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Collecting and analyzing personal data is important in modern information applications. Though the privacy of data providers should be protected, some adversarial users may behave badly under circumstances where they are not identified. However, the privacy of honest users should not be infringed. Thus, detecting anomalies without revealing normal users-identities is quite important for operating information systems using personal data. Though various methods of statistics and machine learning have been developed for detecting anomalies, it is difficult to know in advance what anomaly will come up. Thus, it would be useful to provide a "general" framework that can employ any anomaly detection method regardless of the type of data and the nature of the abnormality. In this paper, we propose a privacy preserving anomaly detection framework that allows an authority to detect adversarial users while other honest users are kept anonymous. By using cryptographic techniques, group signatures with message-dependent opening (GS-MDO) and public key encryption with non-interactive opening (PKENO), we provide a correspondence table that links a user and data in a secure way, and we can employ any anonymization technique and any anomaly detection method. It is particularly worth noting that no big brother exists, meaning that no single entity can identify users, while bad behaviors are always traceable. We also show the result of implementing our framework. Briefly, the overhead of our framework is on the order of dozens of milliseconds.

Chu et al. (ASIACCS 2012) proposed group signature with time-bound keys (GS-TBK) where each signing key is associated to an expiry time τ. In addition to prove the membership of the group, a signer needs to prove that the expiry time has not passed, i.e., t

Let us consider a scenario that a data holder (e.g., a hospital) encrypts a data (e.g., a medical record) which relates a keyword (e.g., a disease name), and sends its ciphertext to a server. We here suppose not only the data but also the keyword should be kept private. A receiver sends a query to the server (e.g., average of body weights of cancer patients). Then, the server performs the homomorphic operation to the ciphertexts of the corresponding medical records, and returns the resultant ciphertext. In this scenario, the server should NOT be allowed to perform the homomorphic operation against ciphertexts associated with different keywords. If such a mis-operation happens, then medical records of different diseases are unexpectedly mixed. However, in the conventional homomorphic encryption, there is no way to prevent such an unexpected homomorphic operation, and this fact may become visible after decrypting a ciphertext, or as the most serious case it might be never detected. To circumvent this problem, in this paper, we propose mis-operation resistant homomorphic encryption, where even if one performs the homomorphic operations against ciphertexts associated with keywords ω' and ω, where ω -ω', the evaluation algorithm detects this fact. Moreover, even if one (intentionally or accidentally) performs the homomorphic operations against such ciphertexts, a ciphertext associated with a random keyword is generated, and the decryption algorithm rejects it. So, the receiver can recognize such a mis-operation happens in the evaluation phase. In addition to mis-operation resistance, we additionally adopt secure search functionality for keywords since it is desirable when one would like to delegate homomorphic operations to a third party. So, we call the proposed primitive mis-operation resistant searchable homomorphic encryption (MR-SHE). We also give our implementation result of inner products of encrypted vectors. In the case when both vectors are encrypted, the running time of the receiver is millisecond order for relatively small-dimensional (e.g., 26) vectors. In the case when one vector is encrypted, the running time of the receiver is approximately 5 msec even for relatively high-dimensional (e.g., 213) vectors.

We propose the notion of disavowable public key encryption with non-interactive opening (disavowable PKENO) where, for a ciphertext and a message, the receiver of the ciphertext can issue a proof that the plaintext of the ciphertext is NOT the message, and give a fairly practical construction.

Various techniques need to be combined to realize anonymously authenticated communication. Cryptographic tools enable anonymous user authentication while anonymous communication protocols hide users' IP addresses from service providers. One simple approach for realizing anonymously authenticated communication is their simple combination, but this gives rise to another issue; how to build a secure channel. The current public key infrastructure cannot be used since the user's public key identifies the user. To cope with this issue, we propose a protocol that uses identity-based encryption for packet encryption without sacrificing anonymity, and group signature for anonymous user authentication. Communications in the protocol take place through proxy entities that conceal users' IP addresses from service providers. The underlying group signature is customized to meet our objective and improve its efficiency. We also introduce a proof-of-concept implementation to demonstrate the protocol's feasibility. We compare its performance to SSL communication and demonstrate its practicality, and conclude that the protocol realizes secure, anonymous, and authenticated communication between users and service providers with practical performance., Comment: This is a preprint version of our paper presented in SAC'14, March 24-28, 2014, Gyeongju, Korea. ACMSAC 2014

Group signature with message-dependent opening (GS-MDO) is a kind of group signature in which only the signers who have created group signatures on problematic messages will be identified. In the previous GS-MDO scheme, however, the number of problematic messages is bounded owing to a limitation of the Groth-Sahai proofs. In this paper, we propose the first GS-MDO scheme with the unbounded-MDO functionality in the random oracle model. Our unbounded GS-MDO scheme is based on the short group signature scheme proposed by Boneh, Boyen, and Shacham and the Boneh-Franklin identity-based encryption scheme. To combine these building blocks and to achieve CCA-anonymity, we also construct a special type of multiple encryption. This technique yields an efficient construction compared with the previous bounded GS-MDO scheme: the signature of our scheme contains about 16 group elements (3630 bits), whereas that of the previous scheme has about 450 group elements (75820 bits).

The number of computer security incidents is rising in unison with the development of cyber-society. One reason for this is a lack of users' security awareness. The widespread use of mobile devices further complicates this problem. An approach for raising the awareness level is introducing a system that visualizes and issues alerts of security risks end-users. This paper introduces the architecture of such a system. It analyzes information by monitoring the user's end-to-end communication and its related entities, looks up knowledge bases, and provides alerts by directly visualizing risks to the user. One characteristic of this system is its ability to enable customized visualization for each user, which boosts the user's risk awareness and understanding. This paper also introduces the system's proof-of-concept implementation, which demonstrates the architecture's feasibility. Based on the prototype, the paper discusses the direction of further technical development.