Internet-Draft | Properties of AEAD algorithms | October 2023 |
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Authenticated Encryption with Associated Data (AEAD) algorithms provide confidentiality and integrity of data. The extensive use of AEAD algorithms in various high-level applications has caused the need for AEAD algorithms with additional properties and motivated research in the area. This document gives definitions for the most common of those properties intending to improve consistency in the field.¶
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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.¶
AEAD algorithms are formally defined in [RFC5116]. 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 [RFC8446], IPsec ESP [RFC4303] [RFC8221], and QUIC [RFC9000]. 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.¶
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
In this section we give a general definition of an AEAD algorithm, following [RFC5116].¶
Definition. An AEAD algorithm is defined by two operations --- authenticated encryption and authenticated decryption:¶
A determenistic operation of authenticated encryption has four inputs, each of them is a binary string: a secret key K of a fixed bit lenght, a nonce N, associated data A and a plaintext P. The plaintext cointains the data to be encrypted and autheticated, the associated data cointains 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 determenistic operation of authenticated decryption has four inputs, each of them is a binary string: a secret key K of a fixed bit lenght, 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.¶
For more details on AEAD definition please refer to [RFC5116].¶
Throught 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 strcuture. However, some properties defined in the document are applicable only to some special 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. Some other properties, on the contarary, are defined for algorithms with extended or completely different interfaces. We address that issue in Section Section 4.1.¶
We will call an AEAD algorithm secure, if it providees such properties as Confidentiality and Data integrity, defined in Section Section 4. Even though we are aiming to give high-level definitions, we will sometimes use the adversaial advantage notion. Specifically, we will use the Authenticated Encryption advantage notion for the AEAD algorithm. We adopt the corresponding definition from [I-D.irtf-cfrg-aead-limits].¶
Definition. The probability of an active attacker succeeding in breaking the authenticated-encryption properties of the AEAD algorithm. In this document, the definition of authenticated encryption advantage roughly is the probability that an attacker successfully distinguishes the ciphertext outputs of the AEAD scheme from the outputs of a random function or is able to forge a ciphertext that will be accepted as valid.¶
In this document we use a very high-level classification of properties. That classification is aimed to highlight the differences between the nature of the considered properties. All properties in the document fall into one of these three categories:¶
Definition. An AEAD algorithm guarantees that the plaintext is available only to those authorized to obtain it, i.e. those who are in the posession of the secret key. That property is required for the AEAD algorithm to be called secure.¶
Synonyms. Privacy.¶
Definition. An AEAD algorithm guarantees that the plaintext and the associated data has not been changed or forged by those who are not authorized to, i.e. by those who are not in the posession of the secret key. That property is required for the AEAD algorithm to be called secure.¶
Synonyms. Message authentication.¶
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.¶
Definition. An AEAD algorithm guarantees that it is difficult 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.¶
Definition. An AEAD algorithm guarantees that it is difficult 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.¶
Synonyms. Key-robustness, key collision resistance.¶
Notes. Key commitment follows from full commitment. Full commitment does not follow form key commitment. [BH22]¶
Functional applications. Password-Authenticated Key Exchange [LGR21].¶
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.¶
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.¶
Definition. An AEAD algorithm Authenticated Encryption advantage increases sublinearly in the number of users.¶
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:¶
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.¶
Definition. An AEAD algorithm provides security even if the plaintext is released for every ciphertext, including those with failed integrity verification.¶
Functional applications. Decryption with limited memory [FJMV2004], CMS protocols [RFC5652].¶
Definition. A block cipher-based AEAD algorithm can be securely implemented without evaluating the block cipher inverse.¶
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 [MBTM17].¶
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.¶
Definition. An AEAD algorithm can fully exploit the parallel computation infrastructure.¶
Synonyms. Pipelineable.¶
Definition. An AEAD algorithm encryption (decryption) operation can be implemented with a single pass over the plaintext (ciphertext).¶
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.¶
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¶
Definition. An AEAD algorithm allows encrypting a message, which only partly differs from some other previously encrypted message, faster than processing it from scratch.¶
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.¶
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.¶
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.¶
This document has no IANA actions.¶