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Browsing by Subject "Data encryption (Computer science)"

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    ItemOpen Access
    Analysis of android random number generator
    (2013) Sarıtaş, Serkan
    Randomness is a crucial resource for cryptography, and random number generators are critical building blocks of almost all cryptographic systems. Therefore, random number generation is one of the key parts of secure communication. Random number generation does not guarantee security. Problematic random number generation process may result in breaking the encrypted communication channel, because encryption keys are obtained by using random numbers. For computers and smart devices, generation of random numbers is done by operating systems. Applications which need random numbers for their operation request them from the operating system they are working on. Due to the importance of random number generation, this process should be analyzed deeply and cryptographically for different operating systems. From this perspective, we studied Android random number generation process by looking at the source codes and found that security of random number generation done by Android relies on the security of random number generation of Linux. Then we analyzed Android random number generator by modifying the kernel source code and applying some tests on its entropy estimator. Finally, we looked for possible weaknesses of random number generator during startup of Android devices.
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    ItemOpen Access
    OpenID with certificate-based user authentication on smartcard
    (2013) Kişin, Bahar Berna
    From the point of its users, federated identity systems provide great convenience to log in to varied web sites without bothering of registration in advance. Looking from a vantage point, federated identity management gives the opportunity to users of one IT system to access data and sources of another IT system seamlessly and securely without handling a complete user administration. Single signon mechanisms manage user authentication process of these systems prompting log in once and assure access control across those multiple independent systems. OpenID is a widely used federated identity/single sign-on scheme generally implemented with username-password authentication. In this work, we augment the user authentication phase of OpenID with certi cate-based authentication using smartcard technology. Our solution provides a secure method to authenticate the user with user's digital certi cate written on the smartcard.
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    ItemOpen Access
    Optimal broadcasting encryption with free riders
    (2006) Ak, Murat
    Broadcast encryption schemes allow a center to broadcast encrypted messages so that each particular message can only be decrypted by a set of privileged receivers designated for it. The standard technique is to distribute keys to all receivers at the beginning and to use only the necessary keys to encrypt the message for each particular broadcast. The number of encryptions needed constitutes the transmission cost in broadcast encryption schemes. The most efficient scheme in terms of transmission cost published so far is the subset difference (SD) scheme. It is possible to reduce the transmission overhead of a broadcast encryption scheme by allowing a number of free riders to be able to decrypt the message although they are not privileged. However, unless the free riders are chosen cleverly, the cost may even increase. In this thesis, we deal with the problem of choosing a given number of free riders effectively to reduce the transmission overhead in SD scheme. We first present three greedy algorithms. First algorithm has a fast execution, the second one is slower but has lower transmission overhead, and the last one has a trade-off between running time and transmission cost. Then we present two algorithms which find the minimum transmission overhead possible given a free rider quota. The experiments we conduct show that the transmission cost can be significantly reduced.
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    ItemOpen Access
    Optimization techniques and new methods for boradcast encryption and traitor tracing schemes
    (2012) Ak, Murat
    In the last few decades, the use of digital content increased dramatically. Many forms of digital products in the form of CDs, DVDs, TV broadcasts, data over the Internet, entered our life. Classical cryptography, where encryption is done for only one recipient, was not able to handle this change, since its direct use leads to intolerably expensive transmissions. Moreover, new concerns regarding the commercial aspect arised. Since digital commercial contents are sold to various customers, unauthorized copying by malicious actors became a major concern and it needed to be prevented carefully. Therefore, a new research area called digital rights management (DRM) has emerged. Within the scope of DRM, new cryptographic primitives are proposed. In this thesis, we consider three of these: broadcast encryption (BE), traitor tracing (TT), and trace and revoke (T&R) schemes and propose methods to improve the performances and capabilities of these primitives. Particularly, we first consider profiling the recipient set in order to improve transmission size in the most popular BE schemes. We then investigate and solve the optimal free rider assignment problem for one of the most efficient BE schemes so far. Next, we attempt to close the non-trivial gap between BE and T&R schemes by proposing a generic method for adding traitor tracing capability to BE schemes and thus obtaining a T&R scheme. Finally, we investigate an overlooked problem: privacy of the recipient set in T&R schemes. Right now, most schemes do not keep the recipient set anonymous, and everybody can see who received a particular content. As a generic solution to this problem, we propose a method for obtaining anonymous T&R scheme by using anonymous BE schemes as a primitive.

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