Post-Quantum Security of the CBC, CFB, OFB, CTR, and XTS Modes of Operation
We examine the IND-qCPA security of the wide-spread block cipher modes of operation CBC, CFB, OFB, CTR, and XTS i.e., security against quantum adversaries doing queries in superposition. We show that OFB and CTR are secure assuming that the underlying block cipher is a standard secure PRF a pseudorandom function secure under classical queries. We give counterexamples that show that CBC, CFB, and XTS are not secure under the same assumption. And we give proofs that CBC and CFB mode are secure if we assume a quantum secure PRF secure under queries in superposition.