Public-Key Encryption with Delegated Search

In public-key setting, Alice encrypts email with public key of Bob, so that only Bob will be able to learn contents of email. Consider scenario when computer of Alice is infected and unbeknown to Alice it also embeds malware into message. Bob's company, Carol, cannot scan his email for malicious content as it is encrypted so burden is on Bob to do scan. This is not efficient. We construct mechanism that enables Bob to provide trapdoors to Carol such that Carol, given encrypted data and malware signature, is able to check whether encrypted data contains malware signature, without decrypting it. We refer to this mechanism as Public-Key Encryption with Delegated Search SPKE.\ud \ud We formalize SPKE and give construction based on ElGamal public-key encryption (PKE). proposed scheme has ciphertexts which are both searchable and decryptable. This property of scheme is crucial since entity can search entire content of message, in contrast to existing searchable public-key encryption schemes where search is done only in metadata part. We prove in standard model that scheme is ciphertext indistinguishable and trapdoor indistinguishable under Symmetric External Diffie-Hellman (sxdh) assumption. We prove also ciphertext one-wayness of scheme under modified Computational Diffie-Hellman (mcdh) assumption. We show that our PKEDS scheme can be used in different applications such as detecting encrypted malwares and forwarding encrypted emails

An Identity-Based Group Signature with Membership Revocation in the Standard Model

Group signatures allow group members to sign an arbitrary number\ud of messages on behalf of the group without revealing their\ud identity. Under certain circumstances the group manager holding a\ud tracing key can reveal the identity of the signer from the\ud signature. Practical group signature schemes should support\ud membership revocation where the revoked member loses the\ud capability to sign a message on behalf of the group without\ud influencing the other non-revoked members. A model known as\ud \emph{verifier-local revocation} supports membership revocation.\ud In this model the trusted revocation authority sends revocation\ud messages to the verifiers and there is no need for the trusted\ud revocation authority to contact non-revoked members to update\ud their secret keys. Previous constructions of verifier-local\ud revocation group signature schemes either have a security proof in the\ud random oracle model or are non-identity based. A security proof\ud in the random oracle model is only a heuristic proof and\ud non-identity-based group signature suffer from standard Public Key\ud Infrastructure (PKI) problems, i.e. the group public key is not\ud derived from the group identity and therefore has to be certified.\ud \ud \ud In this work we construct the first verifier-local revocation group\ud signature scheme which is identity-based and which has a security proof in the standard model. In\ud particular, we give a formal security model for the proposed\ud scheme and prove that the scheme has the\ud property of selfless-anonymity under the decision Linear (DLIN)\ud assumption and it is fully-traceable under the\ud Computation Diffie-Hellman (CDH) assumption. The proposed scheme is based on prime order bilinear\ud groups

On Proactive Verifiable Secret Sharing Schemes

The paper has been presented at the International Conference Pioneers of Bulgarian Mathematics, Dedicated to Nikola Obreshkoff and Lubomir Tschakaloff , Sofia, July, 2006. The material in this paper was presented in part at the 11th Workshop on Selected Areas in Cryptography (SAC) 2004This paper investigates the security of Proactive Secret Sharing Schemes. We first consider the approach of using commitment to 0 in the renewal phase in order to refresh the player's shares and we present two types of attacks in the information theoretic case. Then we prove the conditions for the security of such a proactive scheme. Proactivity can be added also using re-sharing instead of commitment to 0. We investigate this alternative approach too and describe two protocols. We also show that both techniques are not secure against a mobile adversary. To summarize we generalize the existing threshold protocols to protocols for general access structure. Besides this, we propose attacks against the existing proactive verifiable secret sharing schemes, and give modifications of the schemes that resist these attacks

On Distributed Oblivious Transfer

The paper has been presented at the International Conference Pioneers of Bulgarian Mathematics, Dedicated to Nikola Obreshkoff and Lubomir Tschakaloff , Sofia, July, 2006. The material in this paper was presented in part at INDOCRYPT 2002This paper is about unconditionally secure distributed protocols for oblivious transfer, as proposed by Naor and Pinkas and generalized by Blundo et al. In this setting a Sender has ζ secrets and a Receiver is interested in one of them. The Sender distributes the information about the secrets to n servers, and a Receiver must contact a threshold of the servers in order to compute the secret. We present a non-existence result and a lower bound for the existence of one-round, threshold, distributed oblivious transfer protocols, generalizing the results of Blundo et al. A threshold based construction implementing 1-out-of-ζ distributed oblivious transfer achieving this lower bound is described. A condition for existence of distributed oblivious transfer schemes based on general access structures is proven. We also present a general access structure protocol implementing 1-out-of-ζ distributed oblivious transfer

The Random Fault Model

In this work, we introduce the random fault model - a more advanced fault model inspired by the random probing model, where the adversary can fault all values in the algorithm but the probability for each fault to occur is limited. The new adversary model is used to evaluate the security of side-channel and fault countermeasures such as Boolean masking, error detection techniques, error correction techniques, multiplicative tags, and shuffling methods. The results of the security analysis reveal new insights both in the novel random fault model as well as in the established random probing model including: shuffling masked implementations does not significantly improve the random probing security over regular masking; error correction providing little security when faults target more bits (versus the significant improvement when using error detection); and the order in which masking and duplication are applied providing a trade-off between random probing and fault security. Moreover, the results also explain the experimental results from CHES 2022 and find weaknesses in the shuffling method from SAMOS 2021

On a Relation Between Verifiable Secret Sharing Schemes and a Class of Error-Correcting Codes

In this paper we try to shed a new insight on Verifiable Secret Sharing Schemes (VSS). We first define a new ``metric (with slightly different properties than the standard Hamming metric). Using this metric we define a very particular class of codes that we call {\it error-set correcting codes}, based on a set of forbidden distances which is a monotone decreasing set. Next we redefine the packing problem for the new settings and generalize the notion of error-correcting capability of the error-set correcting codes accordingly (taking into account the new metric and the new packing). Then we consider burst-error interleaving codes proposing an efficient burst-error correcting technique, which is in fact the well known VSS and Distributed Commitments (DC) pair-wise checking protocol and we prove the error-correcting capability of the error-set correcting interleaving codes. Using the known relationship, due to Van Dijk, between a Monotone Span Program (MSP) and a generator matrix of the code generated by the suitable set of vectors, we prove that the error-set correcting codes in fact has the allowed (opposite to forbidden) distances of the dual access structure of the access structure that the MSP computes. We give an efficient construction for them based on this relation and as a consequence we establish a link between Secret Sharing Schemes (SSS) and the error-set correcting codes. Further we give a necessary and sufficient condition for the existence of linear SSS (LSSS), to be secure against $(\Delta,\Delta_A)$-adversary expressed in terms of an error-set correcting code. Finally, we present necessary and sufficient conditions for the existence of a VSS scheme, based on an error-set correcting code, secure against $(\Delta,\Delta_A)$-adversary. Our approach is general and covers all known linear VSS/DC. It allows us to establish the minimal conditions for security of VSSs. Our main theorem states that the security of a scheme is equivalent to a pure geometrical (coding) condition on the linear mappings describing the scheme. Hence the security of all known schemes, e.g. all known bounds for existence of unconditionally secure VSS/DC including the recent result of Fehr and Maurer, can be expressed as certain (geometrical) coding conditions

Whirlwind: a new cryptographic hash function

A new cryptographic hash function Whirlwind is presented. We give the full specification and explain the design rationale. We show how the hash function can be implemented efficiently in software and give first performance numbers. A detailed analysis of the security against state-of-the-art cryptanalysis methods is also provided. In comparison to the algorithms submitted to the SHA-3 competition, Whirlwind takes recent developments in cryptanalysis into account by design. Even though software performance is not outstanding, it compares favourably with the 512-bit versions of SHA-3 candidates such as LANE or the original CubeHash proposal and is about on par with ECHO and MD6

VerMI: Verification Tool for Masked Implementations

Masking is a widely used countermeasure against Side-Channel Attacks (SCA), but the implementation of these countermeasures is challenging. Experimental security evaluation requires special equipment, a considerable amount of time and extensive technical knowledge. So, to automate and to speed up this process, a formal verification can be performed to asses the security of a design. Multiple theoretical approaches and verification tools have been proposed in the literature. The majority of them are tailored for software implementations, not applicable to hardware since they do not take into account glitches. Existing hardware verification tools are limited either to combinational logic or to small designs due to the computational resources needed. In this work we present VerMI, a verification tool in the form of a logic simulator that checks the properties defined in Threshold Implementations to address the security of a hardware implementation for meaningful orders of security. The tool is designed so that any masking scheme can be evaluated. It accepts combinational and sequential logic and is able to analyze an entire cipher in short time. With the tool we have managed to spot a flaw in the round-based Keccak implementation by Gross et al., published in DSD 2017

Guards in Action: First-Order SCA Secure Implementations of Ketje without Additional Randomness

Recently the CAESAR competition has announced several finalists among the submitted authenticated encryption algorithms, after an open selection process during the last 5 years. Applications using these algorithms are rapidly increasing today. Devices implementing these applications are enormously susceptible to physical attacks, which are able to retrieve secret data through side-channel information such as the power consumption or the electromagnetic radiations. In this work we present a Side-Channel Analysis resistant hardware implementation of the whole family of authenticated encryption schemes KETJE. By changing just one parameter, any of the KETJE designs can be obtained, and tailored for different applications, either lightweight or high throughput. We introduce a new protected KECCAK implementation, as well as unprotected and protected KETJE implementations, which allow both encryption and decryption modes in the same module. In order to secure these implementations we make use of the masking scheme known as Threshold Implementations and complement it with the technique of “Changing of the Guards”, achieving a first-order Side-Channel Analysis protected implementation with zero extra randomness needed. This way, no dedicated PRNG needs to be additionally implemented, avoiding issues such as the security of the PRNG itself or the quality of the randomness

Covering Radius of the (n−3)(n-3)-rd Order Reed-Muller Code in the Set of Resilient Functions

In this paper, we continue the study of the covering radius in the set of resilient functions, which has been defined by Kurosawa. This new concept is meaningful to cryptography especially in the context of the new class of algebraic attacks on stream ciphers proposed by Courtois and Meier at Eurocrypt 2003 and Courtois at Crypto 2003. In order to resist such attacks the combining Boolean function should be at high distance from lower degree functions. Using a result from coding theory on the covering radius of $(n-3)$-rd Reed-Muller codes, we establish exact values of the the covering radius of $RM(n-3,n)$ in the set of $1$-resilient Boolean functions of $n$ variables, when $\lfloor n/2 \rfloor = 1 \mod\;2$. We also improve the lower bounds for covering radius of the Reed-Muller codes $RM(r,n)$ in the set of $t$-resilient functions, where $\lceil r/2 \rceil = 0 \mod\;2$, $t \leq n-r-2$ and $n\geq r+3$