Private set intersection (PSI) evaluation is a well-known secure multiparty computation problem that has received much attention. Authors generally tend to state improved performance as a goal and justification for their proposed PSI schemes. In this paper, we propose an original and new approach to two-party PSI evaluation for small sets that is based on small RSA subgroups. Contrary to previous approaches that at best incur linear communication and computational complexity to the set size, our scheme is the first to achieve constant communication and computational complexity incurring only 2 group elements and 5 exponentiations. An extended PSI scheme handling larger sets is also proposed.

Continuous authentication utilizes automatic recognition of certain user features for seamless and passive authentication without requiring user attention. Such features can be divided into categories of physiological biometrics and behavioral biometrics. Keystroke dynamics is proposed for behavioral biometrics-oriented authentication by recognizing users by means of their typing patterns. However, it has been pointed out that continuous authentication using physiological biometrics and behavior biometrics incur privacy risks, revealing personal characteristics and activities. In this paper, we consider a previously proposed keystroke dynamics-based authentication scheme that has no privacy-preserving properties. In this regard, we propose a generic privacy-preserving version of this authentication scheme in which all user features are encrypted -- preventing disclosure of those to the authentication server. Our scheme is generic in the sense that it assumes homomorphic cryptographic primitives. Authentication is conducted on the basis of encrypted data due to the homomorphic cryptographic properties of our protocol.

Threshold broadcast encryption (TBE) is a class of threshold cryptographic schemes that allow a sender to compute ciphertexts to ad hoc user groups. The plaintext can only be recovered if t of the pertaining recipients collaborate by each producing a partial decryption share. Existing TBE schemes require that the partial decryptions are transferred through secure channels to a single combiner that restores the plaintext. Thus, the single combiner becomes the eventual target for the deciphered plaintext, and not the addressed group. As such, a single combiner and explicit secure channels are inconsistent with broadcasting.

In this paper, we present the novel concept of fractional public keys and an efficient zero-round multi-party Diffie-Hellman key agreement scheme that is based on fractional public keys. Shared group keys are computed highly efficiently by using the fractional public keys of multiple participants as exponents. The scheme provides therefore an efficient and elegant way of multi-party key agreement without key establishment data transmissions. The presented cryptographic scheme is collusion resistant to any number of users.

Electronic patient records (EPR) may contain highly confidential and sensitive medical data, and it is therefore essential that such information is properly protected. Medical teams that are providing care to a patient has a legitimate need to access the medical data of the concerning patient, and this could be a valid criteria for medical professionals to obtain access to such data. Moreover, since teams consist of more than one individual, the consent or agreement among a number of the members of a medical team could by itself be a proper basis for trust and therefore a legitimate basis for medical teams to acquire access to medical data. In this paper, we present three closely related cryptographic protocols for secure team-based EPR access acquisition where the crypto graphically verifiable mutual consent from some minimum number of participants of a medical team is the granting criteria for the team to acquire EPR access. The schemes are based on threshold cryptography and are moreover broadcast-oriented, and are thus well-suited for wireless networks. All schemes do also provide secure transfer of medical data.

In this paper, we present three related and efficient cryptographic schemes for secure communication for hierarchically composed user groups. In such a group, each user is associated with a hierarchical level, so that the group members are divided into two or more hierarchical security classes. Most existing hierarchical cryptographic schemes provide hierarchical deduction of hierarchically arranged predefined keys. In contrast, the first scheme provides secure establishment of hierarchically ordered session keys. Since it prohibits long-term key deduction for underlying (and overlying) classes, such keys do not need to be updated session-wise. An essential security property is that the users can only obtain hierarchical session keys for their own and underlying levels, while it is prevented that a user can obtain hierarchical session keys of overlying levels.This scheme is extended to a hierarchical public key cryptosystem based on the ElGamal cryptosystem, and furthermore to an ElGamal-based threshold decryption scheme. Due to the threshold security requirement, at least t arbitrary group members are required to carry out decryption. The threshold scheme requires only one round of broadcasting in the decryption phase, and is thuswell-suitable for wireless networks.

A common property of practically all multi-party key agreement protocols is that they are non-hierarchical, i.e., they do not support user hierarchies. However, in real life, it is likely that members of groups and organizations differ in ranking according to their job positions so that people of higher rankings are privileged the same and higher privileged information than people of lower rankings. Hierarchical access control schemes address this issue, but they assume rather fixed scenarios based on centralized key distribution with predefined root keys and key parameters set by a trusted center. In this paper, we propose a decentralized efficient hierarchical multi-party key agreement protocol well-suited for ad-hoc wireless networks that ensures secure user hierarchies.

Electronic patient records contains highly personal and confidential information that it is essential to keep private. Thus, only the medical professionals providing care to a patient should access the patient record of the concerning patient. As personal medical data can be considered to be the property of the corresponding patient, it is justified that patients should have the opportunity to exert control over their own data. In this paper, we propose a cryptographic access control scheme allowing patients to grant medical teams authorizations to access their medical data. Moreover, the hierarchical aspects of teams are taken into account so that the modules of the patient record are to be accessed according to the individual privileges of the medical professionals of the team. Thus, more privileged users obtain larger portions of the data than less privileged users