RADIS: Remote Attestation of Distributed IoT Services

Remote attestation is a security technique through which a remote trusted party (i.e., Verifier) checks the trustworthiness of a potentially untrusted device (i.e., Prover). In the Internet of Things (IoT) systems, the existing remote attestation protocols propose various approaches to detect the modified software and physical tampering attacks. However, in an interoperable IoT system, in which IoT devices interact autonomously among themselves, an additional problem arises: a compromised IoT service can influence the genuine operation of other invoked service, without changing the software of the latter. In this paper, we propose a protocol for Remote Attestation of Distributed IoT Services (RADIS), which verifies the trustworthiness of distributed IoT services. Instead of attesting the complete memory content of the entire interoperable IoT devices, RADIS attests only the services involved in performing a certain functionality. RADIS relies on a control-flow attestation technique to detect IoT services that perform an unexpected operation due to their interactions with a malicious remote service. Our experiments show the effectiveness of our protocol in validating the integrity status of a distributed IoT service.Comment: 21 pages, 10 figures, 2 table

Usalduse vähendamine ja turvalisuse parandamine zk-SNARK-ides ja kinnitusskeemides

Väitekirja elektrooniline versioon ei sisalda publikatsioonezk-SNARK-id on tõhusad ja praktilised mitteinteraktiivsed tõestussüsteemid, mis on konstrueeritud viitestringi mudelis ning tänu kompaktsetele tõestustele ja väga tõhusale verifitseeritavusele on need laialdaselt kasutusele võetud suuremahulistes praktilistes rakendustes. Selles töös uurime zk-SNARK-e kahest vaatenurgast: nende usalduse vähendamine ja turvalisuse tugevdamine. Esimeses suunas uurime kui palju saab vähendada usaldust paaristuspõhiste zk-SNARK-ide puhul ilma nende tõhusust ohverdamata niiviisi, et kasutajad saavad teatud turvataseme ka siis kui seadistusfaas tehti pahatahtlikult või kui avalikustati seadistusfaasi salajane teave. Me pakume välja mõned tõhusad konstruktsioonid, mis suudavad takistada zk-SNARK-i seadistusfaasi ründeid ja mis saavutavad senisest tugevama turvataseme. Näitame ka seda, et sarnased tehnikad võimaldavad leevendada usaldust tagauksega kinnitusskeemides, mis on krüptograafiliste primitiivide veel üks silmapaistev perekond ja mis samuti nõub usaldatud seadistusfaasi. Teises suunas esitame mõned tõhusad konstruktsioonid, mis tagavad parema turvalisuse minimaalsete lisakuludega. Mõned esitatud konstruktsioonidest võimaldavad lihtsustada praegusi TK-turvalisi protokolle, nimelt privaatsust säilitavate nutilepingusüsteemide Hawk ja Gyges konstruktsiooni, ja parandada nende tõhusust. Uusi konstruktsioone saab aga otse kasutada uutes protokollides, mis soovivad kasutada zk-SNARK-e. Osa väljapakutud zk-SNARK-e on implementeeritud teegis Libsnark ja empiirilised tulemused kinnitavad, et usalduse vähendamiseks või suurema turvalisuse saavutamiseks on arvutuslikud lisakulud väikesed.Zero-knowledge Succinct Non-interactive ARguments of Knowledge (zk-SNARKs) are an efficient family of NIZK proof systems that are constructed in the Common Reference String (CRS) model and due to their succinct proofs and very efficient verification, they are widely adopted in large-scale practical applications. In this thesis, we study zk-SNARKs from two perspectives, namely reducing trust and improving security in them. In the first direction, we investigate how much one can mitigate trust in pairing-based zk-SNARKs without sacrificing their efficiency. In such constructions, the parties of protocol will obtain a certain level of security even if the setup phase was done maliciously or the secret information of the setup phase was revealed. As a result of this direction, we present some efficient constructions that can resist against subverting of the setup phase of zk-SNARKs and achieve a certain level of security which is stronger than before. We also show that similar techniques will allow us to mitigate the trust in the trapdoor commitment schemes that are another prominent family of cryptographic primitives that require a trusted setup phase. In the second direction, we present some efficient constructions that achieve more security with minimal overhead. Some of the presented constructions allow to simplify the construction of current UC-secure protocols and improve their efficiency. New constructions can be directly deployed in any novel protocols that aim to use zk-SNARKs. Some of the proposed zk-SNARKs are implemented in Libsnark, the state-of-the-art library for zk-SNARKs, and empirical experiences confirm that the computational cost to mitigate the trust or to achieve more security is practical.https://www.ester.ee/record=b535927

Algorithm Substitution Attacks: Detecting ASAs Using State Reset and Making ASAs Asymmetric

The field of cryptography has made incredible progress in the last several decades. With the formalization of security goals and the methods of provable security, we have achieved many privacy and integrity guarantees in a great variety of situations. However, all guarantees are limited by their assumptions on the model's adversaries. Edward Snowden's revelations of the participation of the National Security Agency (NSA) in the subversion of standardized cryptography have shown that powerful adversaries will not always act in the way that common cryptographic models assume. As such, it is important to continue to expand the capabilities of the adversaries in our models to match the capabilities and intentions of real world adversaries, and to examine the consequences on the security of our cryptography. In this thesis, we study Algorithm Substitution Attacks (ASAs), which are one way to model this increase in adversary capability. In an ASA, an algorithm in a cryptographic scheme Λ is substituted for a subverted version. The goal of the adversary is to recover a secret that will allow them to compromise the security of Λ, while requiring that the attack is undetectable to the users of the scheme. This model was first formally described by Bellare, Paterson, and Rogaway (Crypto 2014), and allows for the possibility of a wide variety of cryptographic subversion techniques. Since their paper, many successful ASAs on various cryptographic primitives and potential countermeasures have been demonstrated. We will address several shortcomings in the existing literature. First, we formalize and study the use of state resets to detect ASAs. While state resets have been considered as a possible detection method since the first papers on ASAs, future works have only informally reasoned about the effect of state resets on ASAs. We show that many published ASAs that use state are detectable with simple practical methods relying on state resets. Second, we add to the study of asymmetric ASAs, where the ability to recover secrets is restricted to the attacker who implemented the ASA. We describe two asymmetric ASAs on symmetric encryption based on modifications to previous ASAs. We also generalize this result, allowing for any symmetric ASA (on any cryptographic scheme) satisfying certain properties to be transformed into an asymmetric ASA. This work demonstrates the broad application of the techniques first introduced by Bellare, Paterson, and Rogaway (Crypto 2014) and Bellare, Jaeger, and Kane (CCS 2015) and reinforces the need for precise definitions surrounding detectability of stateful ASAs

Algorithm Substitution Attacks: State Reset Detection and Asymmetric Modifications

In this paper, we study algorithm substitution attacks (ASAs), where an algorithm in a cryptographic scheme is substituted for a subverted version. First, we formalize and study the use of state resets to detect ASAs, and show that many published stateful ASAs are detectable with simple practical methods relying on state resets. Second, we introduce two asymmetric ASAs on symmetric encryption, which are undetectable or unexploitable even by an adversary who knows the embedded subversion key. We also generalize this result, allowing for any symmetric ASA (on any cryptographic scheme) satisfying certain properties to be transformed into an asymmetric ASA. Our work demonstrates the broad application of the techniques first introduced by Bellare, Paterson, and Rogaway (Crypto 2014) and Bellare, Jaeger, and Kane (CCS 2015) and reinforces the need for precise definitions surrounding detectability of stateful ASAs

Subverting Deniability

Deniable public-key encryption (DPKE) is a cryptographic primitive that allows the sender of an encrypted message to later claim that they sent a different message. DPKE\u27s threat model assumes powerful adversaries who can coerce users to reveal plaintexts; it is thus reasonable to consider other advanced capabilities, such as the ability to subvert algorithms in a so-called Algorithm Substitution Attack (ASA). An ASA replaces a trusted algorithm with a subverted version that undermines security from the point of view of the adversary while remaining undetected by users. ASAs have been considered against a number of primitives including digital signatures, symmetric encryption and pseudo-random generators. However, public-key encryption has presented a less fruitful target, as the sender\u27s only secrets are plaintexts and ASA techniques generally do not provide sufficient bandwidth to leak these. In this work, we show that subversion attacks against deniable encryption schemes present an attractive opportunity for an adversary. We note that whilst the notion is widely accepted, there are as yet no practical deniable PKE schemes; we demonstrate the feasibility of ASAs targeting deniable encryption using a representative scheme as a proof of concept. We also provide a formal model and discuss how to mitigate ASAs targeting deniable PKE schemes. Our results strengthen the security model for deniable encryption and highlight the necessity of considering subversion in the design of practical schemes

Security of Symmetric Encryption against Mass Surveillance

Motivated by revelations concerning population-wide surveillance of encrypted communications, we formalize and investigate the resistance of symmetric encryption schemes to mass surveillance. The focus is on algorithm-substitution attacks (ASAs), where a subverted encryption algorithm replaces the real one. We assume that the goal of ``big~brother\u27\u27 is undetectable subversion, meaning that ciphertexts produced by the subverted encryption algorithm should reveal plaintexts to big~brother yet be indistinguishable to users from those produced by the real encryption scheme. We formalize security notions to capture this goal and then offer both attacks and defenses. In the first category we show that successful (from the point of view of big brother) ASAs may be mounted on a large class of common symmetric encryption schemes. In the second category we show how to design symmetric encryption schemes that avoid such attacks and meet our notion of security. The lesson that emerges is the danger of choice: randomized, stateless schemes are subject to attack while deterministic, stateful ones are not

Subversion-Resilient Signatures without Random Oracles

In the aftermath of the Snowden revelations in 2013, concerns about the integrity and security of cryptographic systems have grown significantly. As adversaries with substantial resources might attempt to subvert cryptographic algorithms and undermine their intended security guarantees, the need for subversion-resilient cryptography has become paramount. Security properties are preserved in subversion-resilient schemes, even if the adversary implements the scheme used in the security experiment. This paper addresses this pressing concern by introducing novel constructions of subversion-resilient signatures and hash functions while proving the subversion-resilience of existing cryptographic primitives. Our main contribution is the first construction of subversion-resilient signatures under complete subversion in the offline watchdog model (with trusted amalgamation) without relying on random oracles. We demonstrate that one-way permutations naturally yield subversion-resilient one-way functions, thereby enabling us to establish the subversion-resilience of Lamport signatures, assuming a trusted comparison is available. Additionally, we develop subversion-resilient target-collision-resistant hash functions using a trusted XOR. By leveraging this approach, we expand the arsenal of cryptographic tools that can withstand potential subversion attacks. Our research builds upon previous work in the offline watchdog model with trusted amalgamation (Russell et al. ASIACRYPT\u2716) and subversion-resilient pseudo-random functions (Bemmann et al. ACNS\u2723), culminating in the formal proof of subversion-resilience for the classical Naor-Yung signature construction

Subvert KEM to Break DEM: Practical Algorithm-Substitution Attacks on Public-Key Encryption

Motivated by the currently widespread concern about mass surveillance of encrypted communications, Bellare \emph{et al.} introduced at CRYPTO 2014 the notion of Algorithm-Substitution Attack (ASA) where the legitimate encryption algorithm is replaced by a subverted one that aims to undetectably exfiltrate the secret key via ciphertexts. Practically implementable ASAs on various cryptographic primitives (Bellare \emph{et al.}, CRYPTO\u2714 \& ACM CCS\u2715; Ateniese \emph{et al.}, ACM CCS\u2715; Berndt and Liśkiewicz, ACM CCS\u2717) have been constructed and analyzed, leaking the secret key successfully. Nevertheless, in spite of much progress, the practical impact of ASAs (formulated originally for symmetric key cryptography) on public-key (PKE) encryption operations remains unclear, primarily since the encryption operation of PKE does not involve the secret key, and also previously known ASAs become relatively inefficient for leaking the plaintext due to the logarithmic upper bound of exfiltration rate (Berndt and Liśkiewicz, ACM CCS\u2717). In this work, we formulate a practical ASA on PKE encryption algorithm which, perhaps surprisingly, turns out to be much more efficient and robust than existing ones, showing that ASAs on PKE schemes are far more effective and dangerous than previously believed. We mainly target PKE of hybrid encryption which is the most prevalent way to employ PKE in the literature and in practice. The main strategy of our ASA is to subvert the underlying key encapsulation mechanism (KEM) so that the session key encapsulated could be efficiently extracted, which, in turn, breaks the data encapsulation mechanism (DEM) enabling us to learn the plaintext itself. Concretely, our non-black-box yet quite general attack enables recovering the plaintext from only two successive ciphertexts and minimally depends on a short state of previous internal randomness. A widely used class of KEMs is shown to be subvertible by our powerful attack. Our attack relies on a novel identification and formalization of certain properties that yield practical ASAs on KEMs. More broadly, it points at and may shed some light on exploring structural weaknesses of other ``composed cryptographic primitives,\u27\u27 which may make them susceptible to more dangerous ASAs with effectiveness that surpasses the known logarithmic upper bound (i.e., reviewing composition as an attack enabler)