Key Substitution Attacks on the CFS Signature

Abstract

At Asiacrypt'2001, Courtois, Finiasz and Sendrier proposed the first coding-based signature scheme which is also known as the CFS signature. The CFS signature is seen as one of the candidates of quantum immune signatures. In this letter, we show that the CFS signature is susceptible to both strong-key substitution attacks and weak-key substitution attacks. We also discuss potential countermeasures.