International Association
for Cryptologic Research

In this work, we define and construct fully homomorphic non-interactive zero knowledge (FH-NIZK) and non-interactive witness-indistinguishable (FH-NIWI) proof systems.     We focus on the NP complete language L, where, for a boolean circuit C and a bit b, the pair $$(C,b)\in L$$ if there exists an input $$\mathbf {w}$$ such that $$C(\mathbf {w})=b$$. For this language, we call a non-interactive proof system fully homomorphic if, given instances $$(C_i,b_i)\in L$$ along with their proofs $$\varPi _i$$, for $$i\in \{1,\ldots ,k\}$$, and given any circuit $$D:\{0,1\}^k\rightarrow \{0,1\}$$, one can efficiently compute a proof $$\varPi $$ for $$(C^*,b)\in L$$, where $$C^*(\mathbf {w}^{(1)},\ldots ,\mathbf {w}^{(k)})=D(C_1(\mathbf {w}^{(1)}),\ldots ,C_k(\mathbf {w}^{(k)}))$$ and $$D(b_1,\ldots ,b_k)=b$$. The key security property is unlinkability: the resulting proof $$\varPi $$ is indistinguishable from a fresh proof of the same statement.     Our first result, under the Decision Linear Assumption (DLIN), is an FH-NIZK proof system for L in the common random string model. Our more surprising second result (under a new decisional assumption on groups with bilinear maps) is an FH-NIWI proof system that requires no setup.