Abstract. We provide a security analysis for full-state keyed Sponge and full-state Duplex constructions. Our results can be used for making a large class of Sponge-based authenticated encryption schemes more efficient by concurrent absorption of associated data and message blocks. In particular, we introduce and analyze a new variant of SpongeWrap with almost free authentication of associated data. The idea of using full-state message absorption for higher efficiency was first made explicit in the Donkey Sponge MAC construction, but without any formal security proof. Recently, Gaži, Pietrzak and Tessaro (CRYPTO 2015) have provided a proof for the fixed-output-length variant of Donkey Sponge. Yasuda and Sasaki (CT-RSA 2015) have considered partially full-state Sponge-based authenticated encryption schemes for efficient incorporation of associated data. In this work, we unify, simplify, and generalize these results about the security and applicability of full-state keyed Sponge and Duplex constructions; in particular, for designing more efficient authenticated encryption schemes. Compared to the proof of Gaži et al., our analysis directly targets the original Donkey Sponge construction as an arbitrary-output-length function. Our treatment is also more general than that of Yasuda and Sasaki, while yielding a more efficient authenticated encryption mode for the case that associated data might be longer than messages.

Highly efficient encryption and authentication of short messages is an essential requirement for enabling security in constrained scenarios such as the CAN FD in automotive systems (max. message size 64 bytes), massive IoT, critical communication domains of 5G, and Narrowband IoT, to mention a few. In addition, one of the NIST lightweight cryptography project requirements is that AEAD schemes shall be "optimized to be efficient for short messages (e.g., as short as 8 bytes)". In this work we introduce and formalize a novel primitive in symmetric cryptography called a forkcipher. A forkcipher is a keyed function expanding a fixed-length input to a fixed-length output. We define its security as indistinguishability under chosen ciphertext attack. We give a generic construction validation via the new iterate-fork-iterate design paradigm. We then propose ForkSkinny as a concrete forkcipher instance with a public tweak and based on SKINNY: a tweakable lightweight block cipher constructed using the TWEAKEY framework. We conduct extensive cryptanalysis of ForkSkinny against classical and structurespecific attacks. We demonstrate the applicability of forkciphers by designing three new provably-secure, nonce-based AEAD modes which offer performance and security tradeoffs and are optimized for efficiency of very short messages. Considering a reference block size of 16 bytes, and ignoring possible hardware optimizations, our new AEAD schemes beat the best SKINNY-based AEAD modes. More generally, we show forkciphers are suited for lightweight applications dealing with predominantly short messages, while at the same time allowing handling arbitrary messages sizes. Furthermore, our hardware implementation results show that when we exploit the inherent parallelism of ForkSkinny we achieve the best performance when directly compared with the most efficient mode instantiated with the SKINNY block cipher.

Abstract.A definition of online authenticated-encryption (OAE), call it OAE1, was given by Fleischmann, Forler, and Lucks (2012). It has become a popular definitional target because, despite allowing encryption to be online, security is supposed to be maintained even if nonces get reused. We argue that this expectation is effectively wrong. OAE1 security has also been claimed to capture best-possible security for any online-AE scheme. We claim that this understanding is wrong, too. So motivated, we redefine OAE-security, providing a radically different formulation, OAE2. The new notion effectively does capture best-possible security for a user's choice of plaintext segmentation and ciphertext expansion. It is achievable by simple techniques from standard tools. Yet even for OAE2, nonce-reuse can still be devastating. The picture to emerge is that no OAE definition can meaningfully tolerate nonce-reuse, but, at the same time, OAE security ought never have been understood to turn on this question.

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International audienceWe propose the Offset Merkle-Damgård (OMD) scheme, a mode of operation to use a compression function for building a nonce-based authenticated encryption with associated data. In OMD, the parts responsible for privacy and authenticity are tightly coupled to minimize the total number of compression function calls: for processing a message of ℓ blocks and associated data of a blocks, OMD needs ℓ+a+2 calls to the compression function (plus a single call during the whole lifetime of the key). OMD is provably secure based on the standard pseudorandom function (PRF) property of the compression function. Instantiations of OMD using the compression functions of SHA-256 and SHA-512, called OMD-SHA256 and OMD-SHA512, respectively, provide much higher quantitative level of security compared to the AES-based schemes. OMD-SHA256 can benefit from the new Intel SHA Extensions on next-generation processors