shí)間間隔。大部分的指引可以采取強(qiáng)制執(zhí)行的制度。例如，密碼的程序可以需要很長的密碼，可以核對密碼，選擇了一個(gè)明顯的密碼或報(bào)告錯(cuò)誤消息像字典的東西如果是相同的登錄（PC用戶常用的做法）。登錄程序還可以告知用戶是時(shí)候改密碼了。密碼文件存儲在系統(tǒng)可能像其他文件一樣受損。因此，它并不是好的做法存儲密碼在硬盤中。相反，一個(gè)單向函數(shù)（一個(gè)函數(shù)在計(jì)算機(jī)上是不可行的逆確定）用于確定密碼，并且存儲密碼在文件中。當(dāng)用戶的密碼在10、登錄時(shí)經(jīng)過函數(shù)運(yùn)算與存儲的結(jié)果比較，通過使用單向函數(shù)登錄文件可以公開密碼。允許訪問該對象。一個(gè)主題的能力定義或環(huán)境或主題可以直接訪問。讀者應(yīng)注意，訪問列表中的列對應(yīng)的訪問和功能對應(yīng)一行。兩種方法的一個(gè)重要方面是，無論是能力和訪問的元素必須是不可偽造的，否則，整個(gè)保護(hù)機(jī)制就會終端。一個(gè)保證這些元素的不可偽造的方法是通過一個(gè)立即通過限制訪問這些值得信賴的一段代碼。下面介紹的參考監(jiān)視器就是這樣一個(gè)機(jī)制。訪問控制訪問控制機(jī)制執(zhí)行政策汪汪包含訪問層次。也就是說，主題可能從最高到最低權(quán)限，其中自動獲得更多的特權(quán)用戶的權(quán)限的用戶的權(quán)利，至少在不同的行列。例如在UNIX系統(tǒng)的超級用戶一個(gè)主題權(quán)限可以訪問系統(tǒng)中11、的任何對象。附:英文原文An Overview of Computer SecurityAbstracComputer security consists largely of defensive methods used to detect and thwartwould-be intruders. The principles of computer security thus arise from the kinds of threats intruders can impose. This paper begins by giving examples of known securit12、y threats in existing systems. The second section presents a classification of security threats, and the last section presents some protection mechanisms and techniques for ensuring security of a computer system.This paper doesnt address the topics of physical security, communication security, and b13、reaches of trust by personnel with the access to sensitive information.Probably the most publicized threat is the result of an intruder guessing a userspassword. With the advent of personal computers, dial-up modems and proxy servers this has become much more of a problem. Penetrators have a list of14、 the commonly used passwords and they can then try them all with the aid of their personal computer. In addition, if passwords are short they are easily found by an exhaustive search 1. There are also standard accounts with default passwords that are distributed with systems, and may not have been c15、hanged.Another common threat is called spoofing. This is accomplished by fooling a user intobelieving that he/she is talking to the system, resulting in information being revealed. For instance, a spoofer can make an unsuspecting user accesses the web funneled through the spoofers machine, allowing 16、the spoofer to monitor all of the victims activities including any passwords or account numbers the victim enters. The spoofer can also cause false or misleading data to be sent to web servers in the victims name, or to the victim in the name of any web server. the attacker observes and controls eve17、rything the victim does on the web.Another example that can be cited for instance, the spoofer may display what looks like the system login prompt on a terminal to make the terminal appear to be idle. Then when an unsuspecting user begins to communicate with the terminal, the spoofer retrieves the l18、ogin name and asks for the users password. After obtaining the information, the spoofer displays a try again message or something and returns ownership that was previously obtained by himAnother threat is user browsing for sensitive information. This occurs when a legitimateuser peruses any files th19、at are available and gleans useful information. For instance, a browser may locate a password inadvertently left in a publicly readable file.A more sophisticated threat, commonly known as the Trojan horse, is the result of aprogram doing more that it is supposed to or its a program that appears to d20、o something good,while its actually doing something nasty in the background. For instance, a backgammon program or some software may be made public. However, when the unsuspecting plays against the program, the Trojan horse, executing with the users own access rights to his files surreptitiously rea21、ds the users files and might even also mail them to the creator of the game, if the user has himself logged onto the net. Recently, we saw reputed internet sites susceptible to what are called distributed denial of service attacks. Hackers using Trojans mastermind these attacks.Another threat is the22、 result of a devious user exhausting a shared resource so thatlegitimate users cannot complete the work. For instance, the devious user of a network front-end might use all of the available message buffers making it impossible for the legitimate users to accomplish any useful work. The intentional c23、rashing of the system causing all work to a halt is a further example of this type of threat.Another class of threats is the result of a user of a statistical database being able to infer sensitive data from non-sensitive information returned by the database. For instance, if Ram is the only psychol24、ogy major in a particular class one can deduce Rams grade from the average grade of the course and the average grade of all non-psychology majors in the class.The reader may be asking , “If there is so much computer crime why havent I hearedabout it?” Statistics show that approximately 1% of all com25、puter crime is detected , 7% of the detected crrimes are reproted , 1 out of 33 criminals reported are convicted, and 1 out of 22,000 ends up in jail 3. One reason crimes are not reported is that a successful attack often reveals vulnerabilities that can be exploited by other potential attackers. Fu26、rthermore, may of the crimes are viewed as pranks, and the people who detect them do not think they are serious enough to report to the police 4.This section attempts to categorise the various threats. The classifications used were first used by Denning 2.Browsing describes the method of searching t27、hrough main and secondary memory forresidue information. The browser is usually not looking for anything in particular, but is alert to possibly useful information. The breowser may find files containing sensitive information or containing information that helps to access other sensisitive informati28、on. The most useful deterrent to browsing is the use of controls that restrict users to only accessing information in their own data space.Enciphering data also deters browsing.Leakage is the transmission of information to an unauthorised user from a process that is allowed to access the data. The p29、ublic backgammon game is this type of threat.An inference threat exists if a user can deduce sensitive information from non-sensitivedata. This is usually the result of correlating information about groups of individuals to obtain information about an individual. The inference controls presented in 30、the next section are used to counter this type of threat.Tampering refers to the processof making unauthorised changestothevalue ofinformation stored in the computer. An example of tampering is a student changing his/her grade in the grade file. Tampering is avoided by allowing users to modify only 31、their files. Cryptography check summing can be used for detecting tampering. This method uses cryptographic techniques, such as cipher block chaining, to generate a check sum for each file. The technique only detects changes; it doesnt prevent them.Accidental data destruction although often innocent32、, can be costly. Accidentaldestruction may be caused by both hardware and software failures. For instance, faulty software could allow a program to write beyond its data space and overwrite another users data. Access control techniques can be used to limit overwriting to the users own data space. Cr33、yptographic check summing can also be used for detecting accidental data destruction.Browsing, leakage and inference are threats to the secrecy of data, tampering andaccidental destructions are threats to the integrity of data. Two threat classifications that fit into neither the secrecy or integrit34、y category are masquerading and denial of service. Masquerading refers to the process where an intruder gains access to the stystem under another users account. Spoofing and pasword guessing are masquerading threats. In the first the intruder is posing as the system, in the second the intruder is po35、sing as a legitimate user.Denial of service threats prevent legitimate users from getting useful work done. Thedevious user exhausting all available resources is an example of this threat.This section intoduces protection mechnasims used to enhance computer security. Themechnasims presented are grou36、ped into authentication mechnasims, access control, and inference control. In additon, the methods of penetration analysis, formal verification techniques, and convert channel analysis are introduced.Authentication Mechanisms Authetication mechanisms primarily address themasquerading threat. The fir37、st mechanism discussed is the secure attention key. This key, when hit by a user at a terminal, kills any process running at the terminal except the true system listener and thus guarantees a trusted path to the system. This will foil attempts at spoofing the unsuspecting user. However, it is import38、ant that users make a habit of always hitting the secure attention key to begin a dialogue with the system. One way of ensuring this for the system to only display the login prompt after the key is depressed.Simple guidelines can be used to deter password guessing. One should choose a longpassword (39、at least 8 characters) that is not obvious, and should not use easily guessable passwords like a spouses name or a login name. In addition, a password should not be written in the obvious place. Furthermore, users should be trained to change their passwords at appropriate intervals. Most of the guid40、elines can be enforced by the system. For instance, password program can require long passwords and can check the password chosen against a dictionary of obvious passwords or something like reporting an error message if the password is the same as the login(a common practise by an average pc user). 41、The login program can also inform the user that it is time to change passwords.Password files stored in the system may be compromised like any other file. Therefore, it is not good practise to store passwords in the clear. Instead, a one way function (i.e., a function whose inverse is computationall42、y infeasible to determine) is used to enchiper passwords and the result is stored in the password file. When a users password is presented at the login time it is enchipered and compared to the stored value. By using one way functions to enchiper passwords the login file can be made public.Access Co43、ntrol Assuming that by using authentication mechanisms and good password practice the system can guarantee that users are who they claim to be, the next step is to provide a means of limiting a users access to only those files that policy determines should be accessed. These controls are referred to44、 as access control.When describing access control policies and mechanisms it is necessary to consider the subjects and objects of the system. Subjects are the users of the system along with any active entities that act on behalf of the user or the system (eg. user processes). Objects are the resourc45、es or entities of the system (eg. files, programs, devices). The access control mechanism determines for each subject what access modes such as read (R), write (W), or execute (X), it has for each object.A convenient way of describing a protection system is with an access matrix . In theaccess matri46、x rows correspond to subjects and columns correspond to objects. Each enrty in the matrix is a set of access rights that indicate the access that the subject associated with the row has for the object associated with the column. The following is an example access matrix. From the matrix one can dete47、rmine that subject S3 has read and write access to the object O2 and execute access to the object O3.An example of access matrix There are two common ways of representing an access matrix in a computer system: access control lists (sometimes called authorization lists) and capability lists (often ca48、lled c-lists). With the access list approach each object has an access list associated with it. This list contains the name of each subject that has access to the object along with the modes of access allowed. In contrast the capability list approach associates a list with each subject. The elements49、 of the list are capabilities which can be thought of as tickets that contain an objects name and the modes ofaccess allowed to the object. A subjects capability defines the environment or domain that the subject may directly access.The reader should note that an access list corresponds to a column 50、in the access and acapability corresponds to a row. An important aspect of either approach is that both thecapabilities and the elements of access must be unforgeable or else the entire protection mechanism breaks down. One way of guaranteeing the unforgeability of these elements is by restricting a51、ccess to them through an intermediatry trusted piece of code. The reference monitor introduced below is one such mechanism. Access control policies enforced by the access control mechanisms often incorporate access hierarchies. That is, subjects may have different ranks ranging from the most to the least privileged, where the more privileged user automatically gets the rights of the least privileged user. For instance, in a UNIX system a subject with the superuserprivilege can access any object in the system.