92 research outputs found
An Inverse-free Single-Keyed Tweakable Enciphering Scheme
In CRYPTO 2003, Halevi and Rogaway proposed CMC, a tweakable enciphering scheme (TES) based on a blockcipher. It requires two blockcipher keys and it is not inverse-free (i.e., the decryption algorithm uses the inverse (decryption) of the underlying blockcipher). We present here a new inverse-free, single-keyed TES. Our construction is a tweakable strong pseudorandom permutation (tsprp), i.e., it is secure against chosen-plaintext-ciphertext adversaries assuming that the underlying blockcipher is a pseudorandom permutation (prp), i.e., secure against chosen-plaintext adversaries. In comparison, sprp assumption of the blockcipher is required for the sprp security of CMC. Our scheme can be viewed as a mixture of type-1 and type-3 Feistel cipher and so we call it FMix or mixed-type Feistel cipher
Tweakable HCTR: A BBB Secure Tweakable Enciphering Scheme
\textsf{HCTR}, proposed by Wang et al., is one of the most efficient candidates of tweakable enciphering schemes that turns an -bit block cipher into a variable input length tweakable block cipher. Wang et al. have shown that \textsf{HCTR} offers a cubic security bound against all adaptive chosen plaintext and chosen ciphertext adversaries. Later in FSE 2008, Chakraborty and Nandi have improved its bound to , where is the total number of blocks queried and is the block size of the block cipher. In this paper, we propose \textbf{tweakable \textsf{HCTR}} that turns an -bit tweakable block cipher to a variable input length tweakable block cipher by replacing all the block cipher calls of \textsf{HCTR} with tweakable block cipher. We show that when there is no repetition of the tweak, tweakable \textsf{HCTR} enjoys the optimal security against all adaptive chosen plaintext and chosen ciphertext adversaries. However, if the repetition of the tweak is limited, then the security of the construction remains close to the security bound in no repetition of the tweak case. Hence, it gives a graceful security degradation with the maximum number of repetition of tweaks
Provable Security of (Tweakable) Block Ciphers Based on Substitution-Permutation Networks
Substitution-Permutation Networks (SPNs) refer to a family
of constructions which build a wn-bit block cipher from n-bit public
permutations (often called S-boxes), which alternate keyless and âlocalâ
substitution steps utilizing such S-boxes, with keyed and âglobalâ permu-
tation steps which are non-cryptographic. Many widely deployed block
ciphers are constructed based on the SPNs, but there are essentially no
provable-security results about SPNs.
In this work, we initiate a comprehensive study of the provable security
of SPNs as (possibly tweakable) wn-bit block ciphers, when the underlying
n-bit permutation is modeled as a public random permutation. When the
permutation step is linear (which is the case for most existing designs),
we show that 3 SPN rounds are necessary and sufficient for security. On
the other hand, even 1-round SPNs can be secure when non-linearity
is allowed. Moreover, 2-round non-linear SPNs can achieve âbeyond-
birthdayâ (up to 2 2n/3 adversarial queries) security, and, as the number
of non-linear rounds increases, our bounds are meaningful for the number
of queries approaching 2 n . Finally, our non-linear SPNs can be made
tweakable by incorporating the tweak into the permutation layer, and
provide good multi-user security.
As an application, our construction can turn two public n-bit permuta-
tions (or fixed-key block ciphers) into a tweakable block cipher working
on wn-bit inputs, 6n-bit key and an n-bit tweak (for any w â„ 2); the
tweakable block cipher provides security up to 2 2n/3 adversarial queries
in the random permutation model, while only requiring w calls to each
permutation, and 3w field multiplications for each wn-bit input
Elastic-Tweak: A Framework for Short Tweak Tweakable Block Cipher
Tweakable block cipher (TBC), a stronger notion than standard
block ciphers, has wide-scale applications in symmetric-key schemes. At a high level, it provides flexibility in design and (possibly) better security bounds. In multi-keyed applications, a TBC with short tweak values can be used to replace multiple keys. However, the existing TBC construction frameworks, including TWEAKEY and XEX, are designed for general purpose tweak sizes. Specifically, they are not optimized for short tweaks, which might render them inefficient for certain resource constrained applications. So a dedicated paradigm to construct short-tweak TBCs (tBC) is highly desirable. In this paper, as a first contribution, we present a dedicated framework, called the Elastic-Tweak framework (ET in short), to convert any reasonably secure SPN block cipher into a secure tBC. We apply the ET framework on GIFT and AES to construct efficient tBCs, named TweGIFT and TweAES. These short-tweak TBCs have already been employed in recent NIST lightweight competition candidates, LOTUS-LOCUS and ESTATE. As our second contribution, we show some concrete applications of ET-based tBCs, which are better than their block cipher counterparts in terms of key size, state size, number of block cipher calls, and short message processing. Some notable applications include, Twe-FCBC (reduces the key size of FCBC and gives better security than CMAC), Twe-LightMAC Plus (better rate than LightMAC Plus), Twe-CLOC, and Twe-SILC (reduces the number of block cipher calls and simplifies the design of CLOC and SILC)
Forkcipher: A New Primitive for Authenticated Encryption of Very Short Messages
This is an extended version of the article with the same title accepted at Asiacrypt 2019.International audienceHighly 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 âoptimized to be efficient for short messages (e.g., as short as 8 bytes)â. 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 structure-specific 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
ZCZ - Achieving n-bit SPRP Security with a Minimal Number of Tweakable-block-cipher Calls
Strong Pseudo-random Permutations (SPRPs) are important for various applications. In general, it is desirable to base an SPRP on a single-keyed primitive for minimizing the implementation costs. For constructions built on classical block ciphers, Nandi showed at ASIACRYPT\u2715 that at least two calls to the primitive per processed message block are required for SPRP security, assuming that all further operations are linear. The ongoing trend of using tweakable block ciphers as primitive has already led to MACs or encryption modes with high security and efficiency properties. Thus, three interesting research questions are hovering in the domain of SPRPs: (1) if and to which extent the bound of two calls per block can be reduced with a tweakable block cipher, (2) how concrete constructions could be realized, and (3) whether full -bit security is achievable from primitives with -bit state size.
The present work addresses all three questions. Inspired by Iwata et al.\u27s ZHash proposal at CRYPTO\u2717, we propose the ZCZ (ZHash-Counter-ZHash) construction, a single-key variable-input-length SPRP based on a single tweakable block cipher whose tweak length is at least its state size. ZCZ possesses close to optimal properties with regards to both performance and security: not only does it require only asymptotically calls to the primitive for -block messages, but we also show that this figure is close to the minimum by an PRP distinguishing attack on any construction with tweak size of bits and fewer than calls to the same primitive. Moreover, it provides optimal -bit security for a primitive with -bit state and tweak size
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