PERKS: Persistent and Distributed Key Acquisition for Secure Storage from Passwords

We investigate how users of instant messaging (IM) services can acquire strong encryption keys to back up their messages and media with strong cryptographic guarantees. Many IM users regularly change their devices and use multiple devices simultaneously, ruling out any long-term secret storage. Extending the end-to-end encryption guarantees from just message communication to also incorporate backups has so far required either some trust in an IM or outsourced storage provider, or use of costly third-party encryption tools with unclear security guarantees. Recent works have proposed solutions for password-protected key material, however all require one or more servers to generate and/or store per-user information, inevitably invoking a cost to the users. We define distributed key acquisition (DKA) as the primitive for the task at hand, where a user interacts with one or more servers to acquire a strong cryptographic key, and both user and server are required to store as little as possible. We present a construction framework that we call PERKS---Password-based Establishment of Random Keys for Storage---providing efficient, modular and simple protocols that utilize Oblivious Pseudorandom Functions (OPRFs) in a distributed manner with minimal storage by the user (just the password) and servers (a single global key for all users). Along the way we introduce a formal treatment of DKA, and provide proofs of security for our constructions in their various flavours. Our approach enables key rotation by the OPRF servers, and for this we incorporate updatable encryption. Finally, we show how our constructions fit neatly with recent research on encrypted outsourced storage to provide strong security guarantees for the outsourced ciphertexts

PASTA: PASsword-based Threshold Authentication

Token-based authentication is commonly used to enable a single-sign-on experience on the web, in mobile applications and on enterprise networks using a wide range of open standards and network authentication protocols: clients sign on to an identity provider using their username/password to obtain a cryptographic token generated with a master secret key, and store the token for future accesses to various services and applications. The authentication server(s) are single point of failures that if breached, enable attackers to forge arbitrary tokens or mount offline dictionary attacks to recover client credentials. Our work is the first to introduce and formalize the notion of password-based threshold token-based authentication which distributes the role of an identity provider among $n$ servers. Any t servers can collectively verify passwords and generate tokens, while no t-1 servers can forge a valid token or mount offline dictionary attacks. We then introduce PASTA, a general framework that can be instantiated using any threshold token generation scheme, wherein clients can sign-on using a two-round (optimal) protocol that meets our strong notions of unforgeability and password-safety. We instantiate and implement our framework in C++ using two threshold message authentication codes (MAC) and two threshold digital signatures with different trade-offs. Our experiments show that the overhead of protecting secrets and credentials against breaches in PASTA, i.e. compared to a naive single server solution, is extremely low (1-5%) in the most likely setting where client and servers communicate over the internet. The overhead is higher in case of MAC-based tokens over a LAN (though still only a few milliseconds) due to public-key operations in PASTA. We show, however, that this cost is inherent by proving a symmetric-key only solution impossible

Secure Management of Personal Health Records by Applying Attribute-Based Encryption

The confidentiality of personal health records is a major problem when patients use commercial Web-based systems to store their health data. Traditional access control mechanisms, such as Role-Based Access Control, have several limitations with respect to enforcing access control policies and ensuring data confidentiality. In particular, the data has to be stored on a central server locked by the access control mechanism, and the data owner loses control on the data from the moment when the data is sent to the requester. Therefore, these mechanisms do not fulfil the requirements of data outsourcing scenarios where the third party storing the data should not have access to the plain data, and it is not trusted to enforce access control policies. In this paper, we describe a new approach which enables secure storage and controlled sharing of patient’s health records in the aforementioned scenarios. A new variant of a ciphertext-policy attribute-based encryption scheme is proposed to enforce patient/organizational access control policies such that everyone can download the encrypted data but only authorized users from the social domain (e.g. family, friends, or fellow patients) or authorized users from the professional\ud domain (e.g. doctors or nurses) are allowed to decrypt it

Updatable Oblivious Key Management for Storage Systems

We introduce Oblivious Key Management Systems (KMS) as a more secure alternative to traditional wrapping-based KMS that form the backbone of key management in large-scale data storage deployments. The new system, that builds on Oblivious Pseudorandom Functions (OPRF), hides keys and object identifiers from the KMS, offers unconditional security for key transport, provides key verifiability, reduces storage, and more. Further, we show how to provide all these features in a distributed threshold implementation that enhances protection against server compromise. We extend this system with updatable encryption capability that supports key updates (known as key rotation) so that upon the periodic change of OPRF keys by the KMS server, a very efficient update procedure allows a client of the KMS service to non-interactively update all its encrypted data to be decryptable only by the new key. This enhances security with forward and post-compromise security, namely, security against future and past compromises, respectively, of the client\u27s OPRF keys held by the KMS. Additionally, and in contrast to traditional KMS, our solution supports public key encryption and dispenses with any interaction with the KMS for data encryption (only decryption by the client requires such communication). Our solutions build on recent work on updatable encryption but with significant enhancements applicable to the remote KMS setting. In addition to the critical security improvements, our designs are highly efficient and ready for use in practice. We report on experimental implementation and performance

Quantum-Resistant Password-Based Threshold Single-Sign-On Authentication with Updatable Server Private Key

Passwords are the most prevalent authentication mechanism and proliferate on nearly every new web service. As users are overloaded with the tasks of managing dozens even hundreds of passwords, accordingly password-based single-sign-on (SSO) schemes have been proposed. In password-based SSO schemes, the authentication server needs to maintain a sensitive password file, which is an attractive target for compromise and poses a single point of failure. Hence, the notion of password-based threshold authentication (PTA) system has been proposed. However, a static PTA system is threatened by perpetual leakage (e.g., the adversary perpetually compromises servers). In addition, most of the existing PTA schemes are built on the intractability of conventional hard problems and become insecure in the quantum era. In this work, we first propose a threshold oblivious pseudorandom function (TOPRF) to harden the password so that PTA schemes can resist offline password guessing attacks. Then, we employ the threshold homomorphic aggregate signature (THAS) over lattices to construct the first quantum-resistant password-based threshold single-sign-on authentication scheme with the updatable server private key. Our scheme resolves various issues arising from user corruption and server compromise, and it is formally proved secure against quantum adversaries. Comparison results show that our scheme is superior to its counterparts

Optimizing Key Management within a Crypto-System using Aggregate Keys

Sharing data with peers is an important functionality in cloud storage. This is a study and analysis of secure, efficient, and flexible sharing of data with other users in cloud storage. The new public key encryptions which produce constant-size ciphertexts in such a way that effective delegation of decryption rights given to any set of ciphertexts are described in this paper. The novelty of the mechanism is that someone can aggregate any number of secret keys and turn them into a small single key, but combine the power of all the keys being grouped. To describe, in other words, the secret key holder could release a constant-size aggregate key for more flexible choices of ciphertext set in cloud storage, but different encrypted files outside of the set remain confidential. The aggregate compact key can be sent to others with ease or saved in a smart card with very less secure storage. In this paper, we discuss various such mechanisms and demonstrate the topic with a project. Some of the papers written by other authors in the area are analyzed in this paper. The project in this paper is a partial implementation of the proposed Crypto System