Paper 2025/1089

Rugged Pseudorandom Permutations with Beyond-Birthday-Bound Security

Abstract

A rugged pseudorandom permutation (RPRP) is a security notion for variable-length tweakable ciphers that is strictly weaker than the traditional notion of a strong pseudorandom permutation. Being a weaker security notion it admits more efficient constructions. Yet the notion is strong enough so that any such construction can lend itself to a number of practical applications. It can be used to construct onion encryption, misuse-resistant AEAD, and AEAD secure under the release of unverified plaintext. Two recent works have introduced the notion, explored some of its applications, and studied a number of constructions that meet this notion. However, no constructions are known to achieve this notion with beyond-birthday-bound security. Current cloud applications are processing amounts of data that go well beyond the traditional $2^{32}$ barrier, and $2^{64}$ is becoming the new target. As such, the need for encryption with beyond-birthday-bound security has become a very practical concern. In this work, we present the first constructions for variable-length tweakable ciphers that satisfy RPRP security beyond the birthday bound. From these constructions, we readily obtain efficient AEAD schemes that are optimally secure against once misuse and the release of unverified plaintext.