12 HGI papers at top conferences USENIX and CRYPTO

Numerous scientists present their work at the conferences.

Logo Usenix Security

Great successes for the HGI and the Cluster of Excellence CASA at a couple of top tier conferences: At this year's USENIX (29th USENIX Security Symposium, virtual event held on August 12–14, 2020) the HGI/CASA will be represented with seven papers. At the IACR CRYPTO (40th Annual International Cryptology Conference, virtual event on August 17-21 2020) five papers by HGI scientists will be presented. Find details on the papers in our overview.



1.    Call Me Maybe: Eavesdropping Encrypted LTE Calls With ReVoLTE

David Rupprecht, Katharina Kohls, and Thorsten Holz, Ruhr University Bochum; Christina Pöpper, NYU Abu Dhabi

This paper is under embargo and will be released to the public on the first day of the symposium, August 12, 2020.

2.     AURORA: Statistical Crash Analysis for Automated Root Cause Explanation

Tim Blazytko, Moritz Schlögel, Cornelius Aschermann, Ali Abbasi, Joel Frank, Simon Wörner, and Thorsten Holz, Ruhr-Universität Bochum

Abstract. Given the huge success of automated software testing techniques, a large amount of crashes is found in practice. Identifying the root cause of a crash is a time-intensive endeavor, causing a disproportion between finding a crash and fixing the underlying software fault. To address this problem, various approaches have been proposed that rely on techniques such as reverse execution and backward taint analysis. Still, these techniques are either limited to certain fault types or provide an analyst with assembly instructions, but no context information or explanation of the underlying fault.
In this paper, we propose an automated analysis approach that does not only identify the root cause of a given crashing input for a binary executable, but also provides the analyst with context information on the erroneous behavior that characterizes crashing inputs. Starting with a single crashing input, we generate a diverse set of similar inputs that either also crash the program or induce benign behavior. We then trace the program's states while executing each found input and generate predicates, i.e., simple Boolean expressions that capture behavioral differences between crashing and non-crashing inputs. A statistical analysis of all predicates allows us to identify the predicate pinpointing the root cause, thereby not only revealing the location of the root cause, but also providing an analyst with an explanation of the misbehavior a crash exhibits at this location. We implement our approach in a tool called AURORA and evaluate it on 25 diverse software faults. Our evaluation shows that AURORA is able to uncover root causes even for complex bugs. For example, it succeeded in cases where many millions of instructions were executed between developer fix and crashing location. In contrast to existing approaches, AURORA is also able to handle bugs with no data dependency between root cause and crash, such as type confusion bugs.

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3.    ETHBMC: A Bounded Model Checker for Smart Contracts

Joel Frank, Cornelius Aschermann, and Thorsten Holz, Ruhr-Universität Bochum

Abstract. The introduction of smart contracts has significantly advanced the state-of-the-art in cryptocurrencies. Smart contracts are programs who live on the blockchain, governing the flow of money. However, the promise of monetary gain has attracted miscreants, resulting in spectacular hacks which resulted in the loss of millions worth of currency. In response, several powerful static analysis tools were developed to address these problems. We surveyed eight recently proposed static analyzers for Ethereum smart contracts and found that none of them captures all relevant features of the Ethereum ecosystem. For example, we discovered that a precise memory model is missing and inter-contract analysis is only partially supported.
Based on these insights, we present the design and implementation of, a bounded model checker based on symbolic execution which provides a precise model of the Ethereum network. We demonstrate its capabilities in a series of experiments.

First, we compare against the eight aforementioned tools, showing that even relatively simple toy examples can obstruct other analyzers. Further proving that precise modeling is indispensable, we leverage ETHBmc capabilities for automatic vulnerability scanning. We perform a large-scale analysis of roughly 2.2 million accounts currently active on the blockchain and automatically generate 5,905 valid inputs which trigger a vulnerability. From these, 1,989 can destroy a contract at will (so called suicidal contracts) and the rest can be used by an adversary to arbitrarily extract money. Finally, we compare our large-scale analysis against two previous analysis runs, finding significantly  more inputs (22.8%) than previous approaches.

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4.    Analysis of DTLS Implementations Using Protocol State Fuzzing

Paul Fiterau-Brostean and Bengt Jonsson, Uppsala University; Robert Merget, Ruhr-University Bochum; Joeri de Ruiter, SIDN Labs; Konstantinos Sagonas, Uppsala University; Juraj Somorovsky, Paderborn University

Abstract. Recent years have witnessed an increasing number of protocols relying on UDP. Compared to TCP, UDP offers performance advantages such as simplicity and lower latency. This has motivated its adoption in Voice over IP, tunneling technologies, IoT, and novel Web protocols. To protect sensitive data exchange in these scenarios, the DTLS protocol has been developed as a cryptographic variation of TLS. DTLS’s main challenge is to support the stateless and unreliable transport of UDP. This has forced protocol designers to make choices that affect the complexity of DTLS, and to incorporate features that need not be addressed in the numerous TLS analyses.

We present the first comprehensive analysis of DTLS implementations using protocol state fuzzing. To that end, we extend TLS-Attacker, an open source framework for analyzing TLS implementations, with support for DTLS tailored to the stateless and unreliable nature of the underlying UDP layer. We build a framework for applying protocol state fuzzing on DTLS servers, and use it to learn state machine models for thirteen DTLS implementations. Analysis of the learned state models reveals four serious security vulnerabilities, including a full client authentication bypass in the latest JSSE version, as well as several functional bugs and non-conformance issues. It also uncovers considerable differences between the models, confirming the complexity of DTLS state machines.

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5.    The Unpatchable Silicon: A Full Break of the Bitstream Encryption of Xilinx 7-Series FPGAs

Maik Ender and Amir Moradi, Horst Goertz Institute for IT Security, Ruhr-Universität Bochum; Christof Paar, Max Planck Institute for Cyber Security and Privacy and Horst Goertz Institute for IT Security, Ruhr-Universität Bochum

Abstract. The security of FPGAs is a crucial topic, as any vulnerability within the hardware can have severe consequences, if they are used in a secure design. Since FPGA designs are encoded in a bitstream, securing the bitstream is of the utmost importance. Adversaries have many motivations to recover and manipulate the bitstream, including design cloning, IP theft, manipulation of the design, or design subversions e.g., through hardware Trojans. Given that FPGAs are often part of cyber-physical systems e.g., in aviation, medical, or industrial devices, this can even lead to physical harm. Consequently, vendors have introduced bitstream encryption, offering authenticity and confidentiality. Even though attacks against bitstream encryption have been proposed in the past, e.g., side-channel analysis and probing, these attacks require sophisticated equipment and considerable technical expertise.
In this paper, we introduce novel low-cost attacks against the Xilinx 7-Series (and Virtex-6) bitstream encryption, resulting in the total loss of authenticity and confidentiality. We exploit a design flaw which piecewise leaks the decrypted bitstream. In the attack, the FPGA is used as a decryption oracle, while only access to a configuration interface is needed. The attack does not require any sophisticated tools and, depending on the target system, can potentially be launched remotely. In addition to the attacks, we discuss several countermeasures.

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6.    HALucinator: Firmware Re-hosting Through Abstraction Layer Emulation

Abraham A Clements, Sandia National Laboratories; Eric Gustafson, UC Santa Barbara and Sandia National Laboratories; Tobias Scharnowski, Ruhr-Universität Bochum; Paul Grosen, UC Santa Barbara; David Fritz, Sandia National Laboratories; Christopher Kruegel and Giovanni Vigna, UC Santa Barbara; Saurabh Bagchi, Purdue University; Mathias Payer, EPFL

Abstract. Given the increasing ubiquity of online embedded devices, analyzing their firmware is important to security, privacy, and safety. The tight coupling between hardware and firmware and the diversity found in embedded systems makes it hard to perform dynamic analysis on firmware. However, firmware developers regularly develop code using abstractions, such as Hardware Abstraction Layers (HALs), to simplify their job. We leverage such abstractions as the basis for the re-hosting and analysis of firmware. By providing high-level replacements for HAL functions (a process termed High-Level Emulation – HLE), we decouple the hardware from the firmware. This approach works by first locating the library functions in a firmware sample, through binary analysis, and then providing generic implementations of these functions in a full-system emulator.

We present these ideas in a prototype system, HALucinator, able to re-host firmware, and allow the virtual device to be used normally. First, we introduce extensions to existing library matching techniques that are needed to identify library functions in binary firmware, to reduce collisions, and for inferring additional function names. Next, we demonstrate the re-hosting process, through the use of simplified handlers and peripheral models, which make the process fast, flexible, and portable between firmware samples and chip vendors. Finally, we demonstrate the practicality of HLE for security analysis, by supplementing HALucinator with the American Fuzzy Lop fuzzer, to locate multiple previously-unknown vulnerabilities in firmware middleware libraries.

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7.    McTiny: Fast High-Confidence Post-Quantum Key Erasure for Tiny Network Servers

Daniel J. Bernstein, University of Illinois at Chicago, Ruhr University Bochum; Tanja Lange, Eindhoven University of Technology

Abstract. Recent results have shown that some post-quantum cryptographic systems have encryption and decryption performance comparable to fast elliptic-curve cryptography (ECC) or even better. However, this performance metric is considering only CPU time and ignoring bandwidth and storage. High-confidence post-quantum encryption systems have much larger keys than ECC. For example, the code-based cryptosystem recommended by the PQCRYPTO project uses public keys of 1MB.
Fast key erasure (to provide "forward secrecy") requires new public keys to be constantly transmitted. Either the server needs to constantly generate, store, and transmit large keys, or it needs to receive, store, and use large keys from the clients. This is not necessarily a problem for overall bandwidth, but it is a problem for storage and computation time on tiny network servers. All straightforward approaches allow easy denial-of-service attacks.
This paper describes a protocol, suitable for today's networks and tiny servers, in which clients transmit their code-based one-time public keys to servers. Servers never store full client public keys but work on parts provided by the clients, without having to maintain any per-client state. Intermediate results are stored on the client side in the form of encrypted cookies and are eventually combined by the server to obtain the ciphertext. Requirements on the server side are very small: storage of one long-term private key, which is much smaller than a public key, and a few small symmetric cookie keys, which are updated regularly and erased after use. The protocol is highly parallel, requiring only a few round trips, and involves total bandwidth not much larger than a single public key. The total number of packets sent by each side is 971, each fitting into one IPv6 packet of less than 1280 bytes.
The protocol makes use of the structure of encryption in code-based cryptography and benefits from small ciphertexts in code-based cryptography.

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1.    Shorter Non-Interactive Zero-Knowledge Arguments and ZAPs for Algebraic Languages

Dominik Hartmann, Geoffroy Couteau; Ruhr University Bochum, Université Paris-Diderot

Abstract. We put forth a new framework for building pairing-based non-interactive zero- knowledge (NIZK) arguments for a wide class of algebraic languages, which are an extension of linear languages, containing disjunctions of linear languages and more. Our approach differs from the Groth-Sahai methodology, in that we rely on pairings to compile a Σ-protocol into a NIZK. Our framework enjoys a number of interesting features: – conceptual simplicity, parameters derive from the Σ-protocol; – proofs as short as resulting from the Fiat-Shamir heuristic applied to the underlying Σ-protocol; – fully adaptive soundness and perfect zero-knowledge in the common random string model with a single random group element as CRS; – yields simple and efficient two-round, public coin, publicly-verifiable perfect witness-indistinguishable (WI) arguments (ZAPs) in the plain model. To our knowledge, this is the first construction of two-rounds statistical witness-indistinguishable arguments from pairing assumptions. Our proof system relies on a new (static, falsifiable) assumption over pairing groups which generalizes the standard kernel Diffie-Hellman assumption in a natural way and holds in the generic group model (GGM) and in the algebraic group model (AGM). Replacing Groth-Sahai NIZKs with our new proof system allows to improve several important cryptographic primitives. In particular, we obtain the shortest tightly-secure structure-preserving signature scheme (which are a core component in anonymous credentials), the shortest tightly-secure quasi-adaptive NIZK with unbounded simulation soundness (which in turns implies the shortest tightly-mCCA-secure cryptosystem), and shorter ring signatures.

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2.    Alzette: a 64-bit ARX-box (feat. CRAX and TRAX)

Christof Beierle, Alex Biryukov, Luan Cardoso dos Santos, Johann Großschädl, Léo Perrin, Aleksei Udovenko, Vesselin Velichkov, Qingju Wang
Ruhr University Bochum, Snt, University of Luxembourg, Inria, CryptoExperts, University of Edinburgh

Abstract. S-boxes are the only source of non-linearity in many symmetric primitives. While they are often defined as being functions operating on a small space, some recent designs propose the use of much larger ones (e.g., 32 bits). In this context, an S-box is then defined as a subfunction whose cryptographic properties can be estimated precisely.
In this paper, we present a 64-bit ARX-based S-box called Alzette, which can be evaluated in constant time using only 12 instructions on modern CPUs. Its parallel application can also leverage vector (SIMD) instructions. One iteration of Alzette has differential and linear properties comparable to those of the AES S-box, while two iterations are at least as secure as the AES super S-box.
Since the state size is much larger than the typical 4 or 8 bits, the study of the relevant cryptographic properties of Alzette is not trivial.

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3.    Improved Differential-Linear Attacks with Applications to ARX Ciphers

Christof Beierle, Gregor Leander, Yosuke Todo, Ruhr University Bochum, NTT Secure Platform Laboratories

Abstract: We present several improvements to the framework of differential-linear attacks with a special focus on ARX ciphers. As a demonstration of their impact, we apply them to Chaskey and ChaCha and we are able to significantly improve upon the best attacks published so far.

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4.    Out of Oddity -- New Cryptanalytic Techniques against Symmetric Primitives Optimized for Integrity Proof Systems

Tim Beyne, Anne Canteaut, Itai Dinur, Maria Eichlseder, Gregor Leander, Gaëtan Leurent, Léo Perrin, María Naya Plasencia, Yu Sasaki, Yosuke Todo, Friedrich Wiemer
KU Leuven, Inria, Ben-Gurion University, TU Graz, Ruhr-Universität Bochum, NTT Security Labs, NTT Secure Platform Laboratories and Ruhr-Universität Bochum

Abstract. The security and performance of many integrity proof systems like SNARKs, STARKs and Bulletproofs highly depend on the underlying hash function. For this reason several new proposals have recently been developed. These primitives obviously require an in-depth security evaluation, especially since their implementation constraints have led to less standard design approaches. This work compares the security levels offered by two recent families of such primitives, namely GMiMC and HadesMiMC. We exhibit low-complexity distinguishers against the GMiMC and HadesMiMC permutations for most parameters proposed in recently launched public challenges for STARK-friendly hash functions. In the more concrete setting of the sponge construction corresponding to the practical use in the ZK-STARK protocol, we present a practical collision attack on a round-reduced version of GMiMC and a preimage attack on some instances of HadesMiMC. To achieve those results, we adapt and generalize several cryptographic techniques to fields of odd characteristic.

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5.    Lattice-Based Blind Signatures, Revisited

Eduard Hauck, Eike Kiltz, Julian Loss, Ngoc Khanh Nguyen, Ruhr University Bochum, University of Maryland, College Park, IBM Research - Zurich

Abstract. We observe that all previously known lattice-based blind signature schemes contain subtle flaws in their security proofs (e.g., R\"uckert, ASIACRYPT '08) or can be attacked (e.g., BLAZE by Alkadri et al., FC '20). Motivated by this, we revisit the problem of constructing blind signatures from standard lattice assumptions.
We propose a new three-round lattice-based blind signature scheme whose security can be proved, in the random oracle model, from the standard SIS assumption. Our starting point is a modified version of the (insecure) BLAZE scheme, which itself is based Lyubashevsky's three-round identification scheme combined with a new aborting technique to reduce the correctness error. Our proof builds upon and extends the recent modular framework for blind signatures of Hauck, Kiltz, and Loss (EUROCRYPT '19). It also introduces several new techniques to overcome the additional challenges posed by the correctness error which is inherent to all lattice-based constructions.
While our construction is mostly of theoretical interest, we believe it to be an important stepping stone for future works in this area.

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