| 1. |
- Austrin, Per, et al.
(författare)
-
Lower bounds for Subset Cover based Broadcast Encryption
- 2008
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Ingår i: PROGRESS IN CRYPTOLOGY - AFRICACRYPT 2008. - Berlin, Heidelberg : Springer Berlin Heidelberg. ; , s. 343-356
-
Konferensbidrag (refereegranskat)abstract
- In this paper, we prove lower bounds for a large class of Subset Cover schemes (including all existing schemes based on pseudo-random sequence generators). In particular, we show that For small r, bandwidth is Omega(r) For some r, bandwidth is Omega(n/log(s)) For large r, bandwidth is n - r where n is the number of users, r is the number of revoked users, and s is the space required per user. These bounds are all tight in the sense that they match known constructions up to small constants.
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| 2. |
- Bodriagov, Oleksandr, et al.
(författare)
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Access Control in Decentralized Online Social Networks : Applying a Policy-Hiding Cryptographic Scheme and Evaluating Its Performance
- 2014
-
Konferensbidrag (refereegranskat)abstract
- Privacy concerns in online social networking services have prompted a number of proposals for decentralized online social networks (DOSN) that remove the central provider and aim at giving the users control over their data and who can access it. This is usually done by cryptographic means. Existing DOSNs use cryptographic primitives that hide the data but reveal the access policies. At the same time, there are privacy-preserving variants of these cryptographic primitives that do not reveal access policies. They are, however, not suitable for usage in the DOSN context because of performance or storage constraints. A DOSN needs to achieve both privacy and performance to be useful. We analyze predicate encryption (PE) and adapt it to the DOSN context. We propose a univariate polynomial construction for access policies in PE that drastically increases performance of the scheme but leaks some part of the access policy to users with access rights. We utilize Bloom filters as a means of decreasing decryption time and indicate objects that can be decrypted by a particular user. We evaluate the performance of the adapted scheme in the concrete scenario of a news feed. Our PE scheme is best suited for encrypting for groups or small sets of separate identities.
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| 3. |
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| 4. |
- Enström, Emma, et al.
(författare)
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Five Years with Kattis – Using an Automated Assessment System in Teaching
- 2011
-
Ingår i: 2011 Frontiers in Education Conference (FIE). - New York : IEEE. - 9781612844695
-
Konferensbidrag (refereegranskat)abstract
- Automated assessment systems have been employed in computer science (CS) courses at a number of different universities. Such systems are especially applicable in teaching algorithmic problem solving since they can automatically test if an algorithm has been correctly implemented, i.e., that it performs its specified function on a set of inputs. Being able to implement algorithms that work correctly is a crucial skill for CS students in their professional role, but it can be difficult to convey the importance of this in a classroom situation. Programming and problem solving education supported by automated grading has been used since 2002 at our department. We study, using action research methodology, different strategies for deploying automated assessment systems in CS courses. Towards this end, we have developed an automated assessment system and both introduced it into existing courses and constructed new courses structured around it. Our primary data sources for evaluation consists of course evaluations, statistics on students' submitted solutions, and experience teaching the courses. Authors of this paper have been participating in teaching all of the courses mentioned here.
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| 5. |
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| 6. |
- Goldmann, Mikael, et al.
(författare)
-
Measurements on the Spotify Peer-Assisted Music-on-Demand Streaming System
- 2011
-
Ingår i: 11th IEEE International Conference on Peer-to-Peer Computing, P2P'11. - 9781457701481 ; , s. 206-211
-
Konferensbidrag (refereegranskat)abstract
- Spotify is a streaming service offering low-latency access to a large library of music. Streaming is performed by a combination of client-server access and a peer-to-peer protocol. The service currently has a user base of over 10~million and is available in seven European countries. We provide a background on the Spotify protocol with emphasis on the formation of the peer-to-peer overlay. Using measurement data collected over a week by instrumenting Spotify clients, we analyze general network properties such as the correspondence between individual user accounts and the number of IP addresses they connect from and the prevalence of Network Address Translation devices (NATs). We also discuss the performance of one of the two peer discovery mechanisms used by Spotify.
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| 7. |
- Greschbach, Benjamin, et al.
(författare)
-
The devil is in the metadata - New privacy challenges in decentralised online social networks
- 2012
-
Ingår i: 2012 IEEE International Conference on Pervasive Computing and Communications Workshops, PERCOM Workshops 2012. - : IEEE. - 9781467309073 ; , s. 333-339
-
Konferensbidrag (refereegranskat)abstract
- Decentralised Online Social Networks (DOSN) are evolving as a promising approach to mitigate design-inherent privacy flaws of logically centralised services such as Facebook, Google+ or Twitter. A common approach to build a DOSN is to use a peer-to-peer architecture. While the absence of a single point of data aggregation strikes the most powerful attacker from the list of adversaries, the decentralisation also removes some privacy protection afforded by the central party's intermediation of all communication. As content storage, access right management, retrieval and other administrative tasks of the service become the obligation of the users, it is non-trivial to hide the metadata of objects and information flows, even when the content itself is encrypted. Such metadata is, deliberately or as a side effect, hidden by the provider in a centralised system. In this work, we aim to identify the dangers arising or made more severe from decentralisation, and show how inferences from metadata might invade users' privacy. Furthermore, we discuss general techniques to mitigate or solve the identified issues.
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| 8. |
- Greschbach, Benjamin, 1983-, et al.
(författare)
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User Search with Knowledge Threshold in Decentralized Online Social Networks
- 2013
-
Ingår i: Proceedings of the 8th International IFIP Summer School on Privacy and Identity Management for Emerging Services and Technologies.
-
Konferensbidrag (refereegranskat)abstract
- User search is one fundamental functionality of an Online Social Network (OSN). When building privacy-preserving Decentralized Online Social Networks (DOSNs), the challenge of protecting user data and making users findable at the same time has to be met. We propose a user-defined knowledge threshold ("find me if you know enough about me") to balance the two requirements. We present and discuss protocols for this purpose that do not make use of any centralized component. An evaluation using real world data suggests that there is a promising compromise with good user performance and high adversary costs.
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| 9. |
- Greschbach, Benjamin, et al.
(författare)
-
User Search with Knowledge Thresholds in Decentralized Online Social Networks
- 2014
-
Ingår i: PRIVACY AND IDENTITY MANAGEMENT FOR EMERGING SERVICES AND TECHNOLOGIES. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 9783642551376 - 9783642551369 ; , s. 188-202
-
Konferensbidrag (refereegranskat)abstract
- User search is one fundamental functionality of an Online Social Network (OSN). When building privacy-preserving Decentralized Online Social Networks (DOSNs), the challenge of protecting user data and making users findable at the same time has to be met. We propose a user-defined knowledge threshold ("find me if you know enough about me") to balance the two requirements. We present and discuss protocols for this purpose that do not make use of any centralized component. An evaluation using real world data suggests that there is a promising compromise with good user performance and high adversary costs.
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| 10. |
- Jimenez, Raul, et al.
(författare)
-
Integrating Smartphones in Spotify's Peer-Assisted Music Streaming Service
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Spotify is a large-scale peer-assisted music streaming service. Spotify’s P2P network serves 80% of music data to desktop clients. On the other hand, the rapidly growing number of mobile clients do not use P2P but instead stream all data from Spotify’s servers.We enable P2P on a Spotify mobile client and empirically eval- uate the impact of P2P protocols (in particular low-bandwidth traffic between peers) on energy consumption, both on 3G andWifi. On 3G, current P2P protocols are highly energy inefficient, but simple modifications bring consumption close to the client-server configuration. On Wifi, the extra energy cost of enabling P2P is much lower.Finally, we propose a protocol modification to further integrate mobile devices in Spotify’s P2P network according to their capa- bilities (power source, access network). This allows us to break the artificial division between desktop and mobile platforms and dynamically adapt as resources become (un)available to the device.
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| 11. |
- Johansson, Mattias, et al.
(författare)
-
Stateful subset cover
- 2006
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Ingår i: APPLIED CRYPTOGRAPHY AND NETWORK SECURITY, PROCEEDINGS. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 3540347038 ; , s. 178-193
-
Konferensbidrag (refereegranskat)abstract
- This paper describes a method to convert stateless key revocation schemes based on the subset cover principle into stateful schemes. The main motivation is to reduce the bandwidth overhead to make broadcast encryption schemes more practical in network environments with limited bandwidth resources, such as cellular networks. This modification is not fully collusion-resistant. A concrete new scheme based on the Subset Difference scheme [1] is presented, accomplishing a bandwidth overhead of Delta m + 2 Delta r + 1 compared to e.g. Logical Key Hierarchy's 2(Delta m + Delta r) log m, where Delta m and Delta r is the number of members added and removed since the last stateful update and m is the number of current members.
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| 12. |
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| 13. |
- Kreitz, Gunnar, 1982-
(författare)
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A Zero-One Law for Secure Multi-party Computation with Ternary Outputs
- 2011
-
Ingår i: Theory Of Cryptography. - Berlin, Heidelberg : Springer Berlin/Heidelberg. - 9783642195709 ; , s. 382-399
-
Konferensbidrag (refereegranskat)abstract
- There are protocols to privately evaluate any function in the passive (honest-but-curious) setting assuming that the honest nodes are in majority. For some specific functions, protocols are known which remain secure even without an honest majority. The seminal work by Chor and Kushilevitz [7] gave a complete characterization of Boolean functions, showing that each Boolean function either requires an honest majority, or is such that it can be privately evaluated regardless of the number of colluding nodes. The problem of discovering the threshold for secure evaluation of more general functions remains an open problem. Towards a resolution, we provide a complete characterization of the security threshold for functions with three different outputs. Surprisingly, the zero-one law for Boolean functions extends to Z3, meaning that each function with range Z3 either requires honest majority or tolerates up to n colluding nodes.
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| 14. |
- Kreitz, Gunnar, 1982-
(författare)
-
Aspects of Secure and Efficient Streaming and Collaboration
- 2011
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Research within the area of cryptography constitutes the core of this the- sis. In addition to cryptography, we also present results in peer-assisted streaming and web security. We present results on two specific cryptographic problems: broadcast encryption and secure multi-party computation. Broad- cast encryption is the problem of efficiently and securely distributing content to a large and changing group of receivers. Secure multi-party computation is the subject of how a number of parties can collaborate securely. All in all, this thesis spans from systems work discussing the Spotify streaming system with millions of users, to more theoretic, foundational results. Streaming is among the largest applications of the Internet today. On- demand streaming services allow users to consume the media content they want, at their convenience. With the large catalogs offered by many services, users can access a wide selection of content. Live streaming provides the means for corporations as well as individuals to broadcast to the world. The power of such broadcasts was shown in the recent (early 2011) revolts in Tunisia and Egypt, where protesters streamed live from demonstrations. To stream media to a large global audience requires significant resources, in particular in terms of the bandwidth needed. One approach to reduce the requirements is to use peer-to-peer techniques, where clients assist in distributing the media. Spotify is a commercial music-on-demand streaming system, using peer-to-peer streaming. In this thesis, we discuss the Spotify protocol and measurements on its performance. In many streaming systems, it is important to restrict access to content. One approach is to use cryptographic solutions from the area of broadcast encryption. Within this area, we present two results. The first is a protocol which improves the efficiency of previous systems at the cost of lowered secu- rity guarantees. The second contains lower-bound proofs, showing that early protocols in the subset cover framework are essentially optimal. Many streaming systems are web-based, where the user accesses content in a web browser. Apart from this usage of the web, subscriptions for streaming services are bought using a web browser. This means that to provide a secure streaming service, we must understand web security. This thesis contains a result on a new type of attack, using an old history detection vulnerability to time the execution of a redirect of a victim’s browser. Within the area of secure multi-party computation, this thesis has three contributions. Firstly, we give efficient protocols for the basic functions of summation and disjunction which adapt to the network they run on. Secondly, we provide efficient protocols for sorting and aggregation, by using techniques from the area of sorting networks. Finally, we prove a dichotomy theorem, showing that all functions with three distinct outputs are either maximally easy or maximally difficult with regards to the security provided.
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| 15. |
- Kreitz, Gunnar
(författare)
-
Flow stealing : A well-timed redirection attack
- 2013
-
Ingår i: Journal of Computer Security. - 0926-227X .- 1875-8924. ; 21:3, s. 371-391
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, we present a Flow Stealing attack, where a victim's browser is redirected in the middle of a browsing session. We detail two attack scenarios. The first is redirecting the victim's browser as it moves from a store to a payment provider, and the second redirects the victim to a phishing page, when she navigates to one of a set of target sites. A key issue in flow stealing is correctly timing the redirect. The main way to accomplish this is to leverage a history detection attack to test whether the victim has visited a target. By repeatedly polling, an attacker learns when the victim navigates to a tested target page. With this application, we demonstrate that the impact of history detection is greater than previously known. Our primary history detection mechanism is a cache timing attack, measuring the time it takes to load an element to determine if it was served from the browser cache. This attack works with present browser versions. We also discuss CSS history detection, based on detecting the styling of visited links, which has been solved in most browsers. Lastly, we also consider a network-based attacker who can mount a man-in-the-middle attack on the victim's network traffic. We discuss several countermeasures against flow stealing. These include two new proposed policies on JavaScript window navigation which can be implemented by browser vendors. We also present mitigations which can be implemented by individual stores or payment providers.
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| 16. |
- Kreitz, Gunnar, 1982-, et al.
(författare)
-
Passwords in Peer-to-Peer
- 2012
-
Ingår i: Peer-to-Peer Computing (P2P), 2012 IEEE 12th International Conference on. - : IEEE. - 9781467328623 ; , s. 167-178
-
Konferensbidrag (refereegranskat)abstract
- One of the differences between typical peer-to-peer (P2P) and client-server systems is the existence of user accounts. While many P2P applications, like public file sharing, are anonymous, more complex services such as decentralized online social networks require user authentication. In these, the common approach to P2P authentication builds on the possession of cryptographic keys. A drawback with that approach is usability when users access the system from multiple devices, an increasingly common scenario. In this work, we present a scheme to support logins based on users knowing a username-password pair. We use passwords, as they are the most common authentication mechanism in services on the Internet today, ensuring strong user familiarity. In addition to password logins, we also present supporting protocols to provide functionality related to password logins, such as resetting a forgotten password via e-mail or security questions. Together, these allow P2P systems to emulate centralized password logins. The results of our performance evaluation indicate that incurred delays are well within acceptable bounds.
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| 17. |
- Kreitz, Gunnar, 1982-, et al.
(författare)
-
Practical Private Information Aggregation in Large Networks
- 2012
-
Ingår i: Information Security Technology For Applications. - Berlin, Heidelberg : Springer Berlin/Heidelberg. - 9783642279362 ; , s. 89-103
-
Konferensbidrag (refereegranskat)abstract
- Emerging approaches to network monitoring involve large numbers of agents collaborating to produce performance or security related statistics on huge, partial mesh networks. The aggregation process often involves security or business-critical information which network providers are generally unwilling to share without strong privacy protection. We present efficient and scalable protocols for privately computing a large range of aggregation functions based on addition, disjunction, and max/min. For addition, we give a protocol that is information-theoretically secure against a passive adversary, and which requires only one additional round compared to non-private protocols for computing sums. For disjunctions, we present both a computationally secure, and an information-theoretically secure solution. The latter uses a general composition approach which executes the sum protocol together with a standard multi-party protocol for a complete subgraph of ``trusted servers''. This can be used, for instance, when a large network can be partitioned into a smaller number of provider domains.
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| 18. |
- Kreitz, Gunnar, 1982-, et al.
(författare)
-
Spotify – Large Scale, Low Latency, P2P Music-on-Demand Streaming
- 2010
-
Ingår i: Peer-to-Peer Computing. ; , s. 1-10
-
Konferensbidrag (refereegranskat)abstract
- Spotify is a music streaming service offering low-latency access to a library of over 8 million music tracks. Streaming is performed by a combination of client-server access and a peer-to-peer protocol. In this paper, we give an overview of the protocol and peer-to-peer architecture used and provide measurements of service performance and user behavior. The service currently has a user base of over 7 million and has been available in six European countries since October 2008. Data collected indicates that the combination of the client-server and peer-to-peer paradigms can be applied to music streaming with good results. In particular, 8.8% of music data played comes from Spotify's servers while the median playback latency is only 265 ms (including cached tracks). We also discuss the user access patterns observed and how the peer-to-peer network affects the access patterns as they reach the server.
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| 19. |
- Kreitz, Gunnar, 1982-
(författare)
-
Timing is Everything : the Importance of History Detection
- 2011
-
Ingår i: Computer Security – ESORICS 2011. - Springer : Springer Berlin Heidelberg. - 9783642238215 ; , s. 117-132
-
Konferensbidrag (refereegranskat)abstract
- In this work, we present a Flow Stealing attack, where a victim's browser is redirected during a legitimate flow. One scenario is redirecting the victim's browser as it moves from a store to a payment provider. We discuss two attack vectors.Firstly, browsers have long admitted an attack allowing a malicious web page to detect whether the browser has visited a target web site by using CSS to style visited links and read out the style applied to a link. For a long time, this CSS history detection attack was perceived as having small impact. Lately, highly efficient implementations of the attack have enabled malicious web sites to extract large amounts of information. Following this, browser developers have deployed measures to protect against the attack. Flow stealing demonstrates that the impact of history detection is greater than previously known.Secondly, an attacker who can mount a man-in-the-middle attack against the victim's network traffic can also perform a flow stealing attack.Noting that different browsers place different restrictions on cross-frame navigation through JavaScript window handles, we suggest a stricter policy based on pop-up blockers to prevent Flow Stealing attacks.
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| 20. |
- Lindholm, Fredrik, et al.
(författare)
-
Key distribution in systems for selective access to information
- 2004
-
Patent (populärvet., debatt m.m.)abstract
- A combination of stateless and state-full techniques enable reduced overall key management messages. A set of encryption keys is created based on a stateless subset cover scheme, where each user is associated with a leaf in the subset cover scheme. A state key and a corresponding state representing a subset of users that have the state key and a subset of users that do not have the state key is established, and a subset cover based on information representative of the established state is determined. Finally, a group key management message can be determined at least partly based on the calculated subset cover. The introduced state opens up for efficient subset cover calculation, reducing the size of the key management message.
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| 21. |
- Rzadca, K., et al.
(författare)
-
Game-theoretic mechanisms to increase data availability in decentralized storage systems
- 2015
-
Ingår i: ACM Transactions on Autonomous and Adaptive Systems. - : ACM Special Interest Group on Computer Science Education. - 1556-4665 .- 1556-4703. ; 10:3
-
Tidskriftsartikel (refereegranskat)abstract
- In a decentralized storage system, agents replicate each other’s data to increase availability. Compared to organizationally centralized solutions, such as cloud storage, a decentralized storage system requires less trust in the provider and may result in smaller monetary costs. Our system is based on reciprocal storage contracts that allow the agents to adopt to changes in their replication partners’ availability (by dropping inefficient contracts and forming new contracts with other partners). The data availability provided by the system is a function of the participating agents’ availability. However, a straightforward system in which agents’ matching is decentralized uses the given agent availability inefficiently. As agents are autonomous, the highly available agents form cliques replicating data between each other, which makes the system too hostile for the weakly available newcomers. In contrast, a centralized, equitable matching is not incentive compatible: it does not reward users for keeping their software running. We solve this dilemma by a mixed solution: an "adoption" mechanism in which highly available agents donate some replication space, which in turn is used to help the worst-off agents. We show that the adoption motivates agents to increase their availability (is incentive-compatible), but also that it is sufficient for acceptable data availability for weakly-available agents.
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| 22. |
- Setty, V., et al.
(författare)
-
Maximizing the number of satisfied subscribers in pub/sub systems under capacity constraints
- 2014
-
Ingår i: Proceedings - IEEE INFOCOM. - 9781479933600 ; , s. 2580-2588
-
Konferensbidrag (refereegranskat)abstract
- Publish/subscribe (pub/sub) is a popular communication paradigm in the design of large-scale distributed systems. A provider of a pub/sub service (whether centralized, peer-assisted, or based on a federated organization of cooperatively managed servers) commonly faces a fundamental challenge: given limited resources, how to maximize the satisfaction of subscribers? We provide, to the best of our knowledge, the first formal treatment of this problem by introducing two metrics that capture subscriber satisfaction in the presence of limited resources. This allows us to formulate matters as two new flavors of maximum coverage optimization problems. Unfortunately, both variants of the problem prove to be NP-hard. By subsequently providing formal approximation bounds and heuristics, we show, however, that efficient approximations can be attained. We validate our approach using real-world traces from Spotify and show that our solutions can be executed periodically in real-time in order to adapt to workload variations.
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| 23. |
- Setty, V., et al.
(författare)
-
The hidden pub/sub of spotify (Industry Article)
- 2013
-
Ingår i: DEBS 2013 - Proceedings of the 7th ACM International Conference on Distributed Event-Based Systems. - New York, NY, USA : ACM. - 9781450317580 ; , s. 231-240
-
Konferensbidrag (refereegranskat)abstract
- Spotify is a peer-assisted music streaming service that has gained worldwide popularity. Apart from providing instant access to over 20 million music tracks, Spotify also enhances its users' music experience by providing various features for social interaction. These are realized by a system using the widely-adopted pub/sub paradigm. In this paper we provide an interesting case study of a hybrid pub/sub system designed for real-time as well as offline notifications for Spo-tify users. We firstly describe a multitude of use cases where pub/sub is applied. Secondly, we study the design of its pub/sub system used for matching, disseminating and persisting billions of publications every day. Finally, we study pub/sub traffic collected from the production system, derive characterizations of the pub/sub workload, and show some interesting findings and trends.
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| 24. |
- Yanggratoke, Rerngvit, 1983-, et al.
(författare)
-
On the performance of the Spotify backend
- 2013
-
Ingår i: Journal of Network and Systems Management. - : Springer-Verlag New York. - 1064-7570 .- 1573-7705.
-
Tidskriftsartikel (refereegranskat)abstract
- We model and evaluate the performance of a distributed key-value storage system that is part of the Spotify backend. Spotify is an on-demand music streaming service, offering low-latency access to a library of over 20 million tracks and serving over 20 million users currently. We first present a simplified model of the Spotify storage architecture, in order to make its analysis feasible. We then introduce an analytical model for the distribution of the response time, a key metric in the Spotify service. We parameterize and validate the model using measurements from two different testbed configurations and from the operational Spotify infrastructure. We find that the model is accurate---measurements are within 11% of predictions---within the range of normal load patterns.In addition, we model the capacity of the Spotify storage system under different object allocation policies and find that measurements on our testbed are within 9% of the model predictions. The model helps us justify the object allocation policy adopted for Spotify storage system.
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| 25. |
- Yanggratoke, Rerngvit, et al.
(författare)
-
Predicting response times for the Spotify backend
- 2012
-
Ingår i: Proceedings of the 2012 8th International Conference on Network and Service Management, CNSM 2012. - Stockholm : IFIP. - 9783901882494 ; , s. 117-125
-
Konferensbidrag (refereegranskat)abstract
- We model and evaluate the performance of a distributed key-value storage system that is part of the Spotify backend. Spotify is an on-demand music streaming service, offering low-latency access to a library of over 16 million tracks and serving over 10 million users currently. We first present a simplified model of the Spotify storage architecture, in order to make its analysis feasible. We then introduce an analytical model for the distribution of the response time, a key metric in the Spotify service. We parameterize and validate the model using measurements from two different testbed configurations and from the operational Spotify infrastructure. We find that the model is accurate - measurements are within 11% of predictions - within the range of normal load patterns. We apply the model to what-if scenarios that are essential to capacity planning and robustness engineering. The main difference between our work and related research in storage system performance is that our model provides distributions of key system metrics, while related research generally gives only expectations, which is not sufficient in our case.
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| 26. |
- Zhang, B., et al.
(författare)
-
Understanding user behavior in Spotify
- 2013
-
Ingår i: 2013 Proceedings - IEEE INFOCOM. - 9781467359467 ; , s. 220-224
-
Konferensbidrag (refereegranskat)abstract
- Spotify is a peer-assisted music streaming service that has gained worldwide popularity in the past few years. Until now, little has been published about user behavior in such services. In this paper, we study the user behavior in Spotify by analyzing a massive dataset collected between 2010 and 2011. Firstly, we investigate the system dynamics including session arrival patterns, playback arrival patterns, and daily variation of session length. Secondly, we analyze individual user behavior on both multiple and single devices. Our analysis reveals the favorite times of day for Spotify users. We also show the correlations between both the length and the downtime of successive user sessions on single devices. In particular, we conduct the first analysis of the device-switching behavior of a massive user base.
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