Properties of AEAD algorithms

Introduction An Authenticated Encryption with Associated Data (AEAD) algorithm is an extension of authenticated encryption, which provides confidentiality for the plaintext to be encrypted and integrity for the plaintext and some Associated Data (sometimes called Header). AEAD algorithms are used in numerous applications and have become an important field in cryptographic research.

Background AEAD algorithms are formally defined in . The main benefit of AEAD algorithms is that they provide both data confidentiality and data integrity and have a simple unified interface. The importance of the AEAD algorithms is mainly explained by their exploitation simplicity: they have a unified interface, easy-to-understand security guarantees, and are much easier to implement properly than MAC and encryption schemes separately. Therefore, their embedding into high-level schemes and protocols is highly transparent since, for example, there is no need for additional key derivation procedures. Apart from that, when using the AEAD algorithm, it is possible to reduce the key and state sizes and improve the data processing speed. For instance, such algorithms are mandatory for TLS 1.3 , IPsec ESP , and QUIC . Hence, the research and standardization efforts in the field are extremely active. Most AEAD algorithms usually come with security guarantees, formal proofs, usage guidelines, and reference implementations. Even though providing core properties of AEAD algorithms is enough for use in many applications, some environments require other unusual cryptographic properties, which commonly require additional analysis and research. With the growing number of such properties and research papers, misunderstanding and confusion inevitably appear. Some properties might be understood in different ways, for some only non-trivial formal security notions are provided, others require modification or extension of the standard AEAD interface to support additional functionality. Therefore, the risk of misuse of AEAD algorithms increases which can lead to security issues.

Scope In the following document, we provide a short overview of the most common properties of AEAD algorithms, by giving high-level definitions of these properties in . The document aims to improve clarity and establish a common language in the field.

Conventions Used in This Document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

AEAD properties

Security properties

Confidentiality Definition. An AEAD algorithm guarantees that data is available only to those authorized to obtain it. That property is required for the AEAD algorithm to be called secure. Synonyms. Privacy. Further reading. ,

Data integrity Definition. An AEAD algorithm guarantees that data has not been changed or forged by those who are not authorized to. That property is required for the AEAD algorithm to be called secure. Synonyms. Message authentication. Further reading. ,

Blockwise security Definition. An AEAD algorithm provides security even if an adversary can adaptively choose the next block of the plaintext (ciphertext) depending on already computed blocks of the ciphertext (plaintext) during an encryption (decryption) operation. Further reading. ,

Key Dependent Messages (KDM) security Definition. An AEAD algorithm provides security even when key-dependent plaintexts are encrypted. Notes. KDM-security is achievable only if nonces are chosen randomly and associated data is key-independent. Further reading.

Key commitment Definition. An AEAD algorithm guarantees that it is difficult to find a tuple of the nonce, associated data, and ciphertext such that it can be decrypted correctly with more than one key. Synonyms. Key-robustness, key collision resistance. Further reading. , ,

Leakage resistance Definition. An AEAD algorithm provides security even if some additional information about computations of an encryption (and possibly decryption) operation is obtained via side-channel leakages. Further reading. ,

Multi-user security Definition. An AEAD algorithm security level degrades sublinearly in the number of users. Here the level of security is understood in the sense of Authenticated Encryption Advantage (AEA) as given in . Further reading.

Nonce misuse Definition. An AEAD algorithm provides security (resilience or resistance) even if an adversary can repeat nonces in its encryption queries.

Nonce misuse resilience Definition. Security is provided only for messages encrypted with unique nonces. Further reading. ,

Nonce misuse resistance Definition. Security is provided for all messages. Further reading.

Reforgeability resilience Definition. An AEAD algorithm guarantees that once a successful forgery for the algorithm has been found, it is still hard to find any subsequent forgery. Further reading. ,

Release of unverified plaintext (RUP) security Definition. An AEAD algorithm provides security even if the plaintext is released for every ciphertext, including those with failed integrity verification. Further reading.

Implementation properties

Inverse-free Definition. A block cipher-based AEAD algorithm can be securely implemented without evaluating the block cipher inverse.

Lightweight Definition. An AEAD algorithm can be efficiently and securely implemented on resource-constrained devices. In particular, it meets the criteria required in the NIST Lightweight Cryptography competition . Further reading.

Online Definition. An AEAD algorithm encryption (decryption) operation can be implemented with a constant memory and a single one-direction pass over the plaintext (ciphertext), writing out the result during that pass. Further reading.

Parallelizable Definition. An AEAD algorithm can fully exploit the parallel computation infrastructure. Further reading.

Single pass Definition. An AEAD algorithm encryption (decryption) operation can be implemented with a single pass over the plaintext (ciphertext).

Static Associated Data Definition. An AEAD algorithm allows pre-computation for static (or repeating) associated data so that static AD doesn't significantly contribute to the computational cost of encryption.

ZK-friendly Definition. An AEAD algorithm operates on binary and prime fields with a low number of non-linear operations (often called multiplicative complexity). Thus, it allows efficient implementation using a domain-specific language (DSL) for writing zk-SNARKs circuits. Synonyms. ZK-focused, Arithmetization-oriented, Low Multiplicative Complexity Further reading.

Additional functionality properties

Incremental Definition. An AEAD algorithm allows encrypting a message, which only partly differs from some other previously encrypted message, faster than processing it from scratch. Further reading. ,

Nonce-hiding Definition. An AEAD algorithm decryption operation doesn't need the nonce value to perform the decryption. Thus, the algorithm provides privacy for the nonce value. Further reading.

Remotely-keyed Definition. An AEAD algorithm can be securely implemented with most of the operations in encryption/decryption performed by an insecure (i.e., it leaks all intermediate values) device, which has no access to the key, while another secure device performs operations involving the key. Further reading. ,

Robust Definition. An AEAD algorithm allows the user to choose an arbitrary value l >= 0 for every plaintext and then encrypts it into a ciphertext, which is l bits longer. Further reading.

Security Considerations This document defines the properties of AEAD algorithms. However, the document does not describe any concrete mechanisms providing these properties, neither it describes how to achieve them. In fact, one can claim that an AEAD algorithm provides any of the defined properties only if its analysis in the relevant models was carried out.

IANA Considerations This document has no IANA actions.