We investigate a general class of (black-box) constructions for range extension of weak pseudoran... more We investigate a general class of (black-box) constructions for range extension of weak pseudorandom functions: a construction based on m independent functions F 1,...,F m is given by a set of strings over {1,...,m}*, where for example {〈2〉, 〈1,2〉} corresponds to the function X ↦[F 2(X),F 2(F 1(X))]. All efficient constructions for range expansion of weak pseudorandom functions that we
ABSTRACT In a digital signature scheme with message recovery, rather than transmitting the messag... more ABSTRACT In a digital signature scheme with message recovery, rather than transmitting the message m and its signature σ, a single enhanced signature τ is transmitted. The verifier is able to recover m from τ and at the same time verify its authenticity. The two most important parameters of such a scheme are its security and overhead |τ| − |m|. A simple argument shows that for any scheme with “n bits security” |τ| − |m| ≥ n, i.e., the overhead is lower bounded by the security parameter n. Currently, the best known constructions in the random oracle model are far from this lower bound requiring an overhead of n + logq h , where q h is the number of queries to the random oracle. In this paper we give a construction which basically matches the n bit lower bound. We propose a simple digital signature scheme with n + o(logq h ) bits overhead, where q h denotes the number of random oracle queries. Our construction works in two steps. First, we propose a signature scheme with message recovery having optimal overhead in a new ideal model, the random invertible function model. Second, we show that a four-round Feistel network with random oracles as round functions is tightly “public-indifferentiable” from a random invertible function. At the core of our indifferentiability proof is an almost tight upper bound for the expected number of edges of the densest “small” subgraph of a random Cayley graph, which may be of independent interest.
A black-box combiner for collision resistant hash functions (CRHF) is a construction which given ... more A black-box combiner for collision resistant hash functions (CRHF) is a construction which given black-box access to two hash func- tions is collision resistant if at least one of the components is collision resistant. In this paper we prove a lower bound on the output length of black-box combiners for CRHFs. The bound we prove is basically tight as it
Tampering attacks are cryptanalytic attacks on the implementation of cryptographic algorithms (e.... more Tampering attacks are cryptanalytic attacks on the implementation of cryptographic algorithms (e.g., smart cards), where an adversary introduces faults with the hope that the tampered device will reveal secret information. Inspired by the work of Ishai et al. [Eurocrypt’06], we propose a compiler that transforms any circuit into a new circuit with the same functionality, but which is resilient against a well-defined and powerful tampering adversary. More concretely, our transformed circuits remain secure even if the adversary can adaptively tamper with every wire in the circuit as long as the tampering fails with some probability δ > 0. This additional requirement is motivated by practical tampering attacks, where it is often difficult to guarantee the success of a specific attack. Formally, we show that a q-query tampering attack against the transformed circuit can be “simulated” with only black-box access to the original circuit and log(q) bits of additional auxiliary information. Thus, if the implemented cryptographic scheme is secure against log(q) bits of leakage, then our implementation is tamper-proof in the above sense. Surprisingly, allowing for this small amount of information leakage allows for much more efficient compilers, which moreover do not require randomness during evaluation. Similar to earlier works our compiler requires small, stateless and computation-independent tamper-proof gadgets. Thus, our result can be interpreted as reducing the problem of shielding arbitrary complex computation to protecting simple components.
ABSTRACT Creep damage due to constant loading at elevated temperatures and damage due to plastic ... more ABSTRACT Creep damage due to constant loading at elevated temperatures and damage due to plastic deformation at room temperature are assessed using destructive and nondestructive methods in steels (40HNMA and P91). In the destructive methods the standard tension tests were carried out after prestraining and variations of the selected tension parameters were taken into account for damage identification. In order to assess damage development during the creep and plastic deformation the tests for both steels were interrupted for a range of selected strain magnitudes. Ultrasonic and magnetic techniques were used as the nondestructive methods for damage evaluation. The last step of the experimental programme contained microscopic observations. The results show that ultrasonic and magnetic parameters can be good indicators of material degradation and can help to locate the regions where material properties are changed due to prestraining. A good correlation of mechanical and selected nondestructive parameters identifying damage was achieved for both tested steels.
We investigate a general class of (black-box) constructions for range extension of weak pseudoran... more We investigate a general class of (black-box) constructions for range extension of weak pseudorandom functions: a construction based on m independent functions F 1,...,F m is given by a set of strings over {1,...,m}*, where for example {〈2〉, 〈1,2〉} corresponds to the function X ↦[F 2(X),F 2(F 1(X))]. All efficient constructions for range expansion of weak pseudorandom functions that we
ABSTRACT In a digital signature scheme with message recovery, rather than transmitting the messag... more ABSTRACT In a digital signature scheme with message recovery, rather than transmitting the message m and its signature σ, a single enhanced signature τ is transmitted. The verifier is able to recover m from τ and at the same time verify its authenticity. The two most important parameters of such a scheme are its security and overhead |τ| − |m|. A simple argument shows that for any scheme with “n bits security” |τ| − |m| ≥ n, i.e., the overhead is lower bounded by the security parameter n. Currently, the best known constructions in the random oracle model are far from this lower bound requiring an overhead of n + logq h , where q h is the number of queries to the random oracle. In this paper we give a construction which basically matches the n bit lower bound. We propose a simple digital signature scheme with n + o(logq h ) bits overhead, where q h denotes the number of random oracle queries. Our construction works in two steps. First, we propose a signature scheme with message recovery having optimal overhead in a new ideal model, the random invertible function model. Second, we show that a four-round Feistel network with random oracles as round functions is tightly “public-indifferentiable” from a random invertible function. At the core of our indifferentiability proof is an almost tight upper bound for the expected number of edges of the densest “small” subgraph of a random Cayley graph, which may be of independent interest.
A black-box combiner for collision resistant hash functions (CRHF) is a construction which given ... more A black-box combiner for collision resistant hash functions (CRHF) is a construction which given black-box access to two hash func- tions is collision resistant if at least one of the components is collision resistant. In this paper we prove a lower bound on the output length of black-box combiners for CRHFs. The bound we prove is basically tight as it
Tampering attacks are cryptanalytic attacks on the implementation of cryptographic algorithms (e.... more Tampering attacks are cryptanalytic attacks on the implementation of cryptographic algorithms (e.g., smart cards), where an adversary introduces faults with the hope that the tampered device will reveal secret information. Inspired by the work of Ishai et al. [Eurocrypt’06], we propose a compiler that transforms any circuit into a new circuit with the same functionality, but which is resilient against a well-defined and powerful tampering adversary. More concretely, our transformed circuits remain secure even if the adversary can adaptively tamper with every wire in the circuit as long as the tampering fails with some probability δ > 0. This additional requirement is motivated by practical tampering attacks, where it is often difficult to guarantee the success of a specific attack. Formally, we show that a q-query tampering attack against the transformed circuit can be “simulated” with only black-box access to the original circuit and log(q) bits of additional auxiliary information. Thus, if the implemented cryptographic scheme is secure against log(q) bits of leakage, then our implementation is tamper-proof in the above sense. Surprisingly, allowing for this small amount of information leakage allows for much more efficient compilers, which moreover do not require randomness during evaluation. Similar to earlier works our compiler requires small, stateless and computation-independent tamper-proof gadgets. Thus, our result can be interpreted as reducing the problem of shielding arbitrary complex computation to protecting simple components.
ABSTRACT Creep damage due to constant loading at elevated temperatures and damage due to plastic ... more ABSTRACT Creep damage due to constant loading at elevated temperatures and damage due to plastic deformation at room temperature are assessed using destructive and nondestructive methods in steels (40HNMA and P91). In the destructive methods the standard tension tests were carried out after prestraining and variations of the selected tension parameters were taken into account for damage identification. In order to assess damage development during the creep and plastic deformation the tests for both steels were interrupted for a range of selected strain magnitudes. Ultrasonic and magnetic techniques were used as the nondestructive methods for damage evaluation. The last step of the experimental programme contained microscopic observations. The results show that ultrasonic and magnetic parameters can be good indicators of material degradation and can help to locate the regions where material properties are changed due to prestraining. A good correlation of mechanical and selected nondestructive parameters identifying damage was achieved for both tested steels.
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