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VirusAre “Good” Computer Viruses Still a Bad Idea?During the past six years, computer viruses have caused unaccountable amount of damage – mostly due to loss of time and resources. For most users, the term “computer virus” is a synonym of the worst nightmares that can happen ontheir system. Yet some well-known researchers keep insisting that it is possible to use the replication mechanism ofthe viral programs for some useful and beneficial purposes.This paper is an attempt to summarize why exactly the general public appreciates computer viruses as somethinginherently bad. It is also considering several of the proposed models of “beneficial” viruses and points out theproblems in them. A set of conditions is listed, which every virus that claims to be beneficial must conform to. At last, a realistic model using replication techniques for beneficial purposes is proposed and directions are given in which this technique can be improved further.The paper also demonstrates that the main reason for the conflict between those supporting the idea of a “beneficial virus” and those opposing it, is that the two sides are assuming a different definition of what a computer virus is.1. What Is a Computer Virus?The general public usually associates the term “computer virus” with a small, nasty program, which aims to destroy the information on their machines. As usual, the general public’s understanding of the term is incorrect. There are many kinds of destructive or otherwise malicious computer programs and computer viruses are only one of them. Suchprograms include backdoors, logic bombs, trojan horses and so on [Bontchev94]. Furthermore, many computerviruses are not intentionally destructive – they simply display a message, play a tune, or even do nothing noticeable at all. The important thing, however, is that even those not intentionally destructive viruses are not harmless – they are causing a lot of damage in the sense of time, money and resources spent to remove them – because they are generallyunwanted and the user wishes to get rid of them.A much more precise and scientific definition of the term “computer virus” has been proposed by Dr. Fred Cohen in his paper [Cohen84]. This definition is mathematical – it defines the computer virus as a sequence of symbols on thetape of a Turing Machine. The definition is rather difficult to express exactly in a human language, but an approximateinterpretation is that a computer virus is a “program that is able to infect other programs by modifying them to include a possibly evolved copy of itself”.Unfortunately, there are several problems with this definition. One of them is that it does not mention the possibility of a virus to infect a program without modifying it – by inserting itself in the execution path. Some typical examples are the boot sector viruses and the companion viruses [Bontchev94]. However, this is a flaw only of the human-language expression of the definition – the mathematical expression defines the terms “program” and “modify”in a way that clearly includes the kinds of viruses mentioned above.A second problem with the above definition is its lack of recursiveness. That is, it does not specify that after infecting a program, a virus should be able to replicate further, using the infected program as a host. Another, much more serious problem with Dr. Cohen’s definition is that it is too broad to be useful for practical purposes. In fact, his definition classifies as “computer viruses” even such cases as a compiler which is compiling its own source, a file manager which is used to copy itself, and even the program DISKCOPY when it is on diskette containing the operating system – because it can be used to produce an exact copy of the programs on this diskette.In order to understand the reason of the above problem, we should pay attention to the goal for which Dr. Cohen’s definition has been developed. His goal has been to prove several interesting theorems about the computationalaspects of computer viruses [Cohen89]. In order to do this, he had to develop a mathematical (formal) model of thecomputer virus. For this purpose, one needs a mathematical model of the computer. One of the most commonly usedmodels is the Turing Machine (TM). Indeed, there are a few others (e.g., the Markoff chains, the Post Machine, etc.),but they are not as convenient as the TM and all of them are proven to be equivalent to it.Unfortunately, in the environment of the TM model, we cannot speak about “programs” which modify “otherprograms” – simply because a TM has only one, single program – the contents of the tape of that TM. That’s whyCohen’s model of a computer virus considers the history of the states of the tape of the TM. If a sequence of symbolson this tape appears at a later moment somewhere else on the tape, then this sequence of symbols is said to be acomputer virus for this particular TM. It is important to note that a computer virus should be always considered asrelated to some given computing environment – a particular TM. It can be proven ([Cohen89]) that for any particularTM there exists a sequences of symbols which is a virus for that particular TM.Finally, the technical computer experts usually use definitions for the term “computer virus”, which are less precisethan Dr. Cohen’s model, while in the same time being much more useful for practical reasons and still being muchmore correct than the general public’s vague understanding of the term. One of the best such definitions is ([Seborg]): “We define a computer ‘virus’ as a self-replicating program that can ‘infect’ other programs by modifying them or their environment such that a call to an ‘infected’ program implies a call to a possibly evolved, and in most cases, functionally similar copy of the ‘virus’.”The important thing to note is that a computer virus is a program that is able to replicate by itself. The definition doesnot specify explicitly that it is a malicious program. Also, a program that does not replicate is not a virus, regardless ofwhether it is malicious or not. Therefore the maliciousness is neither a necessary, nor a sufficient property for aprogram to be a computer virus.Nevertheless, in the past ten years a huge number of intentionally or non intentionally destructive computer viruseshave caused an unaccountable amount of damage – mostly due to loss of time, money, and resources to eradicate them- because in all cases they have been unwanted. Some damage has also been caused by a direct loss of valuableinformation due to an intentionally destructive payload of some viruses, but this loss is relatively minor whencompared to the main one. Lastly, a third, indirect kind of damage is caused to the society – many users are forced tospend money on buying and time on installing and using several kinds of anti-virus protection.Does all this mean that computer viruses can be only harmful? Intuitively, computer viruses are just a kind oftechnology. As with any other kind of technology, they are ethically neutral – they are neither “bad” nor “good” – it isthe purposes that people use them for that can be “bad” or “good”. So far they have been used mostly for bad purposes. It is therefore natural to ask the question whether it is possible to use this kind of technology for good purposes.Indeed, several people have asked this question – with Dr. Cohen being one of the most active proponents of the idea[Cohen91]. Some less qualified people have attempted even to implement the idea, but have failed miserably (seesection 3). It is natural to ask – why? Let’s consider the reasons why the idea of a “good” virus is usually rejected by thegeneral public. In order to do this, we shall consider why people think that a computer virus is always harmful andcannot be used for beneficial purposes.2. Why Are Computer Viruses Perceived as Harmful?About a year ago, we asked the participants of the electronic forum Virus-L/comp.virus, which is dedicated todiscussions about computer viruses, to list all reasons they could think about why do they perceive the idea of a”beneficial” virus as a bad one. What follows is a systematized and generalized list of those reasons.2.1. Technical ReasonsThis section lists the arguments against the “beneficial virus” idea, which have a technical character. They are usuallythe most objective ones.2.1.1. Lack of ControlOnce released, the person who has released a computer virus has no control on how this virus will spread. It jumpsfrom machine to machine, using the unpredictable patterns of software sharing among the users. Clearly, it can easilyreach systems on which it is not wanted or on which it would be incompatible with the environment and would causeunintentional damage. It is not possible for the virus writer to predict on which systems the virus will run andtherefore it is impossible to test the virus on all those systems for compatibility. Furthermore, during its spread, acomputer virus could reach even a system that had not existed when that virus has been created – and therefore it hadbeen impossible to test the virus for compatibility with this system.The above is not always true – that is, it is possible to test the virus for compatibility on a reasonably large number ofsystems that are supposed to run it. However, it is the damaging potential of a program that is spreading out of controlwhich is scaring the users.2.1.2. Recognition DifficultyCurrently a lot of computer viruses already exist, which are either intentionally destructive or otherwise harmful. There are a lot of anti-virus programs designed to detect and stop them. All those harmful viruses are not going todisappear overnight. Therefore, if one develops a class of beneficial viruses and people actually begin to use them,then the anti-virus programs will have to be able to make the difference between the “good” and the “bad” viruses – inorder to let the former in and keep the latter out.Unfortunately, in general it is theoretically impossible even to distinguish between a virus and a non-viral program([Cohen89]). There is no reason to think that distinguishing between “good” and “bad” viruses will be much easier. While it might be possible to distinguish between them using virus-specific anti-virus software (e.g., scanners), weshould not forget that many people are relying on generic anti-virus defenses, for instance based on integrity checking. Such systems are designed to detect modifications, not specific viruses, and therefore will be triggered by the”beneficial” virus too, thus causing an unwanted alert. Experience shows that the cost of such false positives is thesame as of a real infection with a malicious virus – because the users waste a lot of time and resources looking for anon-existing problem.2.1.3. Resource WastingA computer virus would eat up disk space, CPU time, and memory resources during its replication. A computer virusis a self-replicating resource eater. One typical example is the Internet Worm, accidentally released by aCarnegie-Mellon student. It was not designed to be intentionally destructive, but in the process of its replication, themultiple copies of it used so much resources, that they practically brought down a large portion of the Internet.Even when the computer virus uses a limited amount of resources, it is considered as a bad thing by the owner of themachine on which the virus is doing it, if it happens without authorization.2.1.4. Bug ContainmentA computer virus can easily escape the controlled environment and this makes it very difficult to test such programsproperly. And indeed – experience shows that almost all computer viruses released so far suffer from significant bugs,which would either prevent them from working in some environments, or even cause unintentional damage in thoseenvironments.Of course, any program can (and usually does) contain bugs. This is especially true for the large and complex softwaresystems. However, a computer virus is not just a normal buggy program. It is a self-spreading buggy program, which isout of control. Even if the author of the virus discovers the bug at a later time, there is the almost untreatable problemof revoking all existing copies of the virus and replacing them with fixed new versions.2.1.5. Compatibility ProblemsA computer virus that can attach itself to any of the user’s programs would disable the several programs on the marketthat perform a checksum on themselves at runtime and refuse to run if modified. In a sense, the virus will perform adenial-of-service attack and thus cause damage.Another problem arises from some attempts to solve the “lack of control” problem by creating a virus that asks forpermission before infecting. Unfortunately, this causes an interruption of the task being currently executed until theuser provides the proper response. Besides of being annoying for the user, it could be sometimes even dangerous. Consider the following example.It is possible that a computer is used to control some kind of life-critical equipment in a hospital. Suppose that such acomputer gets infected by a “beneficial” computer virus, which asks for permission before infecting any particularprogram. Then it is perfectly possible that a situation arises, when a particular program has to be executed for the firsttime after the virus has appeared on the computer, and that this program has to urgently perform some task which iscritical for the life of a patient. If at that time the virus interrupts the process with the request for permission to infectthis program, then the caused delay (especially if there is no operator around to authorize or deny the request) couldeasily result in the death of the patient.2.1.6. EffectivenessIt is argued that any task that could be performed by a “beneficial” virus could also be performed by a non-replicatingprogram. Since there are some risks following from the capability of self-replication, it would be therefore muchbetter if a non-replicating program is used, instead of a computer virus.2.2. Ethical and Legal ReasonsThe following section lists the arguments against the “beneficial virus” idea, which are of ethical or legal kind. Sinceneither ethics, nor the legal systems are universal among the human society, it is likely that those arguments will have

different strength in the different countries. Nevertheless, they have to be taken into account.2.2.1. Unauthorized Data ModificationIt is usually considered unethical to modify other people’s data without their authorization. In many countries this isalso illegal. Therefore, a virus which performs such actions will be considered unethical and/or illegal, regardless ofany positive outcome it could bring to the infected machines. Sometimes this problem is perceived by the users as “thevirus writer claims to know better than me what software should I run on my machine”.2.2.2. Copyright and Ownership ProblemsIn many cases, modifying a particular program could mean that copyright, ownership, or at least technical supportrights for this program are voided.We have witnessed such an example at the VTC-Hamburg. One of the users who called us for help with a computervirus was a sight-impaired lawyer, who was using special Windows software to display the documents he was workingon with a large font on the screen – so that he could read them. His system was infected by a relatively non-damagingvirus. However, when the producer of the software learned that the machine was infected, they refused any technicalsupport to the user, until the infection was removed and their software – installed from clean originals.2.2.3. Possible MisuseAn attacker could use a “good” virus as a means of transportation to penetrate a system. For instance, a person withmalicious intent could get a copy of a “good” virus and modify it to include something malicious. Admittedly, anattacker could trojanize any program, but a “good” virus will provide the attacker with means to transport hismalicious code to a virtually unlimited population of computer systems. The potential to be easily modified to carrymalicious code is one of the things that makes a virus “bad”.2.2.4. ResponsibilityDeclaring some viruses as “good” and “beneficial” would just provide an excuse to the crowd of irresponsible viruswriters to condone their activities and to claim that they are actually doing some kind of “research”. In fact, this isalready happening – the people mentioned above are often quoting Dr. Fred Cohen’s ideas for beneficial viruses as anexcuse of what they are doing – often without even bothering to understand what Dr. Cohen is talking about.2.3. Psychological ReasonsThe arguments listed in this section are of psychological kind. They are usually a result of some kind ofmisunderstanding and should be considered an obstacle that has to be “worked around”.2.3.1. Trust ProblemsThe users like to think that they have full control on what is happening in their machine. The computer is a verysophisticated device. Most computer users do not understand very well how it works and what is happening inside. The lack of knowledge and uncertainty creates fear. Only the feeling that the reactions of the machine will be alwaysknown, controlled, and predictable could help the users to overcome this fear.However, a computer virus steals the control of the computer from the user. The virus activity ruins the trust that theuser has in his/her machine, because it causes the user to lose his/her belief that s/he can control this machine. Thismay be a source of permanent frustrations.2.3.2. Negative Common MeaningFor most people, the word “computer virus” is already loaded with negative meaning. The media has already widelyestablished the belief that a computer virus is a synonym for a malicious program. In fact, many people call “viruses”many malicious programs that are unable to replicate – like trojan horses, or even bugs in perfectly legitimatesoftware. People will never accept a program that is labelled as a computer virus, even if it claims to do somethinguseful.3. Some Bad Examples of “Beneficial” VirusesRegardless of all the objections listed in the previous section, several people have asked themselves the questionwhether a computer virus could be used for something useful, instead of only for destructive purposes.And several people have tried to positively answer this question. Some of them have even implemented their ideas inpractice and have been experimenting with them in the real world – unfortunately, without success. In this section weshall present some of the unsuccessful attempts to create a beneficial virus so far, and explain why they have beenunsuccessful.3.1. The “Anti-Virus” VirusSome computer viruses are designed to work not only in a “virgin” environment of infectable programs, but also onsystems that include anti-virus software and even other computer viruses. In order to survive successfully in suchenvironments, those viruses contain mechanisms to disable and/or remove the said anti-virus programs and”competitor” viruses. Examples for such viruses in the IBM PC environment are Den_Zuko (removes the Brain virusand replaces it with itself), Yankee_Doodle (the newer versions are able to locate the older ones and “upgrade” theinfected files by removing the older version of the virus and replacing it with the newer one), Neuroquila (disablesseveral anti-virus programs), and several other viruses.Several people have had the idea to develop the above behaviour further and to create an “anti-virus” virus – a viruswhich would be able to locate other (presumably malicious) computer viruses and remove them. Such aself-replicating anti-virus program would have the benefits to spread very fast and update itself automatically.Several viruses have been created as an implementation of the above idea. Some of them locate a few known virusesand remove them from the infected files, others attach themselves to the clean files and issue an error message ifanother piece of code becomes attached after the virus (assuming that it has to be an unwanted virus), and so on. However, all such pieces of “self-replicating anti-virus software” have been rejected by the users, who have consideredthe “anti-virus” viruses just as malicious and unwanted as any other real computer virus. In order to understand why, itis enough to realize that the “anti-virus viruses” matches several of the rules that state why a replicating program isconsidered malicious and/or unwanted. Here is a list of them for this particular idea.First, this idea violates the Control condition. Once the “anti-virus” virus is released, its author has no means tocontrol it.Second, it violates the Recognition condition. A virus that attaches itself to executable files will definitely trigger theanti-virus programs based on monitoring or integrity checking. There is no way for those programs to decide whetherthey have been triggered by a “beneficial” virus or not.Third, it violates the Resource Wasting condition. Adding an almost identical piece of code to every executable file onthe system is definitely a waste – the same purpose can be achieved with a single copy of the code and a single file,containing the necessary data.Fourth, it violates the Bug Containment condition. There is no easy way to locate and update or remove all instancesof the virus.Fifth, it causes several compatibility problems, especially to the selfchecking programs, thus violating theCompatibility condition.Sixth, it is not as effective as a non-viral program, thus violating the Effectiveness condition. A virus-specificanti-virus program has to carry thousands of scan strings for the existing malicious viruses – it would be veryineffective to attach a copy of it to every executable file. Even a generic anti-virus (i.e., based on monitoring orintegrity checking) would be more effective if it exists only in one example and is executed under the control of theuser.Seventh, such a virus modifies other people’s programs without their authorization, thus violating the UnauthorizedModification condition. In some cases such viruses ask the user for permission before “protecting” a file by infectingit. However, even in those cases they cause unwanted interruptions, which, as we already demonstrated, in somesituations can be fatal.Eight, by modifying other programs such viruses violate the Copyright condition.Ninth, at least with the current implementations of “anti-virus” viruses, it is trivial to modify them to carry destructivecode – thus violating the Misuse condition.Tenth, such viruses are already widely being used as examples by the virus writers when they are trying to defend theirirresponsible actions and to disguise them as legitimate research – thus the idea violates the responsibility conditiontoo.As we can see from the above, the idea of a beneficial anti-virus virus is “bad” according to almost any of the criterialisted by the users.3.2. The “File Compressor” VirusThis is one of the oldest ideas for “beneficial” viruses. It is first mentioned in Dr. Cohen’s original work [Cohen84]. The idea consists of creating a self-replicating program, which will compress the files it infects, before attaching itselfto them. Such a program is particularly easy to implement as a shell script for Unix, but it is perfectly doable for thePC too. And it has already been done – there is a family of MS-DOS viruses, called Cruncher, which appends itself tothe executable files, then compresses the infected file using Lempel-Zev-Huffman compression, and then prepends asmall decompressor which would decompress the file in memory at runtime.Regardless of the supposed benefits, this idea also fails the test of the criteria listed in the previous section. Here iswhy.First, the idea violates the Control condition. Once released, the author of the virus has no means to controls itsspread. In the particular implementation of Cruncher, the virus writer has attempted to introduce some kind of control. The virus asks the user for permission before installing itself in memory, causing unwanted interruptions. It is alsopossible to tell the virus to install itself without asking any questions – by the means of setting an environmentvariable. However, there are no means to tell the virus not to install itself and not to ask any questions – which shouldbe the default action.Second, the idea violates the Recognition condition. Several virus scanners detect and recognize Cruncher by name,the process of infecting an executable triggers most monitoring programs, and the infected files are, of course,modified, which triggers most integrity checkers.Third, the idea violates the Resource condition. A copy of the decompressor is present in every infected file, which isobviously unnecessary.Fourth, the idea violates the Bug Containment condition. If bugs are found in the virus, the author has no simplemeans to distribute the fix and to upgrade all existing copies of the virus.Fifth, the idea violates the Compatibility condition. There are many files which stop working after being compressed. Examples include programs that perform a self-check at runtime, self-modifying programs, programs with internaloverlay structure, Windows executables, and so on. Admitedly, those programs stop working even after beingcompressed with a stand-alone (i.e., non-viral) compression program. However, it is much more difficult to compressthem by accident when using such a program – quite unlike the case when the user is running a compression virus.Sixth, the idea violates the Effectiveness condition. It is perfectly possible to use a stand-alone, non-viral program tocompress the executable files and prepend a short decompressor to them. This has the added advantage that the codefor the compressor does not have to reside in every compressed file, and thus we don’t have to worry about its size orspeed – because it has to be executed only once. True, the decompressor code still has to be present in each compressedfile and many programs will still refuse to work after being compressed. The solution is to use not compression at afile level, but at a disk level. And indeed, compressed file systems are available for many operating environments(DOS, Novell, OS/2, Unix) and they are much more effective than a file-level compressor that spreads like a virus.Seventh, the idea still violates the Copyright condition. It could be argued that it doesn’t violate the Data Modificationcondition, because the user is asked to authorize the infection. We shall accept this, with the remark mentioned above -that it still causes unwanted interruptions. It is also not very trivial to modify the virus in order to make it malicious,so we’ll assume that the Misuse condition is not violated too – although no serious attempts are made to ensure thatthe integrity of the virus has not been compromised.Eighth, the idea violates the responsibility condition. This particular virus – Cruncher – has been written by the sameperson who has released many other viruses – far from “beneficial” ones – and Cruncher is clearly used as an attempt tocondone virus writing and to masquerade it as legitimate “research”.3.3. The “Disk Encryptor” VirusThis virus has been published by Mark Ludwig – author of two books and a newsletter on virus writing, and of severalreal viruses, variants of many of which are spreading in the real world, causing real damage.The idea is to write a boot sector virus, which encrypts the disks it infects with a strong encryption algorithm (IDEA inthis particular case) and a user-supplied password, thus ensuring the privacy of the user’s data. Unfortunately, this ideais just as flawed as the previous ones.First, it violates the Control condition. True, the virus author has attempted to introduce some means of control. Thevirus is supposed to ask the user for permission before installing itself in memory and before infecting a disk. However, this still causes unwanted interruptions and reportedly in some cases doesn’t work properly – that is, thevirus installs itself even if the user has told it not to.Second, it violates the Recognition condition. Several virus-specific scanners recognize this virus either by name or asa variant of Stealth_Boot, which it actually is. Due to the fact that it is a boot sector infector, it is unlikely to triggerthe monitoring programs. However, the modification that it causes to the hard disk when infecting it, will trigger mostintegrity checkers. Those that have the capability to automatically restore the boot sector, thus removing any possiblypresent virus, will cause the encrypted disk to become inaccessible and therefore cause serious damage.Third, the idea violates the Compatibility condition. A boot sector virus that is permanently resident in memoryusually causes problems to Windows

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