Symmetric Encryption Solutions to Millionaire's Problem and Its Extension

Millionaire's problem is the base of secure multiparty computation, and its solutions have become basic blocks of many secure multi-party computation solutions. Unfortunately, most solutions to millionaire's problem are based on public key cryptography, and thus are inefficient. Furthermore, all solutions are designed to solve millionaire's problem for natural number case to privately determine which natural number is larger. If the numbers are real, these solutions do not directly work. In this paper, we first propose a symmetric key cryptographic solution to millionaire's problem for natural numbers case, and then generalize it to the case of real numbers, that is to privately determine which real number is larger. We further prove, by simulation paradigm, that these solutions are private. These solutions are really efficient.