Determining the Optimal Random-padding Size for Rabin Cryptosystems

TL;DR

This paper analyzes the shortpad attack on Rabin cryptosystems and derives a formula to determine the optimal padding size that balances security and efficiency.

Contribution

It provides a new formula to calculate the optimal random-padding size to prevent shortpad attacks in Rabin encryption.

Findings

Derived processing-time formula for shortpad attack

Determined optimal padding size for security

Enhanced understanding of padding security criteria

Abstract

Rabin encryption and a secure ownership transfer protocol based on the difficulty of factorization of a public key use a small public exponent. Such encryption requires random number padding. The Coppersmith's shortpad attack works effectively on short padding, thereby allowing an adversary to extract the secret message. However, the criteria for determining the appropriate padding size remains unclear. In this paper, we derived the processing-time formula for the shortpad attack and determined the optimal random-padding size in order to achieve the desired security.