Authenticated Encryption based on finite automata cryptosystems

In this paper, we propose a new “full public verifiability” concept for hybrid public-key encryption schemes. We also present a new hybrid public-key encryption scheme that has this feature, which is based on the decisional bilinear Diffie-Hellman assumption. We have proven that the new hybrid public-key encryption scheme is secure against adaptive chosen ciphertext attack in the standard model. The “full public verifiability” feature means that the new scheme has a shorter ciphertext and reduces the security requirements of the symmetric encryption scheme. Therefore, our new scheme does not need any message authentication code, even when the one-time symmetric encryption scheme is passive attacks secure. Compared with all existing publickey encryption schemes that are secure to the adaptive chosen ciphertext attack, our new scheme has a shorter ciphertext, efficient tight security reduction, and fewer requirements (if the symmetric encryption scheme can resist passive attacks).

One of the most used authentication methods is based on short secrets like password, where usually the hash of the secrets are stored in a central database. In case of server compromise the secrets are vulnerable to theft. A possible solution to this problem to apply distributed systems. We propose a mutual authentication protocol with key agreement, where identity verification is carried out by multiple servers applying secret sharing technology on server side. The protocol results in a session key which provides the confidentiality of the later messages between the participants. In our solution we also achieve robustness and scalability as well. To show that the proposed protocol is provably secure, we apply the threshold hybrid corruption model. We assume that among the randomly chosen k servers, there is always at least one uncorrupted and the authentication server reveals at most the long-lived keys. We prove that the protocol is secure in the random oracle model, if Message Authentication Code (MAC) is universally unforgeable under an adaptive chosen-message attack, the symmetric encryption scheme is indistinguishable under chosen plaintext attack, moreover Elliptic Curve Computational Diffie-Hellman assumption holds in the elliptic curve group.

This work presents the employment of graph theory for cryptographic applications. A Cartesian product graph (CPG) is used to propose new symmetric encryption schemes. On proposed encryption schemes, the ciphertexts of the plaintexts are sent to the receiver entity as the CPGs. The CPG is created based on two graphs G and H that are generated depending on the random choices of the plaintext bits. These bits are used as vertices of graphs G and H. The purpose of using the CPG is to increase the security level of the symmetric encryption schemes in comparison with other encryption schemes. Two study cases of the proposed symmetric Cartesian product graphic encryption (SCPGE) schemes are presented as new experimental results. The security issues of the proposed SCPGE schemes are determined. The SCPGE schemes consider as new insights for more secure communications.

Security amplification is an important problem in Cryptography: starting with a secure variant of some cryptographic primitive, the goal is to build a strongly secure variant of the same primitive. This question has been successfully studied for a variety of important cryptographic primitives, such as one-way functions, collision-resistant hash functions, encryption schemes and weakly verifiable puzzles. However, all these tasks were non-interactive. In this work we study security amplification of interactive cryptographic primitives, such as message authentication codes (MACs), digital signatures (SIGs) and pseudorandom functions (PRFs). In particular, we prove direct product theorems for MACs/SIGs and an XOR lemma for PRFs, therefore obtaining nearly optimal security amplification for these primitives. Our main technical result is a new Chernoff-type theorem for what we call Dynamic Weakly Verifiable Puzzles , which is a generalization of ordinary Weakly Verifiable Puzzles which we introduce in this paper.

Characterization of Security Notions for Probabilistic Private-Key Encryption

Lossy trapdoor functions, introduced by Peikert and Waters (STOC’08), have received a lot of attention in the last years, because of their wide range of applications. The notion has been recently extended to the identity-based setting by Bellare et al. (Eurocrypt’12). An identity-based trapdoor function (IB-TDF) satisfying the lossy property introduced by Bellare et al. can be used to construct other cryptographic primitives in the identity-based setting: encryption schemes with semantic security under chosen-plaintext attacks, deterministic encryption schemes, and hedged encryption schemes that maintain some security when messages are encrypted using randomness of poor quality. However, the constructed primitives can be proved secure only against selective adversaries who select the target identity upfront.Our first contribution is an alternative definition for the lossiness of an identity-based trapdoor function. We prove that an IB-TDF satisfying the new property can be used to construct all the aforementioned primitives, in the identity-based setting, with security against adaptive adversaries. We further consider the new definition and its implications in the more general scenario of hierarchical identity-based cryptography, which has proved very useful both for practical applications and to establish theoretical relations with other cryptographic primitives (including encryption with chosen-ciphertext security or with forward-security).As a second contribution, we describe a pairing-based hierarchical IB-TDF satisfying the new definition of lossiness against either selective or, for hierarchies of constant depth, adaptive adversaries. This is also the first example of hierarchical trapdoor functions based on traditional (i.e., non-lattice-related) number theoretic assumptions. As a direct consequence of our two contributions, we obtain a hierarchical identity-based (HIB) encryption scheme with chosen-plaintext security, a HIB deterministic encryption scheme and a HIB hedged encryption scheme, all of them with security against adaptive adversaries.

Due to the equivalent keys revealed by a chosen-plaintext attack or a chosen-ciphertext attack, most of the existing chaotic image encryption schemes are demonstrated to be insecure. In order to improve security performance, some scholars have recently proposed the plaintext-related chaotic image encryption scheme. Although the equivalent effect of a one-time pad is achieved, an additional secure channel is required to transmit the hash values or other parameters related to the plaintext before the ciphertext can be decrypted at the receiving end. Its main drawback is that an absolutely secure channel is needed to transmit the information related to the plaintext, which is not feasible in practical applications. To further solve this problem, this paper proposes a chaotic image encryption scheme based on global dynamic selection of a multi-parallel structure. First, a chaotic sequence is employed to dynamically select DNA encoding rules. Secondly, the permutation with a multi-parallel structure is performed on the DNA-encoded matrix, and the DNA decoding rules are dynamically selected according to another chaotic sequence. Finally, the diffusion rules obtained by the ciphertext feedback mechanism are introduced to determine the dynamic diffusion. Compared with the existing local dynamic encryption schemes, the main advantage of this scheme is that it can realize global dynamic selection, so as to ensure that there is no equivalent key, and it can resist the chosen-ciphertext attack or chosen-plaintext attack and does not need an additional secure channel to transmit parameters related to plaintext, which is practical. A theoretical analysis and numerical experiments demonstrate the feasibility of the method.

Improvement of Rhouma's attacks on Gao algorithm

We examine a natural, but non-tight, reductionist security proof for deterministic message authentication code (MAC) schemes in the multi-user setting. If security parameters for the MAC scheme are selected without accounting for the non-tightness in the reduction, then the MAC scheme is shown to provide a level of security that is less than desirable in the multi-user setting. We find similar deficiencies in the security assurances provided by non-tight proofs when we analyze some protocols in the literature including ones for network authentication and aggregate MACs. Our observations call into question the practical value of non-tight reductionist security proofs. We also exhibit attacks on authenticated encryption schemes, disk encryption schemes, and stream ciphers in the multi-user setting.

Recently, an image encryption scheme based on a 2D hyperchaotic map is proposed. It adopts the permutation-diffusion architecture and consists of three steps, which are permutation, forward diffusion, and backward diffusion. In this paper, we break this cipher with both the chosen-plaintext attack (CPA) and the chosen-ciphertext attack (CCA). According to our analysis, we found the two complex diffusion processes could be simplified into two simple diffusions and a modular addition operation. Based on this, the equivalent key can be obtained with CPA and CCA. Detailed theoretical derivations and the results of experiments confirmed the feasibility of our attack methods. When the image size was 256×256, the running time of the attacks was less than 2 hours on a laptop with a 2.59 GHz Intel Core i7 and 16 GB DDR3 memory. Other sizes of images were also tested, and some rules were found. In addition, the probability of other attacks has also been discussed, and some suggestions for improvements are given. The source codes are publicly available and can be found online.

The deniable encryption is a type of encryption which can hide the true message while revealing a fake one. Even if the sender or the receiver is coerced to show the plaintext and the used random numbers in encryption, a deniable encryption scheme behaves like only an innocent message is encrypted. Because it protects privacy against malicious coercer, the deniable encryption is very useful in communication systems such as the cloud storage system when the communication channel is eavesdropped by a coercer. Previous deniable encryptions only concern the security under the adversary’s chosen plaintext attack (CPA). For non-interactive deniable encryptions, this paper introduce some security notions under adaptive chosen ciphertext attack (CCA). Furthermore, the first sender-deniable construction with deniability and indistinguishability against CCA attack is constructed.KeywordsDeniable EncryptionChosen Plaintext AttackChosen Ciphertext Attack

Chosen plaintext attacks (CPA) pose a significant security risk to encryption algorithms. However, it can be difficult to perform such an attack without direct access to the encryption process. This paper introduces a new cryptoanalysis method that uses hidden CPA to analyze image encryption schemes based on substitution boxes (S-boxes) Unlike traditional CPA methods, the proposed algorithm does not require that they can directly into the encryption process. Instead, a hidden attack vector is embedded in the natural host image to reduce the risk of attack detection. By asking the owner of the encryption algorithm to encrypt this encryption image and provide a cipher image, the input vector can be compared with its encrypted counterpart This can have an effective S-box and break encryption the algorithm, which does not interact directly with the encryption process. Experimental results demonstrate that the proposed method can completely recover cipher images in cascading S-box encryption schemes, regardless of the number of S-boxes used. Additionally, it conceals the CPA vector within the host image imperceptibly, achieving a high PSNR of 49.47 dB, indicating minimal visual distortion. Furthermore, our CPA significantly outperforms existing techniques in speed, recovering a grayscale image in just 1.2 seconds. This method provides a simple yet effective cryptanalysis tool to evaluate the security of such image encryption schemes against CPAs.

In this paper, we propose a lattice-based encryption scheme for Identity-Based Encryption (IBE) which is secure against Chosen-Ciphertext Attacks (CCA), and it is deduced from a previous Chosen-Plaintext Attack (CPA) secure IBE scheme by applying Fujisaki-Okamoto transform. Also, we present the design of a lattice-based cryptoprocessor that performs the encryption or decryption of the proposed CCA IBE scheme for the security parameters $n=512$ and $n=1024$ . In this case, the cryptoprocessor is mainly composed of one Random Number Generator (RNG), two hash function cores based on the Secure Hash Algorithm-3 (SHA-3) standard, and one Number Theoretical Transform (NTT) core for performing the polynomial ring operations. NTT core is designed using a systolic array and radix-2 multiple-path delay commutator architecture. The cryptoprocessor was synthesized on FPGA EP4SGX230KF40C2 using Quartus Prime 17.1, and verified in hardware using DE4 board and SignalTap tool. According to the obtained synthesis results, for the security parameter $n =1024$ , the IBE cryptoprocessor performs the encryption in $21.10~\mu $ s, and the decryption in $24.87~\mu $ s.

We propose a practical key encapsulation mechanism with a simple and intuitive design concept. Security against chosen-ciphertext attacks can be proved in the standard model under a new assumption, the Gap Hashed Diffie-Hellman (GHDH) assumption. The security reduction is tight and simple. Secure key encapsulation, combined with an appropriately secure symmetric encryption scheme, yields a hybrid public-key encryption scheme which is secure against chosen-ciphertext attacks. The implied encryption scheme is very efficient: compared to the previously most efficient scheme by Kurosawa and Desmedt [Crypto 2004] it has 128 bits shorter ciphertexts, between 25-50% shorter public/secret keys, and it is slightly more efficient in terms of encryption/decryption speed. Furthermore, our scheme enjoys (the option of) public verifiability of the ciphertexts and it inherits all practical advantages of secure hybrid encryption.

The Fujisaki-Okamoto (FO) conversion is a generic conversion to enhance a public key encryption scheme with security of one-way against chosen plaintext attacks (OW-CPA) to security of indistinguishable against adaptive chosen ciphertext attacks (IND-CCA) in the random oracle model. Existing works have shown that the FO conversion also can generically upgrade the security of the identity-based encryption (IBE) schemes or certificateless public key encryption (CL-PKE) schemes. However, it is still unknown if the same holds for certificate-based encryption (CBE) schemes. In this paper, we investigate the generic security of the CBE scheme obtained by applying the FO conversion to an arbitrary underlying OW-CBE-CPA secure CBE scheme and confirm that the FO conversion can generically convert any OW-CBE-CPA secure CBE into an IND-CBE-CCA secure CBE. We also note that the straightforward application of the FO conversion only leads to a CBE scheme with a loose reduction. To solve this problem, we propose an enhanced FO conversion which just introduces minor extra computation overhead, but results in considerably more efficient security reduction.