International Association
for Cryptologic Research

Boneh et al proposed the cryptographic primitive public key encryption with keyword search (PEKS) to search on encrypted data without exposing the privacy of the keyword. Most standard PEKS schemes are vulnerable to inside keyword guessing attacks (KGA), i.e., a malicious server may generate a ciphertext by its own and then to guess the keyword of the trapdoor by testing. Huang et al. solved this problem by proposing the public-key authenticated encryption with keyword search (PAEKS) achieving single trapdoor privacy (TP). Qin et al. defined notion of multi-ciphertext indistinguishability (MCI) security and multi-trapdoor privacy (MTP) security, and proposed the first PAEKS scheme with MCI and TP.

Certificateless public-key authenticated encryption with keyword search (CLPAEKS) is first formally proposed by He et al. as combination of the PAEKS and the certificateless public key cryptography (CLPKC). Lin et al. revised He's work and re-formalize the security requirements for CLPAEKS in terms of trapdoor privacy and ciphertext indistinguishability. However, how to achieve both MCI and MTP security in a CLPAEKS scheme is still unknown.

In this paper, we initially propose a CLPAEKS scheme with both MCI security and MTP security simultaneously. We provide formal proof of our schemes in the random oracle model.

Custodian is a core financial service in which the custodian holds in safekeeping assets on behalf of the client. Although traditional custody service is typically endorsed by centralized authorities, decentralized custody scheme has become technically feasible since the emergence of digital assets, and furthermore it is badly needed by new applications such as blockchain and DeFi (Decentralized Finance). In this work, we propose a framework of decentralized asset custody scheme that is able to support a large number of custodians and safely hold customer assets of multiple times value of the total security deposit. The proposed custody scheme distributes custodians and assets into many custodian groups via combinatorial designs and random sampling, where each group fully controls the assigned assets. Since every custodian group is small, the overhead cost is significantly reduced. The liveness is also improved because even a single alive group would be able to process transactions. The security of this custody scheme is guaranteed in the game-theoretic sense, such that any adversary corrupting a bounded fraction of custodians cannot move assets more than his own security deposit. We further analyze the security and performance of our constructions, and give explicit examples with concrete numbers and figures for a better understanding of our results.

Body Biasing Injection (BBI) uses a voltage applied with a physical probe onto the backside of the integrated circuit die. Compared to other techniques such as electromagnetic fault injection (EMFI) or Laser Fault Injection (LFI), this technique appears less popular in academic literature based on published results. It is hypothesized being due to (1) moderate cost of equipment, and (2) effort required in device preperation.

This work demonstrates that BBI (and indeed many other backside attacks) can be trivially performed on Wafer-Level Chip-Scale Packaging (WLCSP), which inherently expose the die backside. A low-cost ($15) design for the BBI tool is introduced, and validated with faults introduced on a STM32F415OG against code flow, RSA, and some initial results on various hardware block attacks are discussed.

textsf{Tweakable TWINE} (\twine) is the first lightweight dedicated tweakable block cipher family built on Generalized Feistel Structure (GFS). \twine family is an extension of the conventional block cipher \textsf{TWINE} with minimal modification by adding a simple tweak based on the SKINNY's tweakey schedule. Similar to \textsf{TWINE}, \twine has two variants, namely \twine[80] and \twine[128]. The two variants have the same block size of 64 bits and a variable key length of 80 and 128 bits. In this paper, we study the implications for adding the tweak on the security of \twine against the integral cryptanalysis. In particular, we first utilize the bit-based division property to search for the longest integral distinguisher. As a result, we are able to perform a distinguishing attack against 19 rounds using $2^{6} \times 2^{63} = 2^{69}$ chosen tweak-plaintext combinations. We then convert this attack to key recovery attacks against 26 and 27 rounds (out of 36) of \twine[80] and \twine[128], respectively. By prepending one round before the distinguisher and using dynamically chosen plaintexts, we manage to extend the attack one more round without using the full codebook of the plaintext. Therefore, we are able to attack 27 and 28 rounds of \twine[80] and \twine[128], respectively.

This paper proposes a simple synchronous composable security framework as an instantiation of the Constructive Cryptography framework, aiming to capture minimally, without unnecessary artefacts, exactly what is needed to state synchronous security guarantees. The objects of study are specifications (i.e., sets) of systems, and traditional security properties like consistency and validity can naturally be understood as specifications, thus unifying composable and property-based definitions. The framework's simplicity is in contrast to current composable frameworks for synchronous computation which are built on top of an asynchronous framework (e.g. the UC framework), thus not only inheriting artefacts and complex features used to handle asynchronous communication, but adding additional overhead to capture synchronous communication.

As a second, independent contribution we demonstrate how secure (synchronous) multi-party computation protocols can be understood as constructing a computer that allows a set of parties to perform an arbitrary, on-going computation. An interesting aspect is that the instructions of the computation need not be fixed before the protocol starts but can also be determined during an on-going computation, possibly depending on previous outputs.

Secure Multi-party Computation (MPC) allows a set of mutually distrusting parties to jointly evaluate a function on their private inputs while maintaining input privacy. In this work, we improve semi-honest secure two-party computation (2PC) over rings, with a focus on the efficiency of the online phase.

We propose an efficient mixed-protocol framework, outperforming the state-of-the-art 2PC framework of ABY. Moreover, we extend our techniques to multi- input multiplication gates without inflating the online communication, i.e., it remains independent of the fan-in. Along the way, we construct efficient protocols for several primitives such as scalar product, matrix multiplication, comparison, maxpool, and equality testing. The online communication of our scalar product is two ring elements irrespective of the vector dimension, which is a feature achieved for the first time in the 2PC literature.

The practicality of our new set of protocols is showcased with four applications: i) AES S-box, ii) Circuit-based Private Set Intersection, iii) Biometric Matching, and iv) Privacy- preserving Machine Learning (PPML). Most notably, for PPML, we implement and benchmark training and inference of Logistic Regression and Neural Networks over LAN and WAN networks. For training, we improve online runtime (both for LAN and WAN) over SecureML (Mohassel et al., IEEE S&P’17) in the range 1.5x-6.1x, while for inference, the improvements are in the range of 2.5x-754.3x.

Functional Encryption (FE) allows users who hold a specific secret key (known as the functional key) to learn a specific function of encrypted data whilst learning nothing about the content of the underlying data. Considering this functionality and the fact that the field of FE is still in its infancy, we sought a route to apply this potent tool to design efficient applications. To this end, we first built a symmetric FE scheme for the $\ell_1$ norm of a vector space, which allows us to compute the sum of the components of an encrypted vector. Then, we utilized our construction, to design an Order-Revealing Encryption (ORE) scheme and a privately encrypted database. While there is room for improvement in our schemes, this work is among the first attempts that seek to utilize FE for the solution of practical problems that can have a tangible effect on people's daily lives.

Somewhat Homomorphic Encryption (SHE) allows arbitrary computation with nite multiplicative depths to be performed on encrypted data, but its overhead is high due to memory transfer incurred by large ciphertexts. Recent research has recognized the shortcomings of general-purpose computing for high-performance SHE, and has begun to pioneer the use of hardware-based SHE acceleration with hardware including FPGAs, GPUs, and Compute-Enabled RAM (CE-RAM). CERAM is well-suited for SHE, as it is not limited by the separation between memory and processing that bottlenecks other hardware. Further, CE-RAM does not move data between dierent processing elements. Recent research has shown the high eectiveness of CE-RAM for SHE as compared to highly-optimized CPU and FPGA implementations. However, algorithmic optimization for the implementation on CE-RAM is underexplored. In this work, we examine the eect of existing algorithmic optimizations upon a CE-RAM implementation of the B/FV scheme, and further introduce novel optimization techniques for the Full RNS Variant of B/FV. Our experiments show speedups of up to 784x for homomorphic multiplication, 143x for decryption, and 330x for encryption against a CPU implementation. We also compare our approach to similar work in CE-RAM, FPGA, and GPU acceleration, and note general improvement over existing work. In particular, for homomorphic multiplication we see speedups of 506.5x against CE-RAM, 66.85x against FPGA, and 30.8x against GPU as compared to existing work in hardware acceleration of B/FV.

In this work, we introduce the first practical post-quantum verifiable random function (VRF) that relies on well-known (module) lattice problems, namely Module-SIS and Module-LWE. Our construction, named LB-VRF, results in a VRF value of only 84 bytes and a proof of around only 5 KB (in comparison to several MBs in earlier works), and runs in about 3 ms for evaluation and about 1 ms for verification.

In order to design a practical scheme, we need to restrict the number of VRF outputs per key pair, which makes our construction few-time. Despite this restriction, we show how our few-time LB-VRF can be used in practice and, in particular, we estimate the performance of Algorand using LB-VRF. We find that, due to the significant increase in the communication size in comparison to classical constructions, which is inherent in all existing lattice-based schemes, the throughput in LB-VRF-based consensus protocol is reduced, but remains practical. In particular, in a medium-sized network with 100 nodes, our platform records a 1.16x to 4x reduction in throughput, depending on the accompanying signature used. In the case of a large network with 500 nodes, we can still maintain at least 66 transactions per second. This is still much better than Bitcoin, which processes only about 5 transactions per second.

Most functional encryption schemes implicitly assume that inputs to decryption algorithms, i.e., secret keys and ciphertexts, are generated honestly. However, they may be tampered by malicious adversaries. Thus, verifiable functional encryption (VFE) was proposed by Badrinarayanan et al. in ASIACRYPT 2016 where anyone can publicly check the validity of secret keys and ciphertexts. They employed indistinguishability-based (IND-based) security due to an impossibility result of simulation-based (SIM-based) VFE even though SIM-based security is more desirable. In this paper, we propose a SIM-based VFE scheme. To bypass the impossibility result, we introduce a trusted setup assumption. Although it appears to be a strong assumption, we demonstrate that it is reasonable in a hardware-based construction, e.g., Fisch et al. in ACM CCS 2017. Our construction is based on a verifiable public-key encryption scheme (Nieto et al. in SCN 2012), a signature scheme, and a secure hardware scheme, which we refer to as VFE-HW. Finally, we discuss an our implementation of VFE-HW using Intel Software Guard Extensions (Intel SGX).

This report contains analysis and discussion on different versions of the TopoSign (Topographic Signature) protocol proposed by Matelski.

In this paper, we comprehensively study aggregate signatures with detecting functionality, that have functionality of both keyless aggregation of multiple signatures and identifying an invalid message from the aggregate signature, in order to reduce a total amount of signature-size for lots of messages. Our contribution is (i) to formalize strong security notions for both non-interactive and interactive protocols by taking into account related work such as fault-tolerant aggregate signatures and (non-)interactive aggregate MACs with detecting functionality (i.e., symmetric case); and (ii) to construct aggregate signatures with the functionality from group testing-protocols in a generic and comprehensive way. As instantiations, pairing-based constructions are provided.

In this paper, we propose a formal security model and a construction methodology of interactive aggregate message authentication with detecting functionality (IAMD). The IAMD is an interactive aggregate MAC protocol which can identify invalid messages with a small amount of tag-size. Several aggregate MAC schemes that can specify invalid messages has been proposed so far by using non-adaptive group testing in the prior work. In this paper, we utilize adaptive group testing to construct IAMD scheme, and we show that the resulting IAMD scheme can identify invalid messages with a small amount of tag-size compared to the previous schemes.

NIST is currently conducting the 3rd round of a survey to find post-quantum class asymmetric protocols (PQC) [1]. We participated in a joint-team with a fellow researcher of the Interamerican Open University (UAI) with a Key-Exchange Protocol (KEP) called HK17 [2]. The proposal was flawed because Bernstein [3] found a weakness, which was later refined by Li [4] using a quadratic reduction of octonions and quaternions, albeit no objection about the published non-commutative protocol and the one-way trapdoor function (OWTF). This fact promoted the search for a suitable algebraic platform. HK17 had its interest because it was the only first-round offer strictly based on canonical group theory [5]. At last, we adapted the original protocol with the R-propping solution of 3-dimensional tensors [6], yielding Bernstein attack fruitless. Therefore, an El Gamal IND-CCA2 cipher security using Cao [7] arguments are at hand.

This paper proposes two different methods to perform NTT-based polynomial multiplication in polynomial rings that do not naturally support such a multiplication. We demonstrate these methods on the NTRU Prime key-encapsulation mechanism (KEM) proposed by Bernstein, Chuengsatiansup, Lange, and Vredendaal, which uses a polynomial ring that is, by design, not amenable to use with NTT. One of our approaches is using Good's trick and focuses on speed and supporting more than one parameter set with a single implementation. The other approach is using a mixed-radix NTT and focuses on the use of smaller multipliers and less memory. On an ARM Cortex-M4 microcontroller, we show that our three NTT-based implementations, one based on Good's trick and two mixed-radix NTTs, provide between 32% and 17% faster polynomial multiplication. For the parameter-set ntrulpr761, this results in between 16% and 9% faster total operations (sum of key generation, encapsulation, and decapsulation) and requires between 15% and 39% less memory than the current state-of-the-art NTRU Prime implementation on this platform, which is using Toom-Cook-based polynomial multiplication.

This short report contains results of a brief cryptanalysis of the initialisation phase of ZUC-256. We find IV differentials that persist for 26 of the 33 initialisation rounds, and Key differentials that persist for 28 of the 33 rounds.

Boneh-Durfee proposed (at Eurocrypt 1999) a polynomial time attacks on RSA small decryption exponent which exploits lattices and sub-lattice structure to obtain an optimized bounds d < N^0.284 and d < N^0.292 respectively using lattice based Coppersmith’s method. In this paper we propose a special case of Boneh-Durfee’s attack with respect to large private exponent (i.e. d = N^&#949; > e = N^&#945; where &#949; and &#945; are the private and public key exponents respectively) for some &#945; &#8804; &#949;, which satisfy the condition d > &#966;(N) &#8722; N^&#949;. We analyzed lattices whose basis matrices are triangular and non-triangular using large decryption exponent and focus group attacks respectively. The core objective is to explore RSA polynomials underlying algebraic structure so that we can improve the performance of weak key attacks. In our solution, we implemented the attack and perform several experiments to show that an RSA cryptosystem successfully attacked and revealed possible weak keys which can ultimately enables an adversary to factorize the RSA modulus.

This paper studies concrete security with respect to expected-time adversaries. Our first contribution is a set of generic tools to obtain tight bounds on the advantage of an adversary with expected-time guarantees. We apply these tools to derive bounds in the random-oracle and generic-group models, which we show to be tight.

As our second contribution, we use these results to derive concrete bounds on the soundness of public-coin proofs and arguments of knowledge. Under the lens of concrete security, we revisit a paradigm by Bootle at al. (EUROCRYPT '16) that proposes a general Forking Lemma for multi-round protocols which implements a rewinding strategy with expected-time guarantees. We give a tighter analysis, as well as a modular statement. We adopt this to obtain the first quantitative bounds on the soundness of Bulletproofs (Bünz et al., S&P 2018), which we instantiate with our expected-time generic-group analysis to surface inherent dependence between the concrete security and the statement to be proved.

The only known non-interactive zero-knowledge (NIZK) protocol that is secure against adaptive corruption of the prover is based on that of Groth-Ostrovsky-Sahai (JACM'11) (GOS). However that protocol does not guarantee full adaptive soundness. Abe and Fehr (TCC'07) construct an adaptively sound variant of the GOS protocol under a knowledge-of-exponent assumption, but knowledge assumptions of this type are inherently incompatible with universally composable (UC) security.

We show the first NIZK which is triply adaptive: it is a UC NIZK protocol in a multi-party, multi-instance setting, with adaptive corruptions and no data erasures. Furthermore, the protocol provides full adaptive soundness. Our construction is very different than that of GOS: it is based on the recent NIZK of Canetti et al (STOC'19), and can be based on a variety of assumptions (e.g. LWE, or LPN and DDH). We also show how to get a succinct reference string assuming LWE or DDH from GOS-like techniques.

Public key encryption with keyword search (PEKS) is first introduced by Boneh et al. enabling a cloud server to search on encrypted data without leaking any information of the keyword. In almost all PEKS schemes, the privacy of trapdoor is vulnerable to inside keyword guessing attacks (KGA), i.e., the server can generate the ciphertext by its own and then run the test algorithm to guess the keyword contained in the trapdoor. To sole this problem, Huang et al. proposed the public-key authenticated encryption with keyword search (PAEKS) achieving trapdoor privacy (TP) security, in which data sender not only encrypts the keyword but also authenticates it by using his/her secret key. Qin et al. introduced the notion of multi-ciphertext indistinguishability (MCI) security to capture outside chosen multi-ciphertext attacks, in which the adversary needs to distinguish two tuples of ciphertexts corresponding with two sets of keywords. They analysed that Huang's work cannot achieve MCI security, so they proposed an improved scheme to match both the MCI security and trapdoor privacy (TP) security. In addition, they also defined the notion of multi-trapdoor privacy (MTP) security, which requires to distinguish two tuples of trapdoors corresponding with two sets of keywords.

Unfortunately, trapdoor generation algorithms of all above works are deterministic, which means they are unable to capture the security requirement of MTP. How to achieve MTP security against inside multi-keyword guessing attacks,i.e., designing a probabilistic trapdoor generation algorithm, is still an open problem.

In this paper, we solve this problem. We initially propose two public-key authenticated encryption with keyword search schemes achieving both MCI security and MTP security simultaneously. We provide formal proof of our schemes in the random oracle model.