We reinvestigate the general distributed secure source coding based on the common key cryptosystem proposed by Oohama and Santoso (ITW 2021). They proposed a framework of distributed source encryption and derived the necessary and sufficient conditions to have reliable and secure transmission. However, the bounds of the rate region, which specifies both necessary and sufficient conditions to have reliable and secure transmission under the proposed cryptosystem, were derived based on a self-tailored non-standard security criterion. In this paper we adopt the standard security criterion, i.e., standard mutual information. We successfully establish the bounds of the rate region based on this security criterion. Information spectrum method and a variant of Birkhoff-von Neumann theorem play important roles in deriving our result.

We study the universal coding problem for the general framework of source encryption with a symmetric key under the side-channel attacks, which is posed and investigated by Oohama and Santoso (2022). The reliable and secure rate region indicating the trade off between the compression rate of the ciphertext and the rate constraint imposed on the adversary for secure source transmission was established by Oohama and Santoso (2022). In this paper we focus on our attention to strengthening the direct coding theorem. We prove the existence of encryption/decryption schemes, which are universal in the sense that they work effectively for any distributions of the plain text, any noisy channels through which the adversary observe the corrupted version of the key, and any measurement device used for collecting the physical information. Those schemes have a good performance such that if we compress the ciphertext with rate within the reliable and secure rate region, then: (1) anyone with secret key will be able to decrypt and decode the ciphertext correctly, but (2) any adversary who obtains the ciphertext and also the side physical information will not be able to obtain any information about the hidden source as long as the leaked physical information is encoded with a rate within the rate constraint.

Any person can post arbitrary strings on the bulletin board. Following publication on bulletin board, a party receives a "evidence" that the intended data were posted. The bulletin board is open to the public, which means that anyone can view its contents. The fundamental security criteria for a BB are that its contents cannot be deleted, and that no evidence of publication can be falsified. BB was commonly perceived as a trusted and publicly verifiable channel. It formed the backbone of many important protocols such as e-voting, secure multi-party computation etc. and quite often it was assumed to exist to support the execution of the main protocol. After the advent of blockchain technology, BB systems has found a strong footing regarding its implementation in a distributed manner. In this paper, we aim to provide a concise summary of the evolution of bulletin board, its implementation issues and security analysis with an emphasis on its impact to e-voting systems.

We are interested in investigating the security of source encryption with a symmetric key under side-channel attacks. In this paper, we propose a general framework of source encryption with a symmetric key under the side-channel attacks, which applies to any source encryption with a symmetric key and any kind of side-channel attacks targeting the secret key. We also propose a new security criterion for strong secrecy under side-channel attacks, which is a natural extension of mutual information, i.e., the maximum conditional mutual information between the plaintext and the ciphertext given the adversarial key leakage, where the maximum is taken over all possible plaintext distribution. Under this new criterion, we successfully formulate the rate region, which serves as both necessary and sufficient conditions to have secure transmission even under side-channel attacks. Furthermore, we also prove another theoretical result on our new security criterion, which might be interesting in its own right: in the case of the discrete memoryless source, no perfect secrecy under side-channel attacks in the standard security criterion, i.e., the ordinary mutual information, is achievable without achieving perfect secrecy in this new security criterion, although our new security criterion is more strict than the standard security criterion.

We pose and investigate the distributed secure source coding based on the common key cryptosystem. This cryptosystem includes the secrecy amplification problem for distributed encrypted sources with correlated keys using post-encryption-compression, which was previously studied by Santoso and Oohama. In this paper we propose another new security criterion which is generally more strict compared with the commonly used security criterion based on the upper-bound of mutual information between the plaintext and the ciphertext. Under this criterion, we establish the necessary and sufficient condition for the secure transmission of correlated sources.

We consider the secure communication problem of broadcasting of two encrypted sources against an adversary which launches side-channel attacks. The adversary is not only allowed to eavesdrop the ciphertexts in the public communication channel but is also allowed to obtain the side information on the secret keys via the side-channel. In this paper, we propose a theoretical framework to solve this problem and also propose a countermeasure based on the post-encryption-compression paradigm. We provide an explicit sufficient condition to attain the exponential decay of the information leakage as the block lengths of encrypted sources become large.

In this paper, we are interested in the problem of strengthening the secrecy of distributed encryption in a particular case where the encryption keys are correlated to each other. We show that the post-encryption-compression (PEC) paradigm provides a suitable solution for the problem without requiring any additional secret randomness. More precisely, we show that in the case of one-time-pad encryption, we can use affine encoders with specific properties to strengthen the secrecy by using them to compress the ciphertexts before sending them to public communication channels. We show how to derive those affine encoders using universal code construction paradigm. The universal code construction gives us advantages as follows. 1) We can construct good encoders based on the transmission rates only, without knowing the value of the distribution of sources or keys. 2) Reliability and secrecy are achievable by construction even in particular cases such as follows: 1) only the range of correlation between keys is known but the exact amount is unknown or 2) the correlation between keys are changing time to time within a certain range. We also derive explicit lower bounds on the achievable reliability and secrecy exponents, i.e., the exponential rates of decay of the probability of decoding error and of the information leakage as the block length becomes large.

We study side-channel attacks for the Shannon cipher system. To pose side channel-attacks to the Shannon cipher system, we regard them as a signal estimation via encoded data from two distributed sensors. This can be formulated as the one helper source coding problem posed and investigated by Ahlswede, Korner(1975), and Wyner(1975). We further investigate the posed problem to derive new secrecy bounds. Our results are derived by a coupling of the result Watanabe and Oohama(2012) obtained on bounded storage eavesdropper with the exponential strong converse theorem Oohama(2015) established for the one helper source coding problem.

In this paper, we propose the application of post-encryption-compression (PEC) to strengthen the secrecy in the case of distributed encryption where the encryption keys are correlated to each other, without additional randomness. We show that, in the case of one-time-pad encryption, certain affine encoders are sufficient for strengthening the secrecy by using them to compress the ciphertexts before sending them to public communication channels. We show how to explicitly construct those affine encoders using universal code construction paradigm. The universal code construction gives us advantages as follows. (1) We can construct good encoders based on the transmission rates only, without knowing the value of the distribution of sources or keys. (2) Reliability and secrecy are achievable by construction even in cases such as follows: (1) only the range of correlation between keys is known but the exact amount is unknown, or (2) the correlation between keys are changing time to time within a certain range.We also derive concrete lower bounds on the achievable reliability and secrecy exponents, i.e., the exponential rates of decay of the probability of decoding error and of the information leakage as the block length n becomes large.

In 1996, Stern proposed a three-pass zero-knowledge identification (ID) scheme where the cheating probability, i.e., the success probability of cheating prover, is 2/3. Since then, variants and generalization of Stern's ID have been proposed. However, within two-bit challenge space, all of them are having the cheating probability of more than half or reaching half only in an asymptotic manner with more than three passes in one round. In this paper, we propose the first code-based zero-knowledge three-pass ID scheme with the cheating probability of exactly half even with only two-bit challenge space. Our proposed ID scheme can reduce the necessary number of rounds in order to achieve the targeted security against impersonation. Since rewinding technique cannot be used against a quantum adversary in the security proof, we prove the security using the lossy paradigm and rely on the decisional version of syndrome decoding problem so that we do not have to rewind the adversary.

In this paper, we consider a system where multiple sources are encrypted in separated nodes and sent through their respective public communication channels into a joint sink node. We are interested at the problem on protecting the security of an already existing system such above, which is found out to have correlated encryption keys. In particular, we focus on finding a solution without introducing additional secret keys and with minimal modification to minimize the cost and the risk of bringing down an already running system. We propose a solution under a security model where an eavesdropper obtains all ciphertexts, i.e., encrypted sources, by accessing available public communication channels. Our main technique is to use encoders of certain linear codes to encode the ciphertexts before sending them to public communication channels. We show that if the rates of linear codes are within a certain rate region: (1) the success probability of any eavesdropper to extract the original sources from the encoded ciphertexts without the keys is negligible, while (2) one who has legitimate keys is able to retrieve the original source data with negligible error probability.

Universally Composable (UC) framework provides the strongest security notion for designing fully trusted cryptographic protocols, and it is very challenging on applying UC security in the design of RFID mutual authentication protocols. In this paper, we formulate the necessary conditions for achieving UC secure RFID mutual authentication protocols which can be fully trusted in arbitrary environment, and indicate the inadequacy of some existing schemes under the UC framework. We define the ideal functionality for RFID mutual authentication and propose the first UC secure RFID mutual authentication protocol based on public key encryption and certain trusted third parties which can be modeled as functionalities. We prove the security of our protocol under the strongest adversary model assuming both the tags' and readers' corruptions. We also present two (public) key update protocols for the cases of multiple readers: one uses Message Authentication Code (MAC) and the other uses trusted certificates in Public Key Infrastructure (PKI). Furthermore, we address the relations between our UC framework and the zero-knowledge privacy model proposed by Deng et al. [1].