Homomorphic Time-Lock Puzzles and Applications

Abstract

Time-lock puzzles allow one to encrypt messages for the future, by efficiently generating a puzzle with a solution s that remains hidden until time$$\mathcal {T}$$ has elapsed. The solution is required to be concealed from the eyes of any algorithm running in (parallel) time less than$$\mathcal {T}$$. We put forth the concept of homomorphic time-lock puzzles, where one can evaluate functions over puzzles without solving them, i.e., one can manipulate a set of puzzles with solutions$$(s:1, \dots, s_n)$$ to obtain a puzzle that solves to$$f(s:1, \ldots, s_n)$$, for any function f. We propose candidate constructions under concrete cryptographic assumptions for different classes of functions. Then we show how homomorphic time-lock puzzles overcome the limitations of classical time-lock puzzles by proposing new protocols for applications of interest, such as e-voting, multi-party coin flipping, and fair contract signing.