International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Ajith Ramanathan

Affiliation: Stanford University

Publications

Year
Venue
Title
2008
JOFC
2006
TCC
2006
EPRINT
On the Relationships Between Notions of Simulation-Based Security
Several compositional forms of simulation-based security have been proposed in the literature, including universal composability, black-box simulatability, and variants thereof. These relations between a protocol and an ideal functionality are similar enough that they can be ordered from strongest to weakest according to the logical form of their definitions. However, determining whether two relations are in fact identical depends on some subtle features that have not been brought out in previous studies. We identify the position of a ``master process" in the distributed system, and some limitations on transparent message forwarding within computational complexity bounds, as two main factors. Using a general computational framework, called Sequential Probabilistic Process Calculus (SPPC), we clarify the relationships between the simulation-based security conditions. We also prove general composition theorems in SPPC. Many of the proofs are carried out based on a small set of equivalence principles involving processes and distributed systems. This gives us results that carry over to a variety of computational models.
2005
TCC
2005
EPRINT
Games and the Impossibility of Realizable Ideal Functionality
A cryptographic primitive or a security mechanism can be specified in a variety of ways, such as a condition involving a game against an attacker, construction of an ideal functionality, or a list of properties that must hold in the face of attack. While game conditions are widely used, an ideal functionality is appealing because a mechanism that is indistinguishable from an ideal functionality is therefore guaranteed secure in any larger system that uses it. We relate ideal functionalities to games by defining the \textit{set} of ideal functionalities associated with a game condition and show that under this definition, which reflects accepted use and known examples, bit commitment, a form of group signatures, and some other cryptographic concepts do not have any realizable ideal functionality.