146 research outputs found
Switching and Extension of a [c2]Daisy-Chain Dimer Polymer
We report the synthesis of a [c2]daisy-chain dimer via ruthenium-catalyzed ring-closing olefin metathesis. Confirmation of the interlocked nature of the structure was achieved through single-crystal X-ray diffraction analysis. The dimer could be readily switched from the bound to the unbound conformation by treatment with 3.0 equiv of KOH and subsequently reprotonated by treatment with 3.0 equiv of HPF_6. Azide functionalization of the dimer enabled incorporation in linear step-growth polymer chains using the alkyne-azide “click” reaction. Gel permeation chromatography coupled with multiangle laser light scattering analysis showed the polymers contained 22 dimers and had a radius of gyration of 14.8 nm. Acylation of the amines of the dimers sterically forced elongation of the interlocked units, and MALLS analysis of the polymer showed a 48% increase in the R_g (21.4 nm)
Synthesis of a Molecular Charm Bracelet via Click Cyclization and Olefin Metathesis Clipping
We describe the synthesis of a polycatenated cyclic polymer, a structure that resembles a
molecular charm bracelet. Ruthenium-catalyzed ring-opening metathesis polymerization of an aminocontaining
cyclic olefin monomer in the presence of a chain transfer agent generated an α,ω-diazide
functionalized polyamine. Cyclization of the resulting linear polyamine using pseudo-high-dilution coppercatalyzed
click cyclization produced a cyclic polymer in 19% yield. The click reaction was then further
employed to remove linear contaminants from the cyclic polymer using azide- and alkyne-functionalized
scavenging resins, and the purified cyclic polymer product was characterized by gel permeation
chromatography, ^1H NMR spectroscopy, and IR spectroscopy. Polymer hydrogenation and conversion to
the corresponding polyammonium species enabled coordination and interlocking of diolefin polyether
fragments around the cyclic polymer backbone using ruthenium-catalyzed ring-closing olefin metathesis to
afford a molecular charm bracelet structure. This charm bracelet complex was characterized by ^1H NMR
spectroscopy, and the catenated nature of the small rings was confirmed using two-dimensional diffusion-ordered
NMR spectroscopy
Spartan and Bulletproofs are simulation-extractable (for free!)
Increasing deployment of advanced zero-knowledge proof systems, especially zkSNARKs, has raised critical questions about their security against real-world attacks. Two classes of attacks of concern in practice are adaptive soundness attacks, where an attacker can prove false statements by choosing its public input after generating a proof, and malleability attacks, where an attacker can use a valid proof to create another valid proof it could not have created itself. Prior work has shown that simulation-extractability (SIM-EXT), a strong notion of security for proof systems, rules out these attacks.
In this paper, we prove that two transparent, discrete-log-based zkSNARKs, Spartan and Bulletproofs, are simulation-extractable (SIM-EXT) in the random oracle model if the discrete logarithm assumption holds in the underlying group. Since these assumptions are required to prove standard security properties for Spartan and Bulletproofs, our results show that SIM-EXT is, surprisingly, for free with these schemes. Our result is the first SIM-EXT proof for Spartan and encompasses both linear- and sublinear-verifier variants. Our result for Bulletproofs encompasses both the aggregate range proof and arithmetic circuit variants, and is the first to not rely on the algebraic group model (AGM), resolving an open question posed by Ganesh et al. (EUROCRYPT \u2722). As part of our analysis, we develop a generalization of the tree-builder extraction theorem of Attema et al. (TCC \u2722), which may be of independent interest
Snapshot-Oblivious RAMs: Sub-Logarithmic Efficiency for Short Transcripts
Oblivious RAM (ORAM) is a powerful technique to prevent harmful data breaches. Despite tremendous progress in improving the concrete performance of ORAM, it remains too slow for use in many practical settings; recent breakthroughs in lower bounds indicate this inefficiency is inherent for ORAM and even some natural relaxations.
This work introduces snapshot-oblivious RAMs, a new secure memory access primitive. Snapshot-oblivious RAMs bypass lower bounds by providing security only for transcripts whose length (call it c) is fixed and known ahead of time. Intuitively, snapshot-oblivious RAMs provide strong security for attacks of short duration, such as the snapshot attacks targeted by many encrypted databases.
We give an ORAM-style definition of this new primitive, and present several constructions. The underlying design principle of our constructions is to store the history of recent operations in a data structure that can be accessed obliviously. We instantiate this paradigm with data structures that remain on the client, giving a snapshot-oblivious RAM with constant bandwidth overhead. We also show how these data structures can be stored on the server and accessed using oblivious memory primitives. Our most efficient instantiation achieves O(log c) bandwidth overhead. By extending recent ORAM lower bounds, we show this performance is asymptotically optimal. Along the way, we define a new hash queue data structure—essentially, a dictionary whose elements can be modified in a first-in-first-out fashion—which may be of independent interest
Interoperability in End-to-End Encrypted Messaging
The Digital Markets Act (DMA) is a nascent European Union regulation adopted in May 2022. One of its most controversial provisions is a requirement that so-called “gatekeepers” offering end-to-end encrypted messaging apps, such as WhatsApp, implement “interoperability” with other messaging apps: in essence, encrypted messaging across service providers. This requirement represents a fundamental shift in the design assumptions of existing encrypted messaging systems, most of which are designed to be centralized. Technologists have not really begun thinking about the myriad security, privacy, and functionality questions raised by the interoperability requirement; given that the DMA’s interoperability mandate may take effect as soon as mid-2024, it is critical for researchers to begin understanding the challenges and offering solutions.
In this paper, we take an initial step in this direction. We break down the DMA’s effects on the design of encrypted messaging systems into three main areas: identity, or how to resolve identities across service providers; protocols, or how to establish a secure connection between clients on different platforms; and abuse prevention, or how service providers can detect and take action against users engaging in abuse or spam. For each area, we identify key security and privacy requirements, summarize existing proposals, and examine whether proposals meet our security and privacy requirements. Finally, we propose our own design for an interoperable encrypted messaging system, and point out open problems
Partitioning Oracle Attacks
In this paper we introduce partitioning oracles, a new class of decryption error oracles which, conceptually, take a ciphertext as input and output whether the decryption key belongs to some known subset of keys. We introduce the first partitioning oracles which arise when encryption schemes are not committing with respect to their keys. We detail novel adaptive chosen ciphertext attacks that exploit partitioning oracles to efficiently recover passwords and de-anonymize anonymous communications. The attacks utilize efficient key multi-collision algorithms --- a cryptanalytic goal that we define --- against widely used authenticated encryption with associated data (AEAD) schemes, including AES-GCM, XSalsa20/Poly1305, and ChaCha20/Poly1305.
We build a practical partitioning oracle attack that quickly recovers passwords from Shadowsocks proxy servers. We also survey early implementations of the OPAQUE protocol for password-based key exchange, and show how many could be vulnerable to partitioning oracle attacks due to incorrectly using non-committing AEAD. Our results suggest that the community should standardize and make widely available committing AEAD to avoid such vulnerabilities
Modifying an Enciphering Scheme after Deployment
Assume that a symmetric encryption scheme has been deployed and used with a secret key. We later must change the encryption scheme in a way that preserves the ability to decrypt (a subset of) previously encrypted plaintexts. Frequent real-world examples are migrating from a token-based encryption system for
credit-card numbers to a format-preserving encryption (FPE) scheme, or extending the message space of an already deployed FPE. The ciphertexts may be stored in systems for which it is not easy or not efficient to retrieve them (to re-encrypt the plaintext under the new scheme). We introduce methods for functionality-preserving modifications to encryption, focusing particularly on deterministic, length-preserving ciphers such as those used to perform format-preserving encryption. We provide a new technique, that we refer to as the Zig-Zag construction, that allows one to combine two ciphers using different domains in a way that results in a secure cipher on one domain. We explore its use in the two settings above, replacing token-based systems and extending message spaces. We develop appropriate security goals and prove security relative to them assuming the underlying ciphers are themselves secure as strong pseudorandom permutations
Message Franking via Committing Authenticated Encryption
We initiate the study of message franking, recently introduced in Facebook’s end-to-end encrypted message system. It targets verifiable reporting of abusive messages to Facebook without compromising security guarantees. We capture the goals of message franking via a new
cryptographic primitive: compactly committing authenticated encryption with associated data (AEAD). This is an AEAD scheme for which a small part of the ciphertext can be used as a cryptographic commitment to the message contents. Decryption provides, in addition to the message, a value that can be used to open the commitment. Security for franking mandates more than that required of traditional notions associated with commitment. Nevertheless, and despite the fact that AEAD schemes are in general not committing (compactly or otherwise), we prove that many in-use AEAD schemes can be used for message franking by using secret keys
as openings. An implication of our results is the first proofs that several in-use symmetric encryption schemes are committing in the traditional sense. We also propose and analyze schemes that retain security even after openings are revealed to an adversary. One is a generalization of the scheme implicitly underlying Facebook’s message franking protocol, and another is a new construction that offers improved performance
Authenticated Encryption with Key Identification
Authenticated encryption with associated data (AEAD) forms the core of much of symmetric cryptography, yet the standard techniques for modeling AEAD assume recipients have no ambiguity about what secret key to use for decryption. This is divorced from what occurs in practice, such as in key management services, where a message recipient can store numerous keys and must identify the correct key before decrypting. To date there has been no formal investigation of their security properties or efficacy, and the ad hoc solutions for identifying the intended key deployed in practice can be inefficient and, in some cases, vulnerable to practical attacks.
We provide the first formalization of nonce-based AEAD that supports key identification (AEAD-KI). Decryption now takes in a vector of secret keys and a ciphertext and must both identify the correct secret key and decrypt the ciphertext. We provide new formal security definitions, including new key robustness definitions and indistinguishability security notions. Finally, we show several different approaches for AEAD-KI and prove their security
ClaimChain: Improving the Security and Privacy of In-band Key Distribution for Messaging
The social demand for email end-to-end encryption is barely supported by
mainstream service providers. Autocrypt is a new community-driven open
specification for e-mail encryption that attempts to respond to this demand. In
Autocrypt the encryption keys are attached directly to messages, and thus the
encryption can be implemented by email clients without any collaboration of the
providers. The decentralized nature of this in-band key distribution, however,
makes it prone to man-in-the-middle attacks and can leak the social graph of
users. To address this problem we introduce ClaimChain, a cryptographic
construction for privacy-preserving authentication of public keys. Users store
claims about their identities and keys, as well as their beliefs about others,
in ClaimChains. These chains form authenticated decentralized repositories that
enable users to prove the authenticity of both their keys and the keys of their
contacts. ClaimChains are encrypted, and therefore protect the stored
information, such as keys and contact identities, from prying eyes. At the same
time, ClaimChain implements mechanisms to provide strong non-equivocation
properties, discouraging malicious actors from distributing conflicting or
inauthentic claims. We implemented ClaimChain and we show that it offers
reasonable performance, low overhead, and authenticity guarantees.Comment: Appears in 2018 Workshop on Privacy in the Electronic Society
(WPES'18
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