Single Untrusted Server Research Articles

On the Security of Offloading Post-Processing for Quantum Key Distribution.

Quantum key distribution (QKD) has been researched for almost four decades and is currently making its way to commercial applications. However, deployment of the technology at scale is challenging because of the very particular nature of QKD and its physical limitations. Among other issues, QKD is computationally intensive in the post-processing phase, and devices are therefore complex and power hungry, which leads to problems in certain application scenarios. In this work, we study the possibility to offload computationally intensive parts in the QKD post-processing stack in a secure way to untrusted hardware. We show how error correction can be securely offloaded for discrete-variable QKD to a single untrusted server and that the same method cannot be used for long-distance continuous-variable QKD. Furthermore, we analyze possibilities for multi-server protocols to be used for error correction and privacy amplification. Even in cases where it is not possible to offload to an external server, being able to delegate computation to untrusted hardware components on the device itself could improve the cost and certification effort for device manufacturers.

Private Decision Tree Evaluation by a Single Untrusted Server for Machine Learnig as a Service

Private Decision Tree Evaluation by a Single Untrusted Server for Machine Learnig as a Service

Comment on “An Efficient ABE Scheme With Verifiable Outsourced Encryption and Decryption”

Recently in IEEE Access (DOI: 10.1109/ACCESS.2018.2890565), Li <i>et al.</i> proposed a secure outsourcing algorithm for modular exponentiation in one single untrusted server model and a new method of generating transformation keys. They claimed that their solution can securely outsource encryption and decryption to untrusted ESP (encryption service providers) and DSP (decryption service providers), leaving only a constant number of simple operations for the DO (data owner) and eligible users to perform locally. In addition, both DO and qualified users can check the correctness of the results returned from ESP and DSP, respectively. Although the authors provide security proofs for their scheme, unfortunately, after carefully observing their scheme, we find that the scheme has security vulnerabilities. These vulnerabilities allow the adversary to generate the sub-key for any attribute and replace ciphertext sub-item, which result in the adversary to be able to break their scheme. In response to this problem, we propose an improved solution and proved its security.

An Efficient ABE Scheme With Verifiable Outsourced Encryption and Decryption

Attribute-based encryption (ABE) is a promising cryptographic tool for data owner (DO) to realize fine-grained date sharing in the cloud computing. In the encryption of most existing ABE schemes, a substantial number of modular exponentiations are often required; the computational cost of it is growing linearly with the complexity of the access policy. Besides, in the most existing ABE with outsourced decryption, the computation cost of generating transformation key is growing linearly with the number of attributes associated with user private key; these computations are prohibitively high for mobile device users, which becomes a bottleneck limiting its application. To address the above issues, we propose a secure outsourcing algorithm for modular exponentiation in one single untrusted server model and a new method to generate the transformation key. Based on these techniques and Brent Waters's ciphertext-policy ABE scheme, we propose an ABE scheme with verifiable outsourced both encryption and decryption, which can securely outsource encryption and decryption to untrusted encryption service provider (ESP) and decryption service provider (DSP), respectively, leaving only a constant number of simple operations for the DO and eligible users to perform locally. In addition, both DO and the eligible users can check the correctness of results returned from the ESP and the DSP with a probability, respectively. Finally, we provide the experimental evaluation and security analysis of our scheme, which indicates that our construction is suitable for the mobile environment.

ExpSOS: Secure and Verifiable Outsourcing of Exponentiation Operations for Mobile Cloud Computing

Discrete exponential operation, such as modular exponentiation and scalar multiplication on elliptic curves, is a basic operation of many public-key cryptosystems. However, the exponential operations are considered prohibitively expensive for resource-constrained mobile devices. In this paper, we address the problem of secure outsourcing of exponentiation operations to one single untrusted server. Our proposed secure outsourcing scheme for general exponential (ExpSOS) only requires a very limited number of modular multiplications at local mobile environment, and thus it can achieve significant computational performance gain. ExpSOS also provides a secure verification scheme with probability approximately 1 to ensure that the mobile end users can always receive valid results. The comprehensive analysis as well as the simulation results in real mobile device demonstrates that our proposed ExpSOS can significantly improve the existing schemes in efficiency, security, and result verifiability. We apply ExpSOS to securely outsource several cryptographic protocols to show that ExpSOS can be widely applied to many computation-intensive applications and achieve significant performance improvement.

CExp: secure and verifiable outsourcing of composite modular exponentiation with single untrusted server

Outsourcing computing allows users with resource-constrained devices to outsource their complex computation workloads to cloud servers that may not be honest. In this paper, we propose a new algorithm for securing the outsourcing of composite modular exponentiation, which is one of the most complex computing tasks in discrete-log based cryptographic protocols. Unlike algorithms based on two untrusted servers, we outsource modular exponentiation operation to only a single server, which eliminates the potential for a collusion attack when using two servers. Moreover, our proposed algorithm can hide the base and exponent of the outsourced data, which prevents the exposure of sensitive information to cloud servers. In addition, compared with the state-of-the-art algorithms, our scheme has remarkably better checkability. The user could detect any misbehavior with a probability of one if the server returns a fault result.

Efficient and verifiable algorithms for secure outsourcing of cryptographic computations

Reducing computational cost of cryptographic computations for resource-constrained devices is an active research area. One of the practical solutions is to securely outsource the computations to an external and more powerful cloud server. Modular exponentiations are the most expensive computation from the cryptographic point of view. Therefore, outsourcing modular exponentiations to a single, external and potentially untrusted cloud server while ensuring the security and privacy provides an efficient solution. In this paper, we propose new efficient outsourcing algorithms for modular exponentiations using only one untrusted cloud server. These algorithms cover public-base and private-exponent, private-base and public-exponent, private-base and private-exponent, more generally private-base and private-exponents simultaneous modular exponentiations. Our algorithms are the most efficient solutions utilizing only one single untrusted server with the best checkability probabilities. Furthermore, unlike existing schemes, which have fixed checkability probability, our algorithms provide adjustable predetermined checkability parameters. Finally, we apply our algorithms to outsource oblivious transfer protocols and blind signatures which are expensive primitives in modern cryptography.

Refereed delegation of computation

Consider a weak client that wishes to delegate a computation to an untrusted server, and then verify the correctness of the result. When the client uses only a single untrusted server, current techniques suffer from disadvantages such as computational inefficiency for the client or the server, limited functionality, or high round complexity.We demonstrate relatively efficient and general solutions where the client delegates the computation to several servers, and is guaranteed to determine the correct answer as long as even a single server is honest. We call such protocols Refereed Delegation of Computation (RDoC) and show:1.A computationally secure protocol for any efficiently computable function, with logarithmically many rounds, based on any collision-resistant hash family. In our description of this protocol, we model the computation as running on a Turing Machine, but the protocol can be adapted to other computation models. We present an adaptation for the X86 computation model and a prototype implementation, called Quin, for Windows executables. We describe the architecture of Quin and experiment with several parameters on live cloud servers. We show that the protocol is practical, can work with real-world cloud servers, and is efficient for both the servers and for the client.2.A 1-round statistically secure protocol for any log-space uniform NC circuit. In contrast, in the single server setting all known one-round delegation protocols are computationally sound. The protocol extends the arithemetization techniques of Goldwasser, Kalai and Rothblum (STOC 08) and Feige and Kilian (STOC 97).