An overview on security in networked computer games

Scientia Estudos Interdisciplinares em Computaç~ ao 15(2): 166--173, julho/dezembro 2004 c 2004 by Unisinos An overview on security in networked computer games Börje Felipe Fernandes Karlsson, Bruno Feijó ICAD/IGAMES/VisionLab Computer Science Department - PUC-Rio Rio de Janeiro, RJ - Brazil {borje, bruno}@inf.puc-rio.br Abstract Network security issues and game exploits can undermine the gameplay experience of networked games. With this in mind, this work tries to draw attention of the academic community to the issue and, through a vast bibliography, present: why research in this area matters, the state of current research in the field, a categorization of cheating types and, also, some possible counter measures. And thus help addressing the serious lack of information regarding the subject. Keywords: network security, multiplayer games, cheating, exploits, computer games. Resumo Jogos em rede podem ter sua jogabilidade bastante prejudicada por questões de segurança em redes e pela presença de falhas internas ao jogo. Com isso em mente, este trabalho tenta atrair a atenção da comunidade acadêmica para a área e, através de uma ampla bibliografia, apresentar: o porque da pesquisa nessa área ser relevante, qual o estado atual das pesquisas na área, uma categorização dos tipos de “trapaças” e, também, algumas possı́veis contra-medidas. Deste modo, contribuindo para reverter a séria falta de informação relacionada ao assunto. Palavras–Chave: segurança em redes, jogos multi-usuário, cheating, exploits, jogos de computador; 1 Introduction Networked games, be they multi-player or online in some other way, have been trough a tremendous growth during the last years [IDSA, 2001]. As also have been new network technologies as wideband and wireless [Clarke, 2001] networks. Consequently, attention dedicated to the networking aspects of games has risen drastically, but very few developers and researchers have taken security into account. Another fundamental point is that the player experience when interacting with a game is key to its success [Costikyan, 1999]. Communities are developed around games and are essential to extend their life cycles; games become a service, not only a product as before. And a secure and fair environment is a major factor in maintaining this [Costikyan, 2000]. These issues caused a change of focus from monouser to multi-user games, not only for PC game deVolume 15 · No 2 · julho/dezembro 2004 velopers but also for console game producers [Handy, 2002], and also changed security initiatives focus traditionally related to preventing piracy [Dodd, 2001]. From the point of view of research, multi-user games are the vanguard of network possibilities utilization. Although there is research in related areas (as military simulations and virtual reality systems), the solutions presented differ more than a little from the problems found in networked games. And when the academic research is directly applied to games, most of it is focused on usability and gameplay issues [Schaefer et al ., 2002], network resources management, concepts of distribution and traffic analysis [Färber, 2002] and [LaPointe and Winslow, 2001]. Very little of this research effort is directed to security issues and concerns. The objective of this work is to draw the attention of the academic community and help understanding and dealing with its various aspects, since (as already B. KARLSSON, B. FEIJÓ stated) little research has been focused in the field of networked digital games and there is a serious lack of information regarding the subject. This goal is pursued with the help of an extensive bibliography. A survey of research in networked games is presented in section 2 in order to show where current research is focused, to point some security issues in this kind of environment, and to try to demonstrate the importance of security in networked games. In section 3, a categorization of the common problems (specially cheating) in the field of networked games security is presented. Some suggestions to counter these issues and related available literature, as well as research and industry state of the art efforts are also briefly discussed in section 4. Finally, we stress the importance of future research in networked games security both by industry and academia. 2 Current research Regarding current research on networked games, [IGDA, 2001] presents a general overview of changes on the field of online games, including: distribution scheme changes, patching, community development, the wireless market, more and more casual gamers (lower hardcore player percentage), raise in the availability of networking middleware, on-line payment issues; and also brings the topics of security and cheating to discussion. Cheating (as it’s commonly called the act of gaining “illegal” advantages on a game) is a central issue in networked games, one has to understand the threats and kinds of cheating, and (at the same time) remember that networked games have the same security requirements as more traditional networked applications such as electronic commerce (privacy, financial transaction integrity, etc.) [Davis, 2001]. One should keep the game integrity, offering fair and honest games and protecting users from fraud as much as possible. Only a small percentage of cheaters are vandals, most of them want to reach a dominant position and beat the other players. There are several kinds of incentive to player participation on game sessions (Davis [Davis, 2001] lists: entertainment, prestige, “virtual property”, prizes and money) and as this incentives grow, risks and consequences of cheats also grow. Besides that, one must remember that interacting with the community is a major part of game play ([Greenhill, 1997] shows that usually less than 35% of players cheat and that it is more than enough to deteriorate honest player experience). As pointed by Yan and Choi [Yan and Choi, Volume 15 · No 2 · julho/dezembro 2004 167 2001], to make matters worse, online game security is one of the areas where the domain specialists (game developers) are not experts (security engineers), and security specialists aren’t familiarized with the necessary domain knowledge (which seems to be simple at first, but is not). One relevant literature overview of network research is presented by Smed et al. [Smed et al., 2002] focused on research in military simulation research, NVEs (networked virtual environments) and MCGs (multiplayer computer games). It shows that the focus of scientific research in the area has changed, from military simulations in the 1980s to NVEs in the 1990s and now to MCGs. Since the 1980s, the US Department of Defense has developed military applications using protocols for distributed interactive simulations (DIS) [Macedonia, 1995] and [Neyland, 1997]. Military research focused then in developing a high level architecture (HLA), that tried to provide services and a general architecture for distributed data exchange. While DIS are more closely related to military applications, the ideas behind HLAs can also be applied in non military applications (as games, for example) and although this promises have not been reached, there has been a certain cooperation among the military and the entertainment industry [Capps et al., 2001], but with almost no focus on security. Several issues contribute to the lack of research in games, especially on networked games and even more on networked games security. Computer games generally were viewed as toys and “not serious” as a real application. Lately this scenario has changed and now games research is growing in a large pace. But security remains a subject not very well explored. Most of the available literature related to computer games and networks is headed towards communication and control. [Cronin et al., 2001] gives a comparison and shows how client/server architectures are simple to implement e allow for a good control over game state, peer-to-peer have a bigger latency but eliminates de bottleneck on the server; and proposes a mirrored-servers architecture that requires a complex consistency protocol, but provides low latency and allows for a administrative control over the game state. One advantage of using a server architecture is that it can dynamically filter information before sending it to the clients. In a simple scenario, if a secure server manages the game state, cheating opportunities are greatly reduced. Administrative control is trivial in client/server architectures. In P2P architectures, most of the code (if not all of it) necessary in order to run the game has to be located on the client machines, which are highly vulnerable. A communi- 168 cation architecture based on a publisher/subscriber model is proposed by [Fiedler et al., 2002], paying special attention to scalability (world size, amount of players and network infrastructure) and dynamic system evolution, and naturally being indicated to use in MMOGs (Massive Multiplayer Online Games). When taking into account network usability requirements for multi-user games it is extremely important to have the target network properties in mind. In Internet case, one can not expect too much from bandwidth and latency. An explanation for this is presented in [Ng, 1997] along with showing that the sensation of network lag on the client side is a critical issue during game sessions. In order to try to avoid these sort of problems, many times a complete re-write of the game engine is necessary; as is the case in [Lincroft, 1999] that makes the decision of only sending player-action specific information through the network. These actions are then collected in order to define the current game environment state. Additionally, the game was structured in such way that one of the players’ machines is the gamehost, responsible for gathering this information and broadcasting an identical set of data to the other ones, which must process the info keeping the game environment on the same state. However, this approach introduced synchronization problems. Bettner and Terrano in turn, show in [Bettner and Terrano, 2001] the approach used to counter the fact that each simulation step can consume a highly variable amount of time. Instead of sending the state of every unit, they had the expectative of running exactly the same simulation on each machine, which would need to be tightly synchronized; this solution was reached through a synchronized random number generator and as now the game depended on every host executing exactly the same simulation, it became much harder to tamper with the client software and even with communication packets. If that was not enough, every command was checked if valid both on when sending and receiving. Mauve [Mauve, 2000b] discusses which problems can arise due to state inconsistencies in systems part of digital virtual environments (DVEs) and describes a timewarp approach to handle them. However, no experimental study was done and no quantitative result is shown. The same author also presents a study on consistency in interactive continuous media [Mauve, 2000a]. Pantel and Wolf [Pantel and Wolf, 2002] analyze the dead reckoning technique when applied to digital game environments. This technique is much used when trying to reduce network problems as delays and packet losses by trying to predict objects’ fu- SECURITY IN NETWORKED COMPUTER GAMES ture state/positions. The quality of this prediction and its consistency depend on the difference between the real position and the predicted ones for each object. Bernier [Bernier, 2001] also presents latency compensation methods, and raises the possibility of a cheater tampering with the client software to gain unfair advantages by abusing the implemented compensation measures but states that the presence of jitter is worse than the possible inconsistencies caused by not using compensation. In order to avoid the cheat known as time-cheat (that allows for a player to abuse the system and end up “fortune telling”, what would give him more time to react to the rest of the players movements), Davis presents an approach that tries to correct dead reckoning issues [Cronin et al., 2003]. Discussing multi-peer related architectures, Diot describes the design, implementation and evaluation of the MiMaze architecture [Gautier and Diot, 1998], a simple distributed multi-user game, but the pioneer in analyzing a distributed interactive game using multicast on the Internet [Diot and Gautier, 1999]. He shows that interactivity can indeed be kept if some level of inconsistencies can be tolerated and that it is possible to use a multi-peer approach instead of the client-server one that is commonly used. He goes on further to say that such approach can provide good performance and probably a better scalability. Regarding more commercial approaches, in [Dionne et al., 2000] the Netz architecture is presented, a distributed middleware, that presents fault tolerance and load balancing. And in [Quimby, 2002] Asheron’s call approach is described, concerned with security, availability, scalability and its patching problems. Another area that is gaining popularity is multi-user wireless games; one example of such application is discussed in [Fitzek et al., 2002] and presents an architecture capable of providing some guarantees in this kind of environment. 3 Categories of attacks There is an almost infinite number of possible attacks against networked games, and as such, a categorization of attack types is necessary in order to help determine on what kinds of issues one should focus. Some of these attacks can be countered by traditional computer security but as they affect every kind of networked application are also encompassed here. The following categorization scheme was derived from personal experience, general literature and by other classifications available in the literature ([DaScientia – Estudos Interdisciplinares em Computação B. KARLSSON, B. FEIJÓ vis, 2001; Gray, 2002; Yan and Choi, 2001; Pritchard, 2000]) that did not quite cover all the different kinds of attacks or were too broad for digital games. Attacks against networked games can be categorized as: • Connivance: when communication happens between two players that should not exchange secrets; • Policy abuse: when a player makes use of the system policy to gain an advantage. One example is escaping, where a player that is going to loose pretends to have lost his connection so that the defeat is not registered. Can also happen in the form of scapegoating (or hit-thenhide), where the cheater uses a denial of service attack to try to disconnect his opponent; • Virtual wealth: character or game item exchange, kind of real life commerce created with the popularization of on-line games and that can be intentional or not. If intentional, one can use digital signatures for non-repudiation of such transferences, what would help players in proving their transactions and belongings; and the game must support a Fair-trade system. If not intentional, unfortunately there is not much that can be done, as such commerce usually happens outside the game world; • Service theft: non authorized usage of the system, either by password compromising or authentication failure for example; • Event manipulation: consists in the cheater altering game events in order to gain an advantage, for example: “Compromised Servers” were their configuration is changed. Differs from other attacks for it’s focused basically on servers and tries to make small changes to the environment used by all players; • Lack of secret: the cheater has access to supposedly secret information that should not be available, can be accomplished by sniffing the network or by reading memory positions, by tampering with the client software. Some examples are replacing textures with transparent ones, allowing a cheater to see through walls or removing the fog of war in strategy games; • Internal misuse: bad faith from people within the organization responsible for the game operation; Volume 15 · No 2 · julho/dezembro 2004 169 • Social engineering: usage of social engineering techniques in order to fool others into doing something that helps the cheater. Be it pretending to be from the support team and asking for a user password, to convincing a player to give the cheater some item; • Rule manipulation: spoofing or authoritative clients, altering client software behaviour or network information in order to circumvent game rules or to change a simulation result, for example the usage of an aiming proxy that rewrites network packets in such a way that the cheater always hits his target; another option could be a replay attack; • Bugs and design defects abuse: using breaches in the game implementation in order to obtain unfair advantages, like for instance, abusing dead reckoning and getting more time to react to an event. If there is an exploitable bug, cheaters will find it, exploit it in a matter of days; • Perfect game: utilization of bots, macros, or information collected outside the game world in order to accomplish a better performance. Although in some cases not considered a cheat, it can be considered as such in situations such as reflex augmentation (consists of replacing or enhancing the player skills and response with an automated input) like using aim-bots (software that automatically aims for the player). Just as card counting is strongly discouraged in some card games, tools that help optimize players’ actions are usually considered cheating, even if they don’t otherwise break the rules; • Griefing: as digital networked games are inherently social activities and the systems that implement them assume norms of behavior, some individuals (usually feeling hidden by the relative anonymity of online play) do not play the game, but use it as a medium to attack or disrupt other people fun. Some of these classes of attacks indeed have an intersection, but it only goes to show that attacks can be seen and treated from different perspectives. It is also important to remember that client software is on enemy hands, and the enemy has access to all sorts of “helpers”, network sniffers, memory snoopers, debuggers, etc. 170 4 SECURITY IN NETWORKED COMPUTER GAMES Counter measures Especially because of these kinds of attacks, online game security is getting more attention and becoming an industry concern. Periodically attacks, announcements about “bad behaviours” and cheating are announced [Greenhill, 1997] together with even worse bugs (as the one on the Unreal engine [Luigi, 2003], that can affect more than 20 games and is open for about 5 years, and can even allow for the execution of arbitrary remote code). Some facets of this problem are exclusive of online games, and as such are not focused on related research. Even in the contrary, most of available military simulation research and the standard DIS not even consider cheating and security violations [Smed et al ., 2001]. Authors in [Kirmse and Kirmse, 1997] recognize two main goals for security applied to networked games: protect sensitive information (credit card, personal private info, etc.) and to provide a fair playing environment. One must also not forget the necessity for security even inside the game world [Kirmse, 2000; Fitch, 2001]. Traditional IT security solutions, can for sure help developers and game operators to reach a “good” online security as they range across the entire gamut of attacks. Gray provides a security industry vision [Gray, 2002] on how to improve networked games in this aspect (countering service theft, denial of service, password capturing). Good tips are to make players aware of security concerns, maintain and open and functional communication channel for complaint and audit game server logs. Another tip is: network performance is often the biggest enemy of good game security. One shall always keep an eye on borderline conditions (high latency, server under DoS attack) and try to predict how the game system will behave on such situation. For fighting bugs and design issues, a good approach is too release patches as soon as possible and check for patches applied before letting a player join a game session; if possible, an automatic patching system is recommended. Yet another idea that can catch general attacks is to monitor the game for strange behaviours; either impossible behaviour (as a player moving way too fast, that should cause the player disconnection) or try to use some statistical approach to determine if a player is too good to be true (but beware of false positives). The suspicion of cheating by players can cause serious problems for an online game. A popular solution used in commercial environments is the usage of software security tools, these tools attempt to detect and counter cheating soft- ware by various strategies. Some products in this area are: Cheating-Death [UnitedAdmins, 2004] and PunkBuster [PunkBuster, 2004]. The server portion of such solutions looks for anomalous network traffic that corresponds to various cheating packages. This is very similar to conventional Intrusion Detection Systems (IDS) that have a catalog of signatures that they continuously search for. One must also pay special attention to the integrity of the game executable; there are two main types of game integrity checks: state integrity (verifying whether the current game state is valid) and rule or transition integrity (verifying whether the transition between one state and the next is valid). But as many online games have rules and state information that cannot be known while the game is being played, some means must be used to verify activities as they occur, where possible. Usually cheaters perform network cheats by reverse engineering the game protocol in order to understand the meaning of its packets. A direct way to try to prevent that is to use checksums. However, there are weaknesses that just a checksum will not solve: cheaters may discover the checksum algorithm and bypass it or perform replay attacks. If we cryptograph network packets, there will definitively be less chances of someone forging their payload. However, there is still the necessity to have some state information on the packets, like a timestamp and also ways to make packets with similar payloads look different. Instead of an identifier, one can use, pseudo random numbers, or add a variable amount of padding to the packets in order to difficult their identification by size [Kirmse, 2000]. Other alternative is to encode every communication [Isensee, 2002] and also encode important data in memory and change its position at each execution. Baughman and Levine present a protocol with anti-cheating capabilities [Baughman and Levine, 2001] (during synchronization and in dead-reckoning, and also suppress-correct (intentional packet drops), and use the Xpilot game to test the protocol. Deals also with implementation details, for example, when players are on the same game cell in a MMORPG (the game world is partitioned, and each of its parts is a cell) the cheater can use the available info to try to predict risky situations and change his strategy. Lee [Lee et al., 2002] attacks the same issues as Baughman but faces latency and jitter. Regarding game architectures, Davis proposes a “Secure Game Contract” [Davis, 2001] implemented with the use of middleware and protocols and also stresses the necessity of security as standard and default. Buro describes an anti cheating toolkit of Scientia – Estudos Interdisciplinares em Computação 171 B. KARLSSON, B. FEIJÓ his own, specific for RTS games [Buro, 2002]. Pritchard suggests an approach of loosening local command checks in such way that the cheater has the impression that his cheat is working, but he would be disconnected in a later check for having and invalid game state [Pritchard, 2000]. A different take on the issue would be to use a proxy that would relief the server from filtering the network data [Mauve et al., 2002]. There are also a number of non-technical solutions to security problems in online games. Game monitors (real people) and good customer service are always valuable and they can also be an effective tool to deter cheating. Unfortunately, additional staffing can be very expensive relative to software. Another approach, more fun focused, is to use social techniques, as modeling player reputation [Fitch, 2001; Sanderson, 1999]. However, once again, special attention must be paid to this, as attackers can even use these anti-griefing systems to disrupt games by targeting innocent players’ reputations. One must always remember that good development practices are a major part of addressing security problems. 5 Conclusion The goal of this work is to draw attention of the academic community and help understanding and dealing with its various aspects, since little research has been focused in the field of security in networked digital games and there is a serious lack of information regarding the subject. This goal is pursued with the help of a vast bibliography from game related network to research to current efforts in digital games security, this work provides an overview of security problems faced by networked games. The importance of research in the field is shown along with the state of the art practices in the entertainment industry world; and a categorization of cheats and network attacks and some counter measures are used to try to stress real-life situations and provide some guidance on research topics. It must be remembered that digital networked games are a social activity and that security initiatives must also be looked through this prism. Given networked digital games enormous growth in popularity, market size and complexity (especially in the case of multi-user games), this kind of game provides a wide range of opportunities for future research. And as such, ideas originated in other projects and research scenarios should also be evaluated with games in mind, which can also raise community Volume 15 · No 2 · julho/dezembro 2004 attention to the study of security issues in virtual environment in general. We stress the importance of future research in networked games security both by industry and academia and that although there are no perfectsecurity solutions, security can add a lot to player experience in networked digital games. References Baughman, N. and Levine, B. 2001. Cheat-proof playout for centralized and distributed online games. In Proceedings of the 20th IEEE Computer and Communication Society INFOCOM Conference, Anchorage, EUA. IEEE. Bernier, Y. 2001. Latency compensating methods in client/server in-game protocol design and optimization. In Proceedings of the Game Developers Conference 2001, San Jose, EUA. Bettner, P. and Terrano, M. 2001. 1500 archers on a 28.8: Network programming in age of empires and beyond. In Proceedings of the Game Developers Conference 2001, San Jose, EUA. Buro, M. 2002. Orts: A hack-free rts game environment. In Proceedings of the International Computers and Games Conference 2002, Edmonton, Canada. Capps, M., McDowell, P., and Zyda, M. 2001. A future for entertainment-defense research collaboration. IEEE Computer Graphics and Applications, 21(1):37. Clarke, P. 2001. Mobile giants pick each other for universal games. EE Times. http://www.eetimes.com/ story/OEG20010321S0069 Costikyan, G. 1999. The future of online games. Technical Report, Creative Good. Costikyan, G. 2000. Online games: The past through the future. In Proceedings of IIR London Online Games Conference 2000, London, UK. Cronin, E., Filstrup, B., and Jamin, S. 2003. Cheat-proofing dead reckoned multiplayer games. In Proceedings of the 2nd International Conference on Application and Development of Computer Games, Hong Kong, China. Cronin, E., Filstrup, B., and Kurc, A. 2001. A distributed multiplayer game server system. Technical report, University of Michigan. UM EECS589 Course Project Report. 172 Davis, S. 2001. Why cheating matters. In Proceedings of the Game Developers Conference 2001. San Jose, p. 235–250. Dionne, C., Lavoie, M., and Trueman, K. 2000. Netz - multiplayer architecture for online games. In Proceedings of the Game Developers Conference 2000, San Jose, USA. Diot, C. and Gautier, L. 1999. A distributed architecture for multiplayer interactive applications on the internet. IEEE Networks Magazine, 13(4):6–15. Dodd, G. 2001. Keeping the pirates at bay: Implementing crack protection for spyro: Year of the dragon. Game Developer Magazine, ??(?):??–??. Fiedler, S., Wallner, M., and Weber, M. 2002. A communication architecture for massive multiplayer games. In Proceedings of the 1st Workshop on Network and System Support for Games, Germany, p. 14–22,. Fitch, C. 2001. Cyberspace in the 21st century: Part seven, security is relative. Gamasutra. http://www.gamasutra.com/features/20020805/ fitch 01.htm Fitzek, F., Schulte, G., and Reisslein, M. 2002. System architecture for billing of multiplayer games in a wireless environment using gsm/umts and wlan services. In Proceedings of the 1st Workshop on Network and System Support for Games, Germany. Färber, J. 2002. Network game traffic modeling. In Proceedings of the 1st Workshop on Network and System Support for Games, Germany, p. 53–57. Gautier, L. and Diot, C. 1998. Design and evaluation of mimaze, a multi-player game on the internet. In Proceedings of IEEE International Conference on Multimedia Computing and Systems, pages 23–36, Austin, EUA. Gray, J. 2002. Defense against service theft, internet cheating and denial of service. In Proceedings of the Game Developers Conference 2002, San Jose, USA. Greenhill, R. 1997. Diablo, and online multiplayer game’s future. Games Domain Review , http://www.gamesdomain.com/gdreview/depart/ jun97/diablo.html Handy, A. 2002. Game consoles – the next hacker target? Security Focus. http://www.security focus.com/ news/490 IDSA 2001. State of the industry - Report 20002001. IDSA. http://www.idsa.com SECURITY IN NETWORKED COMPUTER GAMES IGDA 2001. 2003 online games whitepaper . IGDA, http://www.igda.org/online Isensee, P. 2002. Secure sockets. In: Treglia, D., (Ed.), Game Programming Gems 3. Hingham, Charles River Media, p. 546–556. Kirmse, A. 2000. A network protocol for online games. In: DeLoura, M., (Ed.), Game Programming Gems.Hingham, Charles River Media, p. 104–108. Kirmse, A. and Kirmse, C. 1997. Security in online games. Game Developers Magazine, 4(4):20–8. LaPointe, D. and Winslow, J. 2001. Analyzing and simulating network game traffic. Project Report MLC-NG01, CS Department - WPI, Worcester. Lee, H., Kozlowski, E., Lenker, S., and S., J. 2002. Synchronization and cheat-proofing protocol for real-time multiplayer games. In Proceedings of the International Workshop on Entertainment Computing, Makuhari, Japan. Lincroft, P. 1999. The internet sucks: Or, what I learned coding x-wing vs. tie fighter . Gamasutra, http://www.gamasutra.com/features/19990903/ lincroft 01.htm Luigi, A. 2003. Unreal engine vulnerabilities advisory. http://www.pivx.com/luigi/adv/uengadv.txt. (13/11/2003). Macedonia, M. 1995. A Network Software Architecture for Large Scale Virtual Environments. PhD thesis, Naval Postgraduate School, Monterrey, CA. Mauve, M. 2000a. Consistency in replicated continuous interactive media. In Proceedings of the ACM Conference on Computer Supported Cooperative Work 2000, p. 181–190. Mauve, M. 2000b. How to keep a dead man from shooting. In Proceedings of the 7th International Workshop on Interactive Distributed Multimedia Systems and Telecommunication Services, Enschede, The Netherlands, p. 199–204. Mauve, M., Fischer, S., and Wildmer, J. 2002. A generic proxy system for networked computer games. In Proceedings of the 1st Workshop on Network and System Support for Games, Germany. Neyland, D. 1997. Virtual Combat: A Guide to Distributed Interactive Simulation. Mechanicsburg, Stackpole Books. Ng, Y. 1997. Designing fast-action games for the internet. Gamasutra. http://www.gamasutra.com/ features/19970905/ng 01.htm Scientia – Estudos Interdisciplinares em Computação 173 B. KARLSSON, B. FEIJÓ Pantel, L. and Wolf, L. 2002. On the suitability of dead reckoning schemes for games. In Proceedings of the 1st Workshop on Network and System Support for Games, Germany. Pritchard, M. 2000. How to hurt the hackers: The scoop on internet cheating and how you can combat it. Game Developers Magazine, ??(?):??–??. PunkBuster 2004. Punkbuster online countermeasures. PunkBuster, http://www.punkbuster.com/index.php Quimby, J. 2002. Massively multiplayer gameplay system implementation. In: Proceedings of the Game Developers Conference 2002, San Jose, USA. mes 2002, pages 74–78. Smed, J., Kaukoranta, T., and Hakonen, H. 2001. Aspects of networking in multiplayer computer games. In Proceedings of International Conference on Application and Development of Computer Games in the 21st Century, p. 74–81. Smed, J., Kaukoranta, T., and Hakonen, H. 2002. A review on networking and multiplayer computer games. Technical report, TUCS. TUCS Technical Report No 454. UnitedAdmins 2004. Cheating-death. United Admins. http://www.unitedadmins.com/cdeath.php. Sanderson, D. 1999. Online justice systems. Game Developers Mgazine, 6(4):42–49. Yan, J. and Choi, H. 2001. Security issues in online games. In: Proceedings of International Conference on Application and Development of Computer Games in the 21st Century. Schaefer, C., Enderes, T., Ritter, H., and Zitterbart, M. 2002. Subjective quality assessment for multiplayer real-time games. In Proceedigns of the ACM SIGCOMM Workshop on Network and System Support for Games - NetGa- Submitted in 31/02/2004 Accepted in 31/02/2014 Volume 15 · No 2 · julho/dezembro 2004