Self-Guarding Cryptographic Protocols against Algorithm Substitution Attacks

We put forward the notion of self-guarding cryptographic protocols as a countermeasure to algorithm substitution attacks. Such self-guarding protocols can prevent undesirable leakage by subverted algorithms if one has the guarantee that the system has been properly working in an initialization phase. Unlike detection-based solutions they thus proactively thwart attacks, and unlike reverse firewalls they do not assume an online external party. We present constructions of basic primitives for (public-key and private-key) encryption and for signatures. We also argue that the model captures attacks with malicious hardware tokens and show how to self-guard a PUF-based key exchange protocol

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

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

Subversion-Resilient Enhanced Privacy ID

Anonymous attestation for secure hardware platforms leverages tailored group signature schemes and assumes the hardware to be trusted. Yet, there is an ever increasing concern on the trustworthiness of hardware components and embedded systems. A subverted hardware may, for example, use its signatures to exfiltrate identifying information or even the signing key. In this paper we focus on Enhanced Privacy ID (EPID)---a popular anonymous attestation scheme used in commodity secure hardware platforms like Intel SGX. We define and instantiate a \emph{subversion resilient} EPID scheme (or SR-EPID). In a nutshell, SR-EPID provides the same functionality and security guarantees of the original EPID, despite potentially subverted hardware. In our design, a ``sanitizer\u27\u27 ensures no covert channel between the hardware and the outside world both during enrollment and during attestation (i.e., when signatures are produced). We design a practical SR-EPID scheme secure against adaptive corruptions and based on a novel combination of malleable NIZKs and hash functions modeled as random oracles. Our approach has a number of advantages over alternative designs. Namely, the sanitizer bears no secret information---hence, a memory leak does not erode security. Further, the role of sanitizer may be distributed in a cascade fashion among several parties so that sanitization becomes effective as long as one of the parties has access to a good source of randomness. Also, we keep the signing protocol non-interactive, thereby minimizing latency during signature generation