Paper 2026/1227

Chosen Ciphertext Secure Pseudorandom Codes in the Standard Model

Abstract

Pseudorandom codes (PRCs), recently proposed by Christ and Gunn (CRYPTO'24), are encryption schemes that have pseudorandom ciphertexts and a decryption algorithm which is resilient against a bounded number of Hamming errors. This notion provides a significant strengthening over standard PKE and has exciting applications in, e.g., watermarking LLMs. The recent work of Alrabiah et al. (STOC'25) initiated the study of CCA-secure public-key PRCs, where the adversary is additionally given access to a decoding oracle. In terms of realizations, they provide one that can be proven secure in the random oracle model. Constructing CCA-secure public-key PRCs in the standard model remained an open problem. In this work, we resolve this problem and provide the first construction of CCA-secure public-key PRCs in the standard model. Our construction achieves a constant rate and can decode from a constant fraction of adversarial errors. In fact, our construction is a general blueprint that can be instantiated from a broad range of standard cryptographic assumptions. As an additional contribution, we construct a strong adaptively robust public-key pseudorandom code with conjectured sub-exponential security based on a new family of assumptions we call Noisy McEliece. In a nutshell, these assumptions mask a scrambled generator matrix from an efficiently decodable inner-code family with sparse Bernoulli noise; this additional error is meant to obscure the algebraic structure targeted by known attacks and thereby permits candidate instantiations from broader classes of codes.