miTLS: Verifying Protocol Implementations against Real-World Attacks

International audienceThe TLS Internet Standard, previously known as SSL, is the default protocol for encrypting communications between clients and servers on the Web. Hence, TLS routinely protects our sensitive emails, health records, and payment information against network-based eavesdropping and tampering. For the past 20 years, TLS security has been analyzed in various cryptographic and programming models to establish strong formal guarantees for various protocol configurations. However, TLS deployments are still often vulnerable to attacks and rely on security experts to fix the protocol implementations. The miTLS project intends to solve this apparent contradiction between published proofs and real-world attacks, which reveals a gap between TLS theory and practice. To this end, the authors developed a verified reference implementation and a cryptographic security proof that account for the protocol's low-level details. The resulting formal development sheds light on recent attacks, yields security guarantees for typical TLS usages, and informs the design of the protocol's next version

A verification framework for secure machine learning

International audienceWe propose a programming and verification framework to help developers build distributed software applications using composite homomorphic encryption (and secure multi-party computation) protocols, and implement secure machine learning and classification over private data. With our framework, a developer can prove that the application code is functionally correct, that it correctly composes the various cryptographic schemes it uses, and that it does not accidentally leak any secrets (via side-channels, for example.) Our end-to-end solution results in verified and efficient implementations of state-of-the-art secure privacy-preserving learning and classification techniques

Practical Formal Methods for Real World Cryptography

International audienceCryptographic algorithms, protocols, and applications are difficult to implement correctly, and errors and vulnerabilities in their code can remain undiscovered for long periods before they are exploited. Even highly-regarded cryptographic libraries suffer from bugs like buffer overruns, incorrect numerical computations, and timing side-channels, which can lead to the exposure of sensitive data and longterm secrets. We describe a tool chain and framework based on the the F * programming language to formally specify, verify and compile high-performance cryptographic software that is secure by design. This tool chain has been used to build a verified cryptographic library called HACL * , and provably secure implementations of sophisticated secure communication protocols like TLS and Signal. We describe these case studies and conclude with ongoing work on using our framework to build verified implementations of privacy preserving machine learning systems

On the Practical (In-)Security of 64-bit Block Ciphers: Collision Attacks on HTTP over TLS and OpenVPN

International audienceWhile modern block ciphers, such as AES, have a block size of at least 128 bits, there are many 64-bit block ciphers, such as 3DES and Blowfish, that are still widely supported in Internet security protocols such as TLS, SSH, and IPsec. When used in CBC mode, these ciphers are known to be susceptible to collision attacks when they are used to encrypt around 2^32 blocks of data (the so-called birthday bound). This threat has traditionally been dismissed as impractical since it requires some prior knowledge of the plaintext and even then, it only leaks a few secret bits per gigabyte. Indeed, practical collision attacks have never been demonstrated against any mainstream security protocol, leading to the continued use of 64-bit ciphers on the Internet. In this work, we demonstrate two concrete attacks that exploit collisions on short block ciphers. First, we present an attack on the use of 3DES in HTTPS that can be used to recover a secret session cookie. Second, we show how a similar attack on Blowfish can be used to recover HTTP BasicAuth credentials sent over OpenVPN connections. In our proof-of-concept demos, the attacker needs to capture about 785GB of data, which takes between 19-38 hours in our setting. This complexity is comparable to the recent RC4 attacks on TLS: the only fully implemented attack takes 75 hours. We evaluate the impact of our attacks by measuring the use of 64-bit block ciphers in real-world protocols. We discuss mitigations, such as disabling all 64-bit block ciphers, and report on the response of various software vendors to our responsible disclosure of these attacks

On the Practical (In-)Security of 64-bit Block Ciphers: Collision Attacks on HTTP over TLS and OpenVPN

While modern block ciphers, such as AES, have a block size of at least 128 bits, there are many 64-bit block ciphers, such as 3DES and Blowfish, that are still widely supported in Internet security protocols such as TLS, SSH, and IPsec. When used in CBC mode, these ciphers are known to be susceptible to collision attacks when they are used to encrypt around $2^{32}$ blocks of data (the so-called birthday bound). This threat has traditionally been dismissed as impractical since it requires some prior knowledge of the plaintext and even then, it only leaks a few secret bits per gigabyte. Indeed, practical collision attacks have never been demonstrated against any mainstream security protocol, leading to the continued use of 64-bit ciphers on the Internet. In this work, we demonstrate two concrete attacks that exploit collisions on short block ciphers. First, we present an attack on the use of 3DES in HTTPS that can be used to recover a secret session cookie. Second, we show how a similar attack on Blowfish can be used to recover HTTP BasicAuth credentials sent over OpenVPN connections. In our proof-of-concept demos, the attacker needs to capture about 785GB of data, which takes between 19-38 hours in our setting. This complexity is comparable to the recent RC4 attacks on TLS: the only fully implemented attack takes 75 hours. We evaluate the impact of our attacks by measuring the use of 64-bit block ciphers in real-world protocols. We discuss mitigations, such as disabling all 64-bit block ciphers, and report on the response of various software vendors to our responsible disclosure of these attacks

Web-based Attacks on Host-Proof Encrypted Storage

International audienceCloud-based storage services, such as Wuala, and pass- word managers, such as LastPass, are examples of so- called host-proof web applications that aim to protect users from attacks on the servers that host their data. To this end, user data is encrypted on the client and the server is used only as a backup data store. Authorized users may access their data through client-side software, but for ease of use, many commercial applications also offer browser-based interfaces that enable features such as remote access, form-filling, and secure sharing.We describe a series of web-based attacks on popular host-proof applications that completely circumvent their cryptographic protections. Our attacks exploit standard web application vulnerabilities to expose flaws in the encryption mechanisms, authorization policies, and key management implemented by these applications. Our analysis suggests that host-proofing by itself is not enough to protect users from web attackers, who will simply shift their focus to flaws in client-side interfaces

Une preuve cryptographique mécanisée du protocole de réseau privé virtuel WireGuard

WireGuard is a free and open source Virtual Private Network (VPN) that aims to replace IPsec and OpenVPN. It is based on a new cryptographic protocol derived from the Noise Protocol Framework. This paper presents the first mechanised cryptographic proof of the protocolunderlying WireGuard, using the CryptoVerif proof assistant. We analyse the entire WireGuard protocol as it is, including transport data messages, in an ACCE-style model. We contribute proofs for correctness, message secrecy, forward secrecy, mutual authentication, session uniqueness, and resistance against key compromise impersonation, identity mis-binding, and replay attacks. We also discuss the strength of the identity hiding provided by WireGuard. Our work also provides novel theoretical contributions that are reusable beyond WireGuard. First, we extend CryptoVerif to account for the absence of public key validation in popular Diffie-Hellman groups like Curve25519, which is used in many modern protocols including WireGuard. To our knowledge, this is the first mechanised cryptographic proof for any protocol employing such a precise model. Second, we prove several indifferentiability lemmas that are useful to simplify the proofs for sequences of key derivations.WireGuard est un logiciel de réseau privé virtuel (VPN) gratuit et open source qui cherche à remplacer IPsec et OpenVPN. Il est fondé sur un nouveau protocole cryptographique dérivé de la famille de protocoles Noise. Ce document présente la première preuve cryptographique mécanisée du protocole de WireGuard, obtenue avec l’assistant de preuve CryptoVerif. Nous analysons le protocole WireGuard en entier, tel qu’il est, y compris les messages de transport de données, dans un modèle du style ACCE. Nous obtenons des preuves de correction, secret des messages, forward secrecy, authentification mutuelle, unicité des sessions, et résistance contre des attaques d’imposture par compromis de clés, de mauvaise liaison d’identités, et de rejeu. Nous discutons également la robustesse de la protection des identités fournie par WireGuard. Notre travail fournit aussi de nouvelles contributions théoriques, réutilisables au-delà de WireGuard. Premièrement, nous étendons CryptoVerif pour tenir compte de l’absence devalidation des clés publiques dans des groupes Diffie-Hellman populaires comme Curve25519, qui est utilisé dans beaucoup de protocoles modernes dont WireGuard. À notre connaissance, c’est la première preuve cryptographique mécanisée qui utilise un modèle aussi précis. Deuxièmement, nous prouvons plusieurs lemmes d’indifférentiabilité qui sont utiles pour simplifier les preuves de suites de dérivations de clés

Modèles vérifiés et implémentations de référence pour le candidat standard TLS 1.3

TLS 1.3 is the next version of the Transport Layer Security (TLS) protocol. Its clean-slate design is a reaction both to the increasing demand for low-latency HTTPS connections and to a series of recent high-profile attacks on TLS. The hope is that a fresh protocol with modern cryptography will prevent legacy problems; the danger is that it will expose new kinds of attacks, or reintroduce old flaws that were fixed in previous versions of TLS. After 18 drafts, the protocol is nearing completion, and the working group has appealed to researchers to analyze the protocol before publication. This paper responds by presenting a comprehensive analysis of the TLS 1.3 Draft-18 protocol.We seek to answer three questions that have not been fully addressed in previous work on TLS 1.3: (1) Does TLS 1.3 prevent well-known attacks on TLS 1.2, such as Logjam or the Triple Handshake, even if it is run in parallel with TLS 1.2? (2) Can we mechanically verify the computational security of TLS 1.3 under standard (strong) assumptions on its cryptographic primitives? (3) How can we extend the guarantees of the TLS 1.3 protocol to the details of its implementations?To answer these questions, we propose a methodology for developing verified symbolic and computational models of TLS 1.3 hand-in-hand with a high-assurance reference implementation of the protocol. We present symbolic ProVerif models for various intermediate versions of TLS 1.3 and evaluate them against a rich class of attacks to reconstruct both known and previously unpublished vulnerabilities that influenced the current design of the protocol. We present a computational CryptoVerif model for TLS 1.3 Draft-18 and prove its security. We present RefTLS, an interoperable implementation of TLS 1.0-1.3 and automatically analyze its protocol core by extracting a ProVerif model from its typed JavaScript code.TLS 1.3 est la prochaine version du protocole TLS (Transport Layer Security). Sa conception à partir de zéro est une réaction à la fois à la demande croissante de connexions HTTPS à faible latence et à une série d'attaques récentes de haut niveau sur TLS. L'espoir est qu'un nouveau protocole avec de la cryptographie moderne éviterait d'hériter des problèmes des versions précédentes; le danger est que cela pourrait exposer à de nouveaux types d'attaques ou réintroduire d'anciens défauts corrigés dans les versions précédentes de TLS. Après 18 versions préliminaires, le protocole est presque terminé, et le groupe de travail a appelé les chercheurs à analyser le protocole avant publication. Cet article répond en présentant une analyse globale du protocole TLS 1.3 Draft-18.Nous cherchons à répondre à trois questions qui n'ont pas été entièrement traitées dans les travaux antérieurs sur TLS 1.3: (1) TLS 1.3 empêche-t-il les attaques connues sur TLS 1.2, comme Logjam ou Triple Handshake, même s'il est exécuté en parallèle avec TLS 1.2 ? (2) Peut-on vérifier mécaniquement la sécurité calculatoire de TLS 1.3 sous des hypothèses standard (fortes) sur ses primitives cryptographiques? (3) Comment pouvons-nous étendre les garanties du protocole TLS 1.3 aux détails de ses implémentations?Pour répondre à ces questions, nous proposons une méthodologie pour développer des modèles symboliques et calculatoires vérifiés de TLS 1.3 en même temps qu'une implémentation de référence du protocole. Nous présentons des modèles symboliques dans ProVerif pour différentes versions intermédiaires de TLS 1.3 et nous les évaluons contre une riche classe d'attaques, pour reconstituer à la fois des vulnérabilités connues et des vulnérabilités précédemment non publiées qui ont influencé la conception actuelle du protocole. Nous présentons un modèle calculatoire dans CryptoVerif de TLS 1.3 Draft-18 et prouvons sa sécurité. Nous présentons RefTLS, une implémentation interopérable de TLS 1.0-1.3 et analysons automatiquement le coeur de son protocole en extrayant un modèle ProVerif à partir de son code JavaScript typé

A Mechanised Cryptographic Proof of the WireGuard Virtual Private Network Protocol

International audienceWireGuard is a free and open source Virtual Private Network (VPN) that aims to replace IPsec and OpenVPN. It is based on a new cryptographic protocol derived from the Noise Protocol Framework. This paper presents the first mechanised cryptographic proof of the protocol underlying WireGuard, using the CryptoVerif proof assistant. We analyse the entire WireGuard protocol as it is, including transport data messages, in an ACCE-style model. We contribute proofs for correctness, message secrecy, forward secrecy, mutual authentication, session uniqueness, and resistance against key compromise impersonation, identity mis-binding, and replay attacks. We also discuss the strength of the identity hiding provided by WireGuard. Our work also provides novel theoretical contributions that are reusable beyond WireGuard. First, we extend CryptoVerif to account for the absence of public key validation in popular Diffie-Hellman groups like Curve25519, which is used in many modern protocols including WireGuard. To our knowledge, this is the first mechanised cryptographic proof for any protocol employing such a precise model. Second, we prove several indifferentiability lemmas that are useful to simplify the proofs for sequences of key derivations

Noise Explorer: Fully Automated Modeling and Verification for Arbitrary Noise Protocols

International audienceThe Noise Protocol Framework, introduced recently, allows for the design and construction of secure channel protocols by describing them through a simple, restricted language from which complex key derivation and local state transitions are automatically inferred. Noise "Handshake Patterns" can support mutual authentication, forward secrecy, zero round-trip encryption, identity hiding and other advanced features. Since the framework's release, Noise-based protocols have been adopted by WhatsApp, WireGuard and other high-profile applications.We present Noise Explorer, an online engine for designing, reasoning about, formally verifying and implementing arbitrary Noise Handshake Patterns. Based on our formal treatment of the Noise Protocol Framework, Noise Explorer can validate any Noise Handshake Pattern and then translate it into a model ready for automated verification and also into a production-ready software implementation written in Go or in Rust. We use Noise Explorer to analyze more than 57 handshake patterns. We confirm the stated security goals for 12 fundamental patterns and provide precise properties for the rest. We also analyze unsafe handshake patterns and document weaknesses that occur when validity rules are not followed. All of this work is consolidated into a usable online tool that presents a compendium of results and can parse formal verification results to generate detailed-but-pedagogical reports regarding the exact security goals of each message of a Noise Handshake Pattern with respect to each party, under an active attacker and including malicious principals. Noise Explorer evolves alongside the standard Noise Protocol Framework, having already contributed new security goal verification results and stronger definitions for pattern validation and security parameters