Abstract
In this work, we have analyzed ChaCha against Grover’s search algorithm. We designed a reversible quantum circuit of ChaCha and then estimated the resources required to implement Grover. We showed that for MAXDEPTH = \( 2^{40} \), the ChaCha20 256-bit key can be recovered using Grover’s search algorithm with a gate count of \( 1.233 \cdot 2^{251} \), which is less than the NIST’s requirement of \( 2^{258} \). We also showed that implementing Grover’s algorithm greatly depends on the number of rounds in ChaCha. We deduced that ChaCha would require approximately 166 rounds so that implementing a non parallelized Grover would require a \( 2^{298} \) gate count. We implemented a ChaCha-like toy cipher in IBMQ simulator and recovered key using Grover’s algorithm.
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Bathe, B., Anand, R. & Dutta, S. Evaluation of Grover’s algorithm toward quantum cryptanalysis on ChaCha. Quantum Inf Process 20, 394 (2021). https://doi.org/10.1007/s11128-021-03322-7
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DOI: https://doi.org/10.1007/s11128-021-03322-7