Secure Messaging with Strong Compromise Resilience, Temporal Privacy, and Immediate Decryption

Recent years have seen many advances in designing secure messaging protocols, aiming at provably strong security properties in theory or high efficiency for real-world practical deployment. However, important trade-off areas of the design space inbetween these elements have not yet been explored. In this work we design the first provably secure protocol that at the same time achieves (i) strong resilience against finegrained compromise, (ii) temporal privacy, and (iii) immediate decryption with constant-size overhead, notably, in the postquantum (PQ) setting. Besides these main design goals, we introduce a novel definition of offline deniability suitable for our setting, and prove that our protocol meets it, notably when combined with a PQ offline deniable initial key exchange

Secure Messaging with Strong Compromise Resilience, Temporal Privacy, and Immediate Decryption

Recent years have seen many advances in designing secure messaging protocols, aiming at provably strong security properties in theory or high efficiency for real-world practical deployment. However, important trade-off areas of the design space inbetween these elements have not yet been explored. In this work we design the first provably secure protocol that at the same time achieves (i) strong resilience against fine-grained compromise, (ii) temporal privacy, and (iii) immediate decryption with constant-size overhead, notably, in the post-quantum (PQ) setting. Besides these main design goals, we introduce a novel definition of offline deniability suitable for our setting, and prove that our protocol meets it, notably when combined with a PQ offline deniable initial key exchange

Viscum album extract suppresses cell proliferation and induces apoptosis in bladder cancer cells

Purpose: To evaluate the effect of Viscum album (VA) extract on the progression of bladder cancer (BC) and its effect on the proliferation and apoptosis of T24 and J82 bladder cancer cells. Methods: 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay (MTT assay) was conducted to examine the proliferation of bladder cancer cells. Flow cytometry (FCM) was employed to assess changes in the cell cycle of bladder cancer cells. The expression levels of proliferating cell nuclear antigen (PCNA), CLND1 (cyclin D1), p21, and p27 in control and VA extract-treated (100, 200, or 300 μg/mL) T24 and J82 cells were measured by immunoblot assay. The effects of VA extract on T24 or J82 cell apoptosis were evaluated using FCM. Immunoblot assay was performed to evaluate Bcl2, Bax, and cleaved caspase 3 expression in control or VA extract-treated bladder cancer cells. In addition, the effect of VA extract on Axl-AKT pathways was also evaluated by immunoblot assay. Results: Viscum album extract treatment significantly blocked bladder cancer cell proliferation and induced cell cycle arrest. In addition, VA extract stimulated bladder cancer cell apoptosis. Moreover, this study found that VA extract suppressed Axl-AKT pathways in bladder cancer. Conclusion: Viscum album extract exerts anti-proliferation and pro-apoptosis effects on bladder cancer cells. These abilities render Viscum album extract as promising agent in bladder cancer treatment

Multi-Stage Group Key Distribution and PAKEs: Securing Zoom Groups against Malicious Servers without New Security Elements

Video conferencing apps like Zoom have hundreds of millions of daily users, making them a high-value target for surveillance and subversion. While such apps claim to achieve some forms of end-to-end encryption, they usually assume an incorruptible server that is able to identify and authenticate all the parties in a meeting. Concretely this means that, e.g., even when using the “end-to-end encrypted” setting, malicious Zoom servers could eavesdrop or impersonate in arbitrary groups. In this work, we show how security against malicious servers can be improved by changing the way in which such protocols use passwords (known as passcodes in Zoom) and integrating a password-authenticated key exchange (PAKE) protocol. To formally prove that our approach achieves its goals, we formalize a class of cryptographic protocols suitable for this setting, and define a basic security notion for them, in which group security can be achieved assuming the server is trusted to correctly authorize the group members. We prove that Zoom indeed meets this notion. We then propose a stronger security notion that can provide security against malicious servers, and propose a transformation that can achieve this notion. We show how we can apply our transformation to Zoom to provably achieve stronger security against malicious servers, notably without introducing new security elements

Multi-Stage Group Key Distribution and PAKEs: Securing Zoom Groups against Malicious Servers without New Security Elements

Video conferencing apps like Zoom have hundreds of millions of daily users, making them a high-value target for surveillance and subversion. While such apps claim to achieve some forms of end-to-end encryption, they usually assume an incorruptible server that is able to identify and authenticate all the parties in a meeting. Concretely this means that, e.g., even when using the “end-to-end encrypted” setting, malicious Zoom servers could eavesdrop or impersonate in arbitrary groups. In this work, we show how security against malicious servers can be improved by changing the way in which such protocols use passwords (known as passcodes in Zoom) and integrating a password-authenticated key exchange (PAKE) protocol. To formally prove that our approach achieves its goals, we formalize a class of cryptographic protocols suitable for this setting, and define a basic security notion for them, in which group security can be achieved assuming the server is trusted to correctly authorize the group members. We prove that Zoom indeed meets this notion. We then propose a stronger security notion that can provide security against malicious servers, and propose a transformation that can achieve this notion. We show how we can apply our transformation to Zoom to provably achieve stronger security against malicious servers, notably without introducing new security elements

FIDO2, CTAP 2.1, and WebAuthn 2: Provable Security and Post-Quantum Instantiation

The FIDO2 protocol is a globally used standard for passwordless authentication, building on an alliance between major players in the online authentication space. While already widely deployed, the standard is still under active development. Since version 2.1 of its CTAP sub-protocol, FIDO2 can potentially be instantiated with post-quantum secure primitives. We provide the first formal security analysis of FIDO2 with the CTAP 2.1 and WebAuthn 2 sub-protocols. Our security models build on work by Barbosa et al. for their analysis of FIDO2 with CTAP 2.0 and WebAuthn 1, which we extend in several ways. First, we provide a more fine-grained security model that allows us to prove more relevant protocol properties, such as guarantees about token binding agreement, the None attestation mode, and user verification. Second, we can prove post-quantum security for FIDO2 under certain conditions and minor protocol extensions. Finally, we show that for some threat models, the downgrade resilience of FIDO2 can be improved, and show how to achieve this with a simple modification

Automated Analysis of Protocols that use Authenticated Encryption: How Subtle AEAD Differences can impact Protocol Security

Many modern security protocols such as TLS, WPA2, WireGuard, and Signal use a cryptographic primitive called Authenticated Encryption (optionally with Authenticated Data), also known as an AEAD scheme. AEAD is a variant of symmetric encryption that additionally provides authentication. While authentication may seem to be a straightforward additional requirement, it has in fact turned out to be complex: many different security notions for AEADs are still being proposed, and several recent protocol-level attacks exploit subtle behaviors that differ among real-world AEAD schemes. We provide the first automated analysis method for protocols that use AEADs that can systematically find attacks that exploit the subtleties of the specific type of AEAD used. This can then be used to analyze specific protocols with a fixed AEAD choice, or to provide guidance on which AEADs might be (in)sufficient to make a protocol design secure. We develop generic symbolic AEAD models, which we instantiate for the Tamarin prover. Our approach can automatically and efficiently discover protocol attacks that could previously only be found using manual inspection, such as the Salamander attack on Facebook’s message franking, and attacks on SFrame and YubiHSM. Furthermore, our analysis reveals undesirable behaviors of several other protocols

Automated Analysis of Protocols that use Authenticated Encryption: How Subtle AEAD Differences can impact Protocol Security

Many modern security protocols such as TLS, WPA2, WireGuard, and Signal use a cryptographic primitive called Authenticated Encryption (optionally with Authenticated Data), also known as an AEAD scheme. AEAD is a variant of symmetric encryption that additionally provides authentication. While authentication may seem to be a straightforward additional requirement, it has in fact turned out to be complex: many different security notions for AEADs are still being proposed, and several recent protocol-level attacks exploit subtle behaviors that differ among real-world AEAD schemes. We provide the first automated analysis method for protocols that use AEADs that can systematically find attacks that exploit the subtleties of the specific type of AEAD used. This can then be used to analyze specific protocols with a fixed AEAD choice, or to provide guidance on which AEADs might be (in)sufficient to make a protocol design secure. We develop generic symbolic AEAD models, which we instantiate for the Tamarin prover. Our approach can automatically and efficiently discover protocol attacks that could previously only be found using manual inspection, such as the Salamander attack on Facebook’s message franking, and attacks on SFrame and YubiHSM. Furthermore, our analysis reveals undesirable behaviors of several other protocols

The Provable Security of Ed25519: Theory and Practice

A standard requirement for a signature scheme is that it is existentially unforgeable under chosen message attacks (EUF-CMA), alongside other properties of interest such as strong unforgeability (SUF-CMA), and resilience against key substitution attacks. Remarkably, no detailed proofs have ever been given for these security properties for EdDSA, and in particular its Ed25519 instantiations. Ed25519 is one of the most efficient and widely used signature schemes, and different instantiations of Ed25519 are used in protocols such as TLS 1.3, SSH, Tor, ZCash, and WhatsApp/Signal. The differences between these instantiations are subtle, and only supported by informal arguments, with many works assuming results can be directly transferred from Schnorr signatures. Similarly, several proofs of protocol security simply assume that Ed25519 satisfies properties such as EUF-CMA or SUF-CMA. In this work we provide the first detailed analysis and security proofs of Ed25519 signature schemes. While the design of the schemes follows the well-established Fiat-Shamir paradigm, which should guarantee existential unforgeability, there are many side cases and encoding details that complicate the proofs, and all other security properties needed to be proven independently. Our work provides scientific rationale for choosing among several Ed25519 variants and understanding their properties, fills a much needed proof gap in modern protocol proofs that use these signatures, and supports further standardisation efforts