A study of KEM generalizations

The NIST, in its recent competition on quantum-resilient confidentiality primitives, requested the submission of exclusively KEMs. The task of KEMs is to establish secure session keys that can drive, amongst others, public key encryption and TLS-like secure channels. In this work we test the KEM abstraction in the context of constructing cryptographic schemes that are not subsumed in the PKE and secure channels categories. We find that, when used to construct a key transport scheme or when used within a secure combiner, the KEM abstraction imposes certain inconvenient limits, the settling of which requires the addition of auxiliary symmetric primitives. We hence investigate generalizations of the KEM abstraction that allow a considerably simplified construction of the above primitives. In particular, we study VKEMs and KDFEMs, which augment classic KEMs by label inputs, encapsulation handle outputs, and key derivation features, and we demonstrate that they can be transformed into KEM combiners and key transport schemes without requiring auxiliary components. We finally show that all four finalist KEMs of the NIST competition are effectively KDFEMs. Our conclusion is that only very mild adjustments are necessary to significantly increase their versatility

Improving Generic Attacks Using Exceptional Functions

Over the past ten years, there have been many attacks on symmetric constructions using the statistical properties of random functions. Initially, these attacks targeted iterated hash constructions and their combiners, developing a wide array of methods based on internal collisions and on the average behavior of iterated random functions. More recently, Gilbert et al. (EUROCRYPT 2023) introduced a forgery attack on so-called duplex-based Authenticated Encryption modes which was based on exceptional random functions, i.e., functions whose graph admits a large component with an exceptionally small cycle. In this paper, we expand the use of such functions in generic cryptanalysis with several new attacks. First, we improve the attack of Gilbert et al. from $\mathcal{O}(2^{3c/4})$ to $\mathcal{O}(2^{2c/3})$, where $c$ is the capacity. This new attack uses a nested pair of functions with exceptional behavior, where the second function is defined over the cycle of the first one. Next, we introduce several new generic attacks against hash combiners, notably using small cycles to improve the complexities of the best existing attacks on the XOR combiner, Zipper Hash and Hash-Twice. Last but not least, we propose the first quantum second preimage attack against Hash-Twice, reaching a quantum complexity $\mathcal{O}(2^{3n/7})$

Algorithm Substitution Attacks against Receivers

This work describes a class of Algorithm Substitution Attack (ASA) generically targeting the receiver of a communication between two parties. Our work provides a unified framework that applies to any scheme where a secret key is held by the receiver; in particular, message authentication schemes (MACs), authenticated encryption (AEAD) and public key encryption (PKE). Our unified framework brings together prior work targeting MAC schemes and AEAD schemes; we extend prior work by showing that public key encryption may also be targeted. ASAs were initially introduced by Bellare, Paterson and Rogaway in light of revelations concerning mass surveillance, as a novel attack class against the confidentiality of encryption schemes. Such an attack replaces one or more of the regular scheme algorithms with a subverted version that aims to reveal information to an adversary (engaged in mass surveillance), while remaining undetected by users. Previous work looking at ASAs against encryption schemes can be divided into two groups. ASAs against PKE schemes target key generation by creating subverted public keys that allow an adversary to recover the secret key. ASAs against symmetric encryption target the encryption algorithm and leak information through a subliminal channel in the ciphertexts. We present a new class of attack that targets the decryption algorithm of an encryption scheme for symmetric encryption and public key encryption, or the verification algorithm for an authentication scheme. We present a generic framework for subverting a cryptographic scheme between a sender and receiver, and show how a decryption oracle allows a subverter to create a subliminal channel which can be used to leak secret keys. We then show that the generic framework can be applied to authenticated encryption with associated data, message authentication schemes, public key encryption and KEM/DEM constructions. We consider practical considerations and specific conditions that apply for particular schemes, strengthening the generic approach. Furthermore, we show how the hybrid subversion of key generation and decryption algorithms can be used to amplify the effectiveness of our decryption attack. We argue that this attack represents an attractive opportunity for a mass surveillance adversary. Our work serves to refine the ASA model and contributes to a series of papers that raises awareness and understanding about what is possible with ASAs

Post Quantum Design in SPDM for Device Authentication and Key Establishment

The Security Protocol and Data Model (SPDM) defines flows to authenticate hardware identity of a computing device. It also allows for establishing a secure session for confidential and integrity protected data communication between two devices. The present version of SPDM, namely version 1.2, relies on traditional asymmetric cryptographic algorithms that are known to be vulnerable to quantum attacks. This paper describes the means by which support for post-quantum (PQ) cryptography can be added to the SPDM protocol in order to enable SPDM for the upcoming world of quantum computing. We examine SPDM 1.2 protocol and discuss how to negotiate the use of post-quantum cryptography algorithms (PQC), how to support device identity reporting, means to authenticate the device, and how to establish a secure session when using PQC algorithms. We consider so called hybrid modes where both classical and PQC algorithms are used to achieve security properties as these modes are important during the transition period. We also share our experience with implementing PQ-SPDM and provide benchmarks for some of the winning NIST PQC algorithms

FO-like Combiners and Hybrid Post-Quantum Cryptography

Combining several primitives together to offer greater security is an old idea in cryptography. Recently, this concept has resurfaced as it could be used to improve trust in new Post-Quantum (PQ) schemes and smooth the transition to PQ cryptography. In particular, several ways to combine key exchange mechanisms (KEMs) into a secure hybrid KEM have been proposed. In this work, we observe that most PQ KEMs are built using a variant of the Fujisaki-Okamoto (FO) transform. Thus, we propose several efficient combiners that take OW-CPA public-key encryption schemes (PKEs) and directly build hybrid IND-CCA KEMs. Our constructions are secure in the ROM and QROM and can be seen as generalizations of the FO transform. We also study how the hash functions (ROs) used in our transforms can be combined in order to improve efficiency and security. In a second part, we implement a hybrid KEM using one of our combiners as a proof-of-concept and benchmark it. More precisely, we build a hybrid IND-CCA KEM from the CPA-secure versions of HQC and LAC, two NIST Round 2 PQ proposals. We show that the resulting KEM offers comparable performances to HQC, thus improving security at a small cost. Finally, we discuss which PQ schemes should be combined in order to offer the best efficiency/security trade-off

Hybrid post-quantum cryptography in network protocols

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Ciência da Computação, Florianópolis, 2023.A segurança de redes é essencial para as comunicações do dia-a-dia. Protocolos como o Transport Layer Security (TLS) e o Automatic Certificate Management Environment (ACME) permitem comunicações seguras para várias aplicações. O TLS fornece canais seguros com autenticação de pares comunicantes, desde que estes pares já possuam um certificado digital para comprovar sua identidade. Já o protocolo ACME contribui com a adoção de TLS com funcionalidades para envio e gerenciamento de certificados digitais. Tanto o TLS quanto o ACME dependem da Criptografia de Chaves Públicas para autenticação e troca de chaves (Key Exchange - KEX). No entanto, o advento do Computador Quântico Criptograficamente Relevante (CQCR) enfraquece os protocolos de KEX e certificados digitais criados com a criptografia clássica usada atualmente, tais como RSA e Diffie-Hellman. Dada a grande adoção do TLS e ACME, esta ameaça alcança uma escala global. Neste contexto, trata-se de tese dos desafios da adoção da Criptografia Pós-Quântica (CPQ) no TLS e ACME, focando-se na abordagem recomendada chamada de CPQ híbrido (ou modo híbrido). A CPQ é criada usando suposições matemáticas diferentes das em uso atualmente. Essas suposições são viáveis ??para construção de esquemas criptográficos resistentes ao computador quântico, pois não se conhece algoritmo (clássico ou quântico) eficiente. Porém, a transição para CPQ é assunto complexo. No modo híbrido, a transição para CPQ é suavizada, pois ela é combinada com a criptografia tradicional. Assim, esta tese defende uma estratégia de adoção de CPQ pelo modo híbrido com as seguintes contribuições: um estudo secundário classificando e mostrando a eficiência e segurança do modo híbrido; uma ferramenta para verificar as garantias quantum-safe em conexões TLS de usuários; um estudo e uma otimização para a emissão de certificados digitais com CPQ no ACME; o projeto e implementação de uma abordagem híbrida para uma alternativa de TLS chamada KEMTLS; e um conceito híbrido inovador, com implementação, para autenticação usando certificados embrulhados. Na maioria dos cenários de avaliações com modo híbrido propostos neste trabalho, as previsões de desempenho não são significativas quando comparadas com a implantação de CPQ sem o modo híbrido. O conceito inovador da autenticação híbrida também habilitou um plano de contingência para o modo híbrido, contribuindo com a adoção do CPQ. Por meio das propostas e avaliações em diferentes cenários, abordagens e protocolos, esta tese soma esforços em direção ao uso de CPQ híbrido para mitigar os efeitos preocupantes da ameaça quântica à criptografia.Abstract: Network security is essential for today?s communications. Protocols such as Transport Layer Security (TLS) and Automatic Certificate Management Environment (ACME) enable secure communications for various applications. TLS provides secure channels with peer authentication, given that the peer already has a digital certificate to prove its identity. ACME contributes to TLS adoption with facilities for issuing and managing digital certificates. Both protocols depend on Public-Key Cryptography for authentication and Key Exchange (KEX) of symmetric key material. However, the advent of a Cryptographically Relevant Quantum Computer (CRQC) weakens KEX and digital certificates built with today?s classical cryptography (like RSA and Diffie-Hellman). Given the widespread adoption of TLS and ACME, such a threat reaches a global scale. In this context, this thesis aims at the challenges of adopting Post- Quantum Cryptography (PQC) in TLS and ACME, focusing on the recommended approach called Hybrid PQC (or hybrid mode). PQC is created using different mathematical assumptions in which there is no known efficient solution by classical and quantum computers. Hybrids ease the PQC transition by combining it with classical cryptography. This thesis defends the hybrid mode adoption by the following contributions: a secondary study classifying and showing hybrid mode efficiency and security; a tool for users checking their TLS connections for quantum-safe guarantees; a study and an optimized approach for issuance of PQC digital certificates in ACME; a design and implementation of a hybrid approach for the TLS alternative called KEMTLS; and a novel hybrid concept (and implementation) for authentication using wrapped digital certificates. In all proposed hybrid mode evaluations, the penalty in performance was non-significant when compared to PQC-only deployment, except in certain situations. The novel concept for hybrid authentication also allows a contingency plan for hybrids, contributing to the PQC adoption. By proposing and evaluating different scenarios, approaches and protocols, this thesis sums efforts towards using hybrid PQC to mitigate the worrisome effects of the quantum threat to cryptography

Hybrid Signal protocol for post-quantum email encryption

The Signal protocol is used in many messaging applications today. While it is an active research topic to design a post-quantum variant of the protocol, no such variant is currently realized in the real world. In the following document we describe a hybrid version of the Signal protocol, that will be implemented to achieve post-quantum security for Tutanota’s end-to-end encrypted e-mails

Subversion-Resilient Public Key Encryption with Practical Watchdogs

Restoring the security of maliciously implemented cryptosystems has been widely considered challenging due to the fact that the subverted implementation could arbitrarily deviate from the official specification. Achieving security against adversaries that can arbitrarily subvert implementations seems to inherently require trusted component assumptions and/or architectural properties. At ASIACRYPT 2016, Russell et al. proposed an attractive model where a watchdog is used to test and approve individual components of an implementation before or during deployment. Such a detection-based strategy has been useful for designing various cryptographic schemes that are provably resilient to subversion. We consider Russell et al.\u27s watchdog model from a practical perspective regarding watchdog efficiency. We find that the asymptotic definitional framework while permitting strong positive theoretical results, does not yet guarantee practical watchdogs due to the fact that the running time of a watchdog is only bounded by an abstract polynomial. Hence, in the worst case, the running time of the watchdog might exceed the running time of the adversary, which seems impractical for most applications. We adopt Russell et al.\u27s watchdog model to the concrete security setting and design the first subversion-resilient public-key encryption scheme which allows for extremely efficient watchdogs with only linear running time. At the core of our construction is a new variant of a combiner for key encapsulation mechanisms (KEMs) by Giacon et al. (PKC\u2718). We combine this construction with a new subversion-resilient randomness generator that can also be checked by an efficient watchdog, even in constant time, which could be of independent interest for the design of other subversion-resilient cryptographic schemes. Our work thus shows how to apply Russell et al.\u27s watchdog model to design subversion-resilient cryptography with efficient watchdogs. We insist that this work does not intend to show that the watchdog model outperforms other defense approaches but to demonstrate that practical watchdogs are practically achievable. This is the full version of a work published at PKC21. We identify a subtle flaw in the proof of the previous version and show it is impossible to achieve CPA security under subversion with the proposed approach. However, the same construction can achieve one-way security under subversion