Optimized Quantum Implementation and Analysis of CHAM

By employing the Grover’s search algorithm (which reduces the complexity of an otherwise secure cipher by the square root boundary), a quantum computer has the potential to undermine the security of symmetric key cryptography. Recently, studies have proposed analyzing potential attacks using the Grover’s search algorithm in conjunction with quantum circuit implementations for symmetric key cryptography. Analyzing quantum attacks on a cipher (i.e., quantum cryptanalysis) and estimating the necessary quantum resources is related to evaluating post-quantum security for the cipher. In this chapter, we revisit quantum implementations of CHAM, a lightweight cipher family, with a focus on optimizing the linear operations in its key schedule. We represent the linear equations of CHAM as matrices and apply optimization techniques. Using the improved CHAM quantum circuits, we estimate the cost of the Grover’s key search and evaluate its post-quantum security strength.