Paper 2025/1820

On the Plaintext Awareness of AEAD Schemes

, Radboud University, Nijmegen, Netherlands and Nanyang Technological University, Singapore, Singapore

Mustafa Khairallah, Nanyang Technological University

Abstract

Plaintext-awareness of AEAD schemes is one of the more obscure and easily misunderstood notions. Originally proposed by Andreeva et al., Mennink and Talnikar showed in 2025 that the original definitions are vague and leave too much room for interpretation. They presented new definitions and analyzed the three main AEAD compositions relative to the new definitions. In particular, they showed that MAC-then-Encrypt (MtE) is not plaintext-aware. However, they showed that an SIV-style variant is indeed plaintext-aware. In this work, we first show that their analysis contains a gap, voiding their proof. We show this by showing several attacks; against their choice of extractor, with trivial complexity, and against any extractor, with birthday-bound complexity. Next, we re-establish their results by designing a new extractor that captures their intended goal and prove a tight PA1 security bound. We also show that the result is not dependent on the encryption scheme used, by showing that an extractor can also be designed for sMtE[ΘCB3], a variant where the encryption step is done by an ΘCB3-like scheme. Afterwards, we strengthen their results, by revisiting other compositions. In particular, we show that Encrypt-then-MAC (EtM) is in fact PA2-secure. Furthermore, we show that SIV-style MtE cannot be PA2-secure. Additionally, we show that Encode-then-Encipher is also PA2-secure, but not beyond the first successful forgery. More importantly, we show that up to some necessary assumptions, PA2 and RAE are equivalent. This result, while positive, holds only up to the first successful forgery. This indicates that the PA2 formalization maybe too strong for practical schemes, since we believe RAE sufficient for intuitive security. Last but not least, we investigate the value of the notion of plaintext awareness. We look deeper into the relation between plaintext awareness and confidentiality. We show that the problem of confidentiality in the presence of release of unverified plaintext can be seen as a confidentiality-with-leakage problem in the simulatable leakage framework. In this regard, we start, by showing that PA1 security cannot imply confidentiality with leakage. Similarly, we compare our results to the AERUP notion of TOSC 2019. We show that a scheme can be AERUP secure but not secure against a somewhat straightforward left-or-right attack in the same model. This puts into question the meaning and relevance of the PA1 and AERUP security notions. Results based on these notions can be seen as providing security in a two-phase game, where the adversary does not observe any leakage after the first challenge query, as we argue in the paper. On the positive side, we show that if a scheme achieves IND-CPA, INT-RUP and PA2, then it achieves confidentiality with leakage for the appropriate leakage function. This closes a gap in the literature on the relation between confidentiality with leakage and RUP notions.