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General Crypto Forum authenticated encryption, mode of operation, AEAD, properties Authenticated Encryption with Associated Data (AEAD) algorithms provide both confidentiality and integrity of data. The widespread use of AEAD algorithms in various applications has led to an increased demand for AEAD algorithms with additional properties, driving research in the field. This document provides definitions for the most common of those properties, aiming to improve consistency in the terminology used in documentation. This document is a product of the Crypto Forum Research Group.

Introduction An Authenticated Encryption with Associated Data (AEAD) algorithm provides confidentiality for the plaintext to be encrypted and integrity for the plaintext and some associated data (sometimes called Header). AEAD algorithms play a crucial role in various applications and have emerged as a significant focus in cryptographic research.

Background AEAD algorithms are formally defined in . The main benefit of AEAD algorithms is that they simultaneously provide data confidentiality and integrity and have a simple unified interface. In contrast to generic compositions of Message Authentication Code (MAC) and encryption algorithms, an AEAD algorithm allows for a reduction in key and state sizes, improving the data processing speed. Most AEAD algorithms come with security analysis, usage guidelines, and reference implementations. Consequently, their integration into high-level schemes and protocols is highly transparent. For instance, AEAD algorithms are mandatory in TLS 1.3 , IPsec ESP , and QUIC . While confidentiality and data integrity, being the conventional properties of AEAD algorithms, suffice for many applications, some environments demand other uncommon cryptographic properties. These often require additional analysis and research. As the number of such properties and corresponding research papers grows, inevitable misunderstandings and confusion arise. It is a common situation when related but formally different properties are named identically, or some security properties only have folklore understanding and are not formally defined. Consequently, the risk of misusing AEAD algorithms increases, potentially resulting in security issues.

Scope In this document, in , we provide the list of the most common additional properties of AEAD algorithms. The properties are divided into two categories, namely security properties (see ) and implementation properties (see ). We provide a high-level definition for each property; for security properties, we also reference an informative source where a formal game-based security notion is defined. When possible, we offer additional information: synonymous names, examples of algorithms that provide the property, applications that might necessitate such property from an AEAD algorithm, references for further reading, and additional notes containing information outside these categories. The objective of this document is to enhance clarity and establish a common language in the field. In particular, the primary application of the document lies in the following two use cases within the IRTF or the IETF documents development process:

• For an RFC or I-D that defines an AEAD algorithm, it is recommended to use the notations of when listing additional properties of the algorithm.
• For an RFC or I-D that defines a generic protocol based on an AEAD algorithm, it is recommended to use the notations of if any additional properties are required from the algorithm.

This document represents the consensus of the Crypto Forum Research Group (CFRG). This document is not an IETF product and is not a standard.

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 Algorithms This section gives a conventional definition of an AEAD algorithm following . Definition: An AEAD algorithm is defined by two operations, which are authenticated encryption and authenticated decryption:

• A deterministic operation of authenticated encryption has four inputs, each a binary string: a secret key K of a fixed bit length, a nonce N, associated data A, and a plaintext P. The plaintext contains the data to be encrypted and authenticated, and the associated data contains the data to be authenticated only. Each nonce value MUST be unique in every distinct invocation of the operation for any particular value of the key. The authenticated encryption operation outputs a ciphertext C.
• A deterministic operation of authenticated decryption has four inputs, each a binary string: a secret key K of a fixed bit length, a nonce N, associated data A, and a ciphertext C. The operation verifies the integrity of the ciphertext and associated data and decrypts the ciphertext. It returns a special symbol FAIL if the inputs are not authentic; otherwise, the operation returns a plaintext P.

We note that specifications of AEAD algorithms that use authentication tags to ensure integrity MAY define it as an independent output of the encryption operation and as an independent input of the decryption operation. Throughout this document, by default, we will consider the authentication tag as part of the ciphertext unless stated otherwise. For more details on the AEAD definition, please refer to . Throughout this document, by default, we will consider nonce-based AEAD algorithms, which have an interface from the definition above, and give no other restrictions on their structure. However, some properties considered in the document apply only to particular classes of such algorithms, like block cipher-based AEAD algorithms (such algorithms use block cipher as a building block). If that is the case, we explicitly point that out in the corresponding section.

AEAD Properties

Classification of additional AEAD Properties In this document, we employ a high-level classification of additional properties. This classification aims to provide insight into how one can benefit from each property. The additional properties in this section are categorized into one of these two groups:

• Security properties: We classify a property as a security property if it either takes into account new threats or extends adversarial capabilities, in addition to those posed by the typical nonce-respecting adversary whose goal is to compromise confidentiality or data integrity.
• Implementation properties: We classify a property as an implementation property if it enables more efficient implementations of the AEAD algorithm in specific cases or environments.

We note that some additional properties of AEAD algorithms found in the literature could not be allocated to either of these two groups. The observation is that such properties require an extension of the conventional AEAD interface. We refer to these properties as 'additional functionality properties' and define the corresponding group as follows:

• Additional functionality properties: We classify a property as an additional functionality property if it introduces new features in addition to the standard authenticated encryption with associated data.

With the extension of the conventional AEAD interface, each additional functionality property defines a new class of cryptographic algorithms. Consequently, the basic threats and adversarial capabilities must be redefined for each class. As a result, additional functionality properties consider the basic threats and adversarial capabilities for their class of algorithms, in contrast to security properties, which consider the extended ones. For this reason, we do not focus on additional functionality properties in this document. However, for the sake of completeness, in , we briefly present two classes of AEAD algorithms with additional functionality.

Conventional Properties In this section, we recall the conventional properties of an AEAD algorithm. Active nonce-respecting adversaries in a single-key setting are considered. We say that an AEAD algorithm provides security if it provides conventional properties listed in this section.

Confidentiality Definition: An AEAD algorithm guarantees that the plaintext is not available to an active, nonce-respecting adversary. Security notion: IND-CCA (or IND-CCA2 ). Synonyms: Message privacy. Notes: Confidentiality against passive adversaries can also be considered. The corresponding security notion is IND-CPA . Further reading: , , .

Data Integrity Definition: An AEAD algorithm guarantees that the ciphertext and the associated data have not been changed or forged by an active, nonce-respecting adversary. Security notion: IND-CTXT (or AUTH ). Synonyms: Message authentication, authenticity. Further reading: , , .

Authenticated Encryption Security Definition: An AEAD algorithm provides confidentiality and data integrity against active, nonce-respecting adversaries. Security notion: IND-CPA and IND-CTXT (or equivalently IND-CCA3 ). Notes: Please refer to for usage limits on modern AEAD algorithms used in IETF protocols. Further reading: , , .

Security Properties

Blockwise Security Definition: An AEAD algorithm provides security even if an adversary can adaptively choose the next part of the plaintext depending on already computed ciphertext parts during an encryption operation. Security notion: D-LORS-BCPA for confidentiality against passive adversaries, B-INT-CTXT for integrity ; OAE1 (stronger notion; originally, OAE in ). Examples: Deoxys , SAEF . Notes: Blockwise security is highly relevant for streamable AEAD algorithms (see ). OAE1 (OAE) security notion , and OAE2 notion are tailored for streamable AEAD algorithms. Blockwise security follows from security in the OAE notion . For a discussion on security notions for streamable AEAD algorithms see . Applications: Real-time streaming protocols, encryption on resource-constrained devices. Further reading: , , , .

Full Commitment Definition: An AEAD algorithm guarantees that it is hard to find two or more different tuples of the key, nonce, associated data, and plaintext such that they encrypt to the same ciphertext. In other words, an AEAD scheme guarantees that a ciphertext is a commitment to all inputs of an authenticated encryption operation. Security notion: CMT-4 , generalized CMT for a restricted setting (see the notes below) . Examples: Ascon , full committing versions of GCM and GCM-SIV , generic constructions . Notes: Full commitment can be considered in a weaker setting, where certain restrictions on the tuples produced by an adversary are imposed . For instance, an adversary must find tuples that all share the same associated data value. In such cases, an AEAD algorithm is said to provide full commitment in a restricted setting. The imposed restrictions MUST be listed. Applications: Message franking . Further reading: , , .

Key Commitment Definition: An AEAD algorithm guarantees that it is hard to find two or more different keys and the same number of potentially equal triples of nonce, associated data, and plaintext such that they encrypt to the same ciphertext under corresponding keys. In other words, an AEAD scheme guarantees that a ciphertext is a commitment to the key used for an authenticated encryption operation. Security notion: CMT-1 . Synonyms: Key-robustness, key collision resistance. Examples: Ascon , generic constructions from . Notes: Key commitment follows from full commitment. Full commitment does not follow from key commitment . Applications: Password-Authenticated Key Exchange, password-based encryption , key rotation, envelope encryption . 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. Security notion: CIL1 (CIML2 with leakages in decryption) for integrity, CCAL1 (CCAmL2 with leakages in decryption) for Authenticated Encryption security. Examples: Ascon (security under CIML2 and CCAL1 notions ), TEDT . Notes: Leakages during AEAD operation executions are implementation-dependent. It is possible to implement symmetric algorithms in a way that every possible physical leakage is entirely independent of the secret inputs of the algorithm (for example, with a masking technique ), meaning the adversary doesn't gain any additional information about the algorithm's computation via side-channel leakages. We say that an AEAD algorithm doesn't provide leakage resistance if it can achieve leakage resistance only with such an implementation. Leakage-resistant AEAD algorithms aim to place as mild requirements on implementation as possible to achieve leakage resistance. These requirements SHOULD be listed. Confidentiality of plaintext in the presence of leakages in the encryption operation is unachievable if an adversary can repeat the nonce used to encrypt the plaintext in other encryption queries. Confidentiality can be achieved only for plaintexts encrypted with fresh nonces (analogously to nonce-misuse resilience, see ). For further discussions, see and . For primitive-based AEAD algorithms, key evolution (internal re-keying ) can contribute to achieving leakage resistance with leakages in encryption. Confidentiality in the presence of decryption leakages can be achieved by two-pass AEAD algorithms with key evolution, which compute independent ephemeral key values for encryption and tag generation, where the computation of these keys is implemented without any leakages. For more discussions on achieving leakage resistance see . A well-known weaker property, Leakage Resilience, introduced in , can also be considered. However, this document makes a conscious choice to focus on the stronger Leakage Resistance, following the framework established in , , for its enhanced practicality and comprehensiveness. Applications: Encryption on smart cards, Internet-of-things devices, or other constrained devices. Further reading: , , , .

Multi-User Security Definition: An AEAD algorithm security degrades slower than linearly with an increase in the number of users. Security notion: mu-ind . Examples: AES-GCM , ChaCha20-Poly1305 , AES-GCM-SIV , AEGIS . Notes: It holds that for any AEAD algorithm security degrades no worse than linearly with an increase in the number of users . However, for some applications with a significant number of users, better multi-user guarantees are required. For example, in the TLS 1.3 protocol, to address this issue, AEAD algorithms are used with a randomized nonce (deterministically derived from a traffic secret and a sequence number). Using nonce randomization in block cipher counter-based AEAD modes can contribute to multi-user security . Multi-user usage limits for AES-GCM and ChaCha20-Poly1305 are provided in . A weaker security notion, multi-user key recovery, is also introduced and thoroughly studied in . While this document focuses on indistinguishability for security notions, key recovery might be relevant and valuable to study alongside indistinguishability. Applications: Data transmission layer of secure communication protocols (e.g., TLS, IPSec, SRTP, etc.) Further reading: , , , , .

Nonce-Hiding Definition: An AEAD algorithm provides confidentiality for the nonce value used to encrypt plaintext. The algorithm includes information about the nonce in the ciphertext and doesn't require the nonce as input for the decryption operation. Security notion: AE2 . Examples: Hide-Nonce (HN) transforms . Notes: As discussed in , adversary-visible nonces might compromise message and user privacy, similar to the way any metadata might do. As pointed out in , even using a counter as a nonce value might compromise privacy. Designing a privacy-preserving way to manage nonces might be a challenging problem for an application. Applications: Any application that can't rely on a secure 'out-of-band' nonce communication. 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 and resistance are defined as follows:

• Nonce misuse resilience: Security is provided only for messages encrypted with fresh nonces (correctly encrypted messages). Security notion: CPA resilience (confidentiality), authenticity resilience (integrity), CCA resilience (authenticated encryption) . Examples: ChaCha20-Poly1305 , AES-GCM (only confidentiality).
• Nonce misuse resistance: Security is provided for all messages that were not encrypted with the same nonce value more than once. Security notion: MRAE . Examples: AES-GCM-SIV , Deoxys-II . Notes: SIV construction is a generic construction that provides nonce misuse resistance.

Notes: Nonce misuse resilience follows from nonce misuse resistance. Nonce misuse resistance does not follow from nonce misuse resilience. Applications: Any application where nonce uniqueness can't be guaranteed, security against fault-injection attacks and malfunctions, processes parallelization, full disk encryption. Further reading: , .

Quantum Security Definition: An AEAD algorithm provides security (in a Q1 or Q2 model) against a quantum adversary. Q1 and Q2 models are defined as follows:

• Q1 model: An adversary has access to local quantum computational power. It has classical access to encryption and decryption oracles. Synonyms: Post-quantum security. Examples: AES-GCM , ChaCha20-Poly1305 , OCB , MGM , AES-GCM-SIV , AEGIS .
• Q2 model: An adversary has access to local quantum computational power. It has quantum access to encryption and decryption oracles, i.e., it can query encryption and decryption oracles with quantum superpositions of inputs to receive quantum superpositions of the outputs. Synonyms: Superposition-based quantum security. Examples: QCB .

Notes: Most symmetric cryptographic algorithms that are secure in the classical model provide quantum security in the Q1 model, i.e., they are post-quantum secure. Security in the Q1 setting corresponds to security against "harvest now, decrypt later" attacks. Security in Q1 follows from security in Q2, the converse does not hold. For discussions on the relevance of the Q2 model please see . 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. Security notion: j-Int-CTXT . Examples: Deoxys , AEGIS , Ascon . Applications: VoIP, real-time streaming in a lightweight setting, applications that require small ciphertext expansion (i.e., short tags). Further reading: , .

Release of Unverified Plaintext (RUP) Integrity Definition: An AEAD algorithm provides data integrity even if plaintext is released for every ciphertext, including those with failed integrity verification. Security notion: INT-RUP . Examples: GCM-RUP . Applications: Decryption with limited memory , real-time streaming protocols. Notes: In a generic approach to achieve INT-RUP security is introduced. In the provided definition, we only consider integrity in the RUP setting, since confidentiality, in the usual sense, is unachievable under RUP. In , the notion of 'Plaintext Awareness' is introduced, capturing the best possible confidentiality under RUP in the following sense: 'The adversary cannot gain any additional knowledge about the plaintext from decryption queries beyond what it can derive from encryption queries'. Further reading: , .

Implementation Properties

Hardware efficient Definition: An AEAD algorithm ensures optimal performance when operating on hardware that complies with the specified requirements. Notes: Various classes of hardware may be taken into consideration. Certain algorithms are tailored to minimize the area of dedicated hardware implementations, while others are intended to capitalize on general-purpose CPUs, with or without specific instruction sets. It is RECOMMENDED to specify the minimum platform requirements for the AEAD to fulfill its intended purpose, as well as to match its performance and security claims.

Inverse-Free Definition: An AEAD algorithm that is based on some primitive can be implemented without evaluating the inverse of that primitive. Examples: AES-GCM , ChaCha20-Poly1305 , OCB , MGM , AEGIS . Notes: In a sponge-based AEAD algorithm, an underlying permutation is viewed as a primitive.

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 . Examples: OCB , Ascon . Further reading: .

Parallelizable Definition: An AEAD algorithm can fully exploit the parallel computation infrastructure. In other words, a parallelizable AEAD algorithm allows for the computation of ciphertext segments (plaintext segments for decryption) in parallel, meaning that ciphertext segments are computed independently. Synonyms: Pipelineable. Examples: AES-GCM , ChaCha20-Poly1305 , OCB , MGM , AEGIS . Further reading: .

Setup-Free Definition: An AEAD algorithm's operations can be implemented in a way that using a new key incurs either no overhead or negligible overhead compared to the reuse of a previous key. Overhead may involve additional computations or increased storage space, such as precomputing a key schedule for a block cipher. Examples: ChaCha20-Poly1305 , AEGIS , Ascon .

Single Pass Definition: An AEAD algorithm encryption (decryption) operation can be implemented with a single pass over the plaintext (ciphertext). Examples: AES-GCM , ChaCha20-Poly1305 , OCB , MGM , AEGIS .

Static Associated Data Efficient Definition: An AEAD algorithm allows pre-computation for static (or repeating) associated data so that static associated data doesn't significantly contribute to the computational cost of encryption. Examples: AES-GCM , ChaCha20-Poly1305 , OCB .

Streamable Definition: An AEAD algorithm encryption (decryption) operation can be implemented with constant memory usage and a single one-direction pass over the plaintext (ciphertext), writing out the result during that pass. Synonyms: Online. Examples: AES-GCM , ChaCha20-Poly1305 , OCB , MGM , AEGIS , Ascon . Applications: Real-time streaming protocols, resource-constrained devices. Notes: Blockwise security (see ) and RUP integrity (see ) might be relevant security properties for streamable AEAD algorithms in certain applications. Further reading: , .

Security Considerations This document gives high-level definitions of AEAD properties. For each security property, we provide an informational reference to a game-based security notion (or security notions if there are separate notions for integrity and confidentiality) that formalizes the property. We only consider game-based notions and security properties that can be formalized using this approach. However, there are different approaches to formalizing AEAD security, like the indifferentiability framework ; security in such notions should be studied separately. For some properties, examples of AEAD algorithms that provide them are given, with standardized AEAD algorithms preferred for commonly encountered properties. However, for certain properties, only non-standardized algorithms exist. Implementing such algorithms requires careful consideration, and it is advised to contact the algorithm designers for reference implementations and implementation guidelines. Every claimed security property of an AEAD algorithm MUST undergo security analysis within a relevant notion. It's RECOMMENDED to use the security notions referenced in the document. If an alternative notion is used, there MUST exist proof of equivalence or it SHOULD be indicated that a non-equivalent notion is used. For security properties that extend adversarial capabilities, consideration of integrity and confidentiality separately may be relevant. If the algorithm provides only one of these, that SHOULD be indicated. When specifying security requirements for an AEAD algorithm in an application, it SHOULD be indicated, for every required security property, whether only integrity or confidentiality is necessary. Additionally, for each security property, it SHOULD be specified whether an analysis in an alternative security notion is required. We also note that some additional properties come with trade-offs in terms of classical security and efficiency, and may only be supported in non-standardized or modified AEAD algorithms. This immediately implies challenges in deployment and interoperability. In an application, the requirements for additional AEAD properties SHOULD be highly motivated and justified, as should all trade-offs be carefully considered.

IANA Considerations This document has no IANA actions.

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AEAD Algorithms with Additional Functionality In this section, we briefly discuss AEAD algorithms that provide additional functionality. As noted in , each additional functionality requires a redefinition of the conventional AEAD interface; thus, each additional functionality property defines a new class of cryptographic algorithms. Most importantly, for every Additional Functionality AEAD class, conventional security properties must be redefined concerning the targeted additional functionality and the new interface. Although it might be possible to consider a particular Additional Functionality AEAD algorithm as a conventional AEAD algorithm and study it for the conventional confidentiality and integrity, security (or insecurity) in that sense won't be sufficient to label that algorithm as a secure (or insecure) Additional Functionality AEAD. Only security in the sense of the redefined conventional properties would suffice. For the examples given in this section, we leave it out of scope how to concretely redefine conventional security for these classes; we only briefly describe the additional functionality they offer and provide further references.

Incremental Authenticated Encryption Definition: An AEAD algorithm allows re-encrypting and authenticating a message (associated data and a plaintext pair), which only partly differs from some previous message, faster than processing it from scratch. Examples: Incremental AEAD algorithm of . Security notion: Privacy, Authenticity . Notes: The interface of an incremental AEAD algorithm is usually expanded, when compared with conventional AEAD, with several operations, which perform different types of updates. For example, one can consider such operations as "Append" or "Chop", which provide a straightforward additional functionality. A comprehensive definition of an incremental AEAD interface is provided in . Further reading: , , .

Robust Authenticated Encryption Definition: An AEAD algorithm allows users to choose a desired ciphertext expansion (the difference between the length of plaintext and corresponding ciphertext) along with an input to the encryption operation. This feature enables the regulation of desired data integrity guarantees, which depend on ciphertext expansion, for each particular application while using the same algorithm implementation. Examples: AEZ . Security notion: RAE . Notes: The security goal of robust AEAD algorithms is to ensure the best possible security, even with small ciphertext expansion (referred to as stretch). For instance, analyzing any AEAD algorithm with a one-byte stretch for conventional integrity reveals insecurity, as the probability of forging a ciphertext is no less than 1/256. Nonetheless, from the robust AEAD perspective, an algorithm with such forgery probability for a one-byte ciphertext expansion is secure, representing the best achievable security in that scenario. Further reading: .

Acknowledgments This document benefited greatly from the comments received from the CFRG community, for which we are very grateful. We would also like to extend special appreciation to Liliya Akhmetzyanova, Evgeny Alekseev, Alexandra Babueva, Frank Denis, Kirill Kutsenok, Sergey Kyazhin, Samuel Lucas, Grigory Marshalko, Christopher Patton, and Christopher Wood for their thoughtful comments, proposals, and discussions.