Symmetrische kryptographische Verfahren (German Edition)

Kathlén Kohn

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  • Sicherheit von Verschlüsselungsalgorithmen
  • Informationen zu verschiedenen Verschlüsselungsverfahren

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    Zuverlässige Hardware-Sicherheitsmodule

    Share your thoughts with other customers. Now, if we trust schemes that are designed according to the provable security approach, should we then also trust schemes devised via the random oracle methodology?

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    This question has lead to a huge debate within the cryptographic community and has been discussed for more than two decades. The discussion is fueled by results showing that the extension of security models to include a random oracle may produce provably secure schemes that cannot be securely implemented. In , Canetti, Goldreich, and Halevi STOC, showed that schemes exist that are inherently insecure but which should be secure according to the random oracle methodology. In more detail, they present a public-key encryption scheme which an adversary can trivially attack when given the code of the hash function that was used to replace the random oracle.

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    In summary, we develop techniques to work with obfuscation which allow us to show that the existence of indistinguishability obfuscation implies that various random oracle techniques may lead to insecure schemes. IE Ref legal event code: In step S2, the prompt message c BR from the on-board computer BR is radio-based system in the service, that is in transport VM emitted. Random Oracles in the Standard Model. The request message is received in the Customer Media transport customers and the response message is sent by the user in the transport media to the onboard computer that collects the response messages to the shared transmission to the data center of the customer contract partner. In other words, we prove that no efficient adversary can break the security with good probability given that it behaves as defined within the security model. This oracle is called the random oracle.

    Note that this differs from, e. With the scheme presented by Canetti et al. Similarly, for many advanced cryptographic concepts, including IND-secure deterministic public-key encryption, correlated-input secure hash functions, universal hardcore functions, and many others, we so far only have constructions in the random oracle model. One reason for the success of random oracles is that they allow to design very efficient and natural schemes.

    Furthermore, the power of random oracles enables us to realize concepts which we would not know how to implement without random oracles. A third, and very compelling argument in favor of the random oracle methodology is that the random oracle heuristic seems to be a good one: However, if a scheme is, indeed, secure, should we then not be able to understand and pinpoint the underlying source of hardness?

    Sicherheit von Verschlüsselungsalgorithmen

    In this thesis we study random oracles with the help of program obfuscation in particular indistinguishability obfuscation and point-function obfuscation. The study of obfuscation has a long tradition in computer science, and specifically in cryptography, but only recent advancements gave rise to the first candidate constructions of provably secure general-purpose indistinguishability obfuscators Garg et al. Intuitively, a program obfuscator takes as input a program and produces a functionally equivalent but unintelligible program, i.

    Conceptually, this is very close to one of the fundamental abstractions made within the random oracle model where hash functions do not have an explicit and efficient description but can be evaluated only via black-box access to the random oracle. While an obfuscated hash function still has an explicit and efficient description, the description should hide the way the function works and, thus, intuitively, should be of no help to any adversary. Using obfuscation-based techniques we show how to instantiate the random oracle in various cryptographic constructions.

    Amongst others, we obtain the first standard model i. We obtain our positive results by instantiating various forms of universal computational extractors.

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    The universal computational extractor UCE framework was introduced by Bellare, Hoang, and Keelveedhi CRYPTO, to provide very strong standard-model notions of hash functions that allow instantiating random oracles in a wide range of applications. Intriguingly, even though obfuscation allows us to show how to replace random oracles in certain situations, it also allows us to show limitations of the random oracle methodology as well as of UCEs. Using obfuscation-based techniques, we prove that several concrete UCE assumptions including all originally proposed assumptions cannot hold in case indistinguishability obfuscation exists.

    We note that these negative results inspired the weaker UCE notions that lay at the core of our positive constructions. Assuming the existence of indistinguishability obfuscation also allows us to extend the uninstantiability techniques of Canetti et al. STOC, and show that a large class of random-oracle transformations are not sound. This affects the Encrypt-with-Hash transformation Bellare et al.

    An often repeated criticism of random-oracle uninstantiability results is that the schemes only fail to be secure because they are designed to do so and, furthermore, their artificial design conflicts good cryptographic practice see, for example, Koblitz and Menezes; Journal of Cryptology, Similar criticism can be voiced also for our counterexamples to the general applicability of the above mentioned random-oracle transformations. While this does not refute the mathematical validity of such uninstantiability results, we do, however, also present a very different counterexample to the soundness of the random oracle methodology: We note that the same holds also for our negative results for UCEs.

    In summary, we develop techniques to work with obfuscation which allow us to show that the existence of indistinguishability obfuscation implies that various random oracle techniques may lead to insecure schemes.

    Einführung Kryptographie