  Here we introduce an improved approach to Variational Quantum Attack
Algorithms (VQAA) on crytographic protocols. Our methods provide robust quantum
attacks to well-known cryptographic algorithms, more efficiently and with
remarkably fewer qubits than previous approaches. We implement simulations of
our attacks for symmetric-key protocols such as S-DES, S-AES and Blowfish. For
instance, we show how our attack allows a classical simulation of a small
8-qubit quantum computer to find the secret key of one 32-bit Blowfish instance
with 24 times fewer number of iterations than a brute-force attack. Our work
also shows improvements in attack success rates for lightweight ciphers such as
S-DES and S-AES. Further applications beyond symmetric-key cryptography are
also discussed, including asymmetric-key protocols and hash functions. In
addition, we also comment on potential future improvements of our methods. Our
results bring one step closer assessing the vulnerability of large-size
classical cryptographic protocols with Noisy Intermediate-Scale Quantum (NISQ)
devices, and set the stage for future research in quantum cybersecurity.
