Aryabhata Remainder Theorem: Relevance to Public-Key Crypto-Algorithms

Abstract

Public-key crypto-algorithms are widely employed for authentication, signatures, secret-key generation and access control. The new range of public-key sizes for RSA and DSA has gone up to 1024 bits and beyond. The elliptic-curve key range is from 162 bits to 256 bits. Many varied software and hardware algorithms are being developed for implementation for smart-card crypto-coprocessors and for public-key infrastructure. We begin with an algorithm from Aryabhatiya for solving the indeterminate equation a · x + c = b · y of degree one (also known as the Diophantine equation) and its extension to solve the system of two residues X mod mi = Xi (for i = 1,2). This contribution known as the Aryabhatiya algorithm (AA) is very profound in the sense that the problem of two congruences was solved with just one modular inverse operation and a modular reduction to a smaller modulus than the compound modulus. We extend AA to any set of t residues, and this is stated as the Aryabhata remainder theorem (ART). An iterative algorithm is also given to solve for t moduli mi (i = 1, 2,... , t). The ART, which has much in common with the extended Euclidean algorithm (EEA), Chinese remainder theorem (CRT) and Garner's algorithm (GA), is shown to have a complexity comparable to or better than that of the CRT and GA.