PhD Defense: FRONTIERS IN LATTICE CRYPTOGRAPHY AND PROGRAM OBFUSCATION
In this dissertation, we explore the frontiers of theory of cryptography along two lines: (i) Lattice Cryptography, the primary sub-area of post-quantum cryptographic research; and (ii) Cryptographic Program Obfuscation. A program obfuscator is a type of cryptographic software compiler that outputs executable code with the guarantee that "whatever can be hidden about the internal workings of program code, is hidden." Indeed, program obfuscation can be viewed as a "universal and cryptographically-complete" tool.
Our first contribution is the construction of a deniable attribute-based encryption scheme from lattices. A deniable encryption scheme is secure against not only eavesdropping attacks as required by semantic security, but also stronger coercion attacks performed after the fact. An attribute-based encryption scheme allows "fine-grained" access to ciphertexts, allowing for a decryption access policy to be embedded in ciphertexts and keys. We achieve both properties simultaneously for the first time from lattices.
Our second contribution is the construction of a digital signature scheme that enjoys both short signatures and a completely tight security reduction from lattices. As a matter of independent interest, we give an improved method of randomized inversion of the G gadget matrix, which reduces the noise growth rate in homomorphic evaluations performed in a large number of lattice-based cryptographic schemes, without incurring the high cost of sampling discrete Gaussians.
Our third contribution is the first, full-scale implementation of secure program obfuscation in software. Our toolchain takes code written in a C-like programming language, specialized for cryptography, and produces secure, obfuscated software.
Our fourth contribution is a new cryptanalytic attack against a variety of "early" program obfuscation candidates. We provide a general, efficiently-testable property for any two branching programs, called partial inequivalence, which we show is sufficient for launching an "annihilation attack" against several obfuscation candidates based on Garg-Gentry-Halevi multilinear maps.
Chair: Dr. Jonathan Katz
Dean's rep: Dr. Nikhil Chopra
Members: Dr. Bill Gasarch
Dr. Andrew Childs
Dr. Dana Dachman-Soled