There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

We propose a fully functional identity-based encryption (IBE) scheme. The scheme has chosen ciphertext security in the random oracle model assuming a variant of the computational Diffie--Hellman problem. Our system is based on bilinear maps between groups. The Weil pairing on elliptic curves is an example of such a map. We give precise definitions for secure IBE schemes and give several applications for such systems.

/pdf/identity-based-encryption-from-the-weil-pairing-1x4254f2zc.pdf

Identity-Based Encryption from the Weil Pairing

In several distributed systems a user should only be able to access data if a user posses a certain set of credentials or attributes. Currently, the only method for enforcing such policies is to employ a trusted server to store the data and mediate access control. However, if any server storing the data is compromised, then the confidentiality of the data will be compromised. In this paper we present a system for realizing complex access control on encrypted data that we call ciphertext-policy attribute-based encryption. By using our techniques encrypted data can be kept confidential even if the storage server is untrusted; moreover, our methods are secure against collusion attacks. Previous attribute-based encryption systems used attributes to describe the encrypted data and built policies into user's keys; while in our system attributes are used to describe a user's credentials, and a party encrypting data determines a policy for who can decrypt. Thus, our methods are conceptually closer to traditional access control methods such as role-based access control (RBAC). In addition, we provide an implementation of our system and give performance measurements.

/pdf/ciphertext-policy-attribute-based-encryption-q09jxdyx4n.pdf

Ciphertext-Policy Attribute-Based Encryption

As more sensitive data is shared and stored by third-party sites on the Internet, there will be a need to encrypt data stored at these sites. One drawback of encrypting data, is that it can be selectively shared only at a coarse-grained level (i.e., giving another party your private key). We develop a new cryptosystem for fine-grained sharing of encrypted data that we call Key-Policy Attribute-Based Encryption (KP-ABE). In our cryptosystem, ciphertexts are labeled with sets of attributes and private keys are associated with access structures that control which ciphertexts a user is able to decrypt. We demonstrate the applicability of our construction to sharing of audit-log information and broadcast encryption. Our construction supports delegation of private keys which subsumesHierarchical Identity-Based Encryption (HIBE).

/pdf/attribute-based-encryption-for-fine-grained-access-control-3fjqefx4um.pdf

Attribute-based encryption for fine-grained access control of encrypted data

We introduce a short signature scheme based on the Computational Diffie-Hellman assumption on certain elliptic and hyperelliptic curves. The signature length is half the size of a DSA signature for a similar level of security. Our short signature scheme is designed for systems where signatures are typed in by a human or signatures are sent over a low-bandwidth channel.

/pdf/short-signatures-from-the-weil-pairing-5ajupg0cpi.pdf

Short Signatures from the Weil Pairing

We present a runtime environment for executing secure programs via a multi-party computation protocol in the preprocessing model. The runtime environment is general and allows arbitrary reactive computations to be performed. A particularly novel aspect is that it automatically determines the minimum number of rounds needed for a computation, given a specific instruction sequence, and it then uses this to minimize the overall cost of the computation. Various experiments are reported on, on various non-trivial functionalities. We show how, by utilizing the ability of modern processors to execute multiple threads at a time, one can obtain various tradeoffs between latency and throughput.

/pdf/an-architecture-for-practical-actively-secure-mpc-with-59yn19dbzw.pdf

An architecture for practical actively secure MPC with dishonest majority.

This chapter formalises the concept of positional number systems, a way of representing numbers in different bases. Using decimal as a starting point, the goal is to introduce binary: motivation is given by the need to store information on CDs and DVDs. For example, if we can represent any number using 0 and 1 then we can physically store such numbers as pits and lands on the surface of a CD.

Compressing and Correcting Digital Media

A number of previous papers explored the notion of identity-based group signature. We present a generic construction of identity-based group signatures. Our construction is based on the Naor transformation of a identity-based signature out of an identity-based encryption, adjusted to hierarchical identity-based encryption. We identify sufficient conditions on the underlying HIBE so that the scheme that results from our transformation meets our security definitions. Finally, we suggest a couple of extensions enabled by our construction, one of which is to hierarchical identity-based group signatures.

/pdf/identity-based-group-signatures-from-hierarchical-identity-1gkw7ojdr9.pdf

Identity Based Group Signatures from Hierarchical Identity-Based Encryption.

Feta

We study the security of the widely deployed Secure Session Layer/Transport Layer Security (TLS) key agreement protocol. Our analysis identifies, justifies, and exploits the modularity present in the design of the protocol: the application keys offered to higher level applications are obtained from a master key, which in turn is derived, through interaction, from a pre-master key. Our first contribution consists of formal models that clarify the security level enjoyed by each of these types of keys. The models that we provide fall under well established paradigms in defining execution, and security notions. We capture the realistic setting where only one of the two parties involved in the execution of the protocol (namely the server) has a certified public key, and where the same master key is used to generate multiple application keys. The main contribution of the paper is a modular and generic proof of security for the application keys established through the TLS protocol. We show that the transformation used by TLS to derive master keys essentially transforms an arbitrary secure pre-master key agreement protocol into a secure master-key agreement protocol. Similarly, the transformation used to derive application keys works when applied to an arbitrary secure master-key agreement protocol. These results are in the random oracle model. The security of the overall protocol then follows from proofs of security for the basic pre-master key generation protocols employed by TLS.

/pdf/a-modular-security-analysis-of-the-tls-handshake-protocol-3gc1xjkmcj.pdf

Nigel P. Smart

Papers

An architecture for practical actively secure MPC with dishonest majority.

Compressing and Correcting Digital Media

Identity Based Group Signatures from Hierarchical Identity-Based Encryption.

Feta

A Modular Security Analysis of the TLS Handshake Protocol.