A multi-designated verifier signature (MDVS) is a digital signature that empowers a signer to designate specific verifiers capable of verifying signatures. Notably, designated verifiers are allowed to not only verify signatures but also simulate “fake” signatures indistinguishable from real ones produced by the original signer. Since this property is useful for realizing off-the-record (i.e., deniable) communication in group settings, MDVS is attracting attention in secure messaging....

Deniable authentication allows Alice to authenticate a message to Bob, while retaining deniability towards third parties. In particular, not even Bob can convince a third party that Alice authenticated that message. Clearly, in this setting Bob should not be considered trustworthy. Furthermore, deniable authentication is necessary for deniable key exchange, as explicitly desired by Signal and off-the-record (OTR) messaging. In this work we focus on (publicly verifiable) designated...

Cryptography's most common use is secure communication---e.g. Alice can use encryption to hide the contents of the messages she sends to Bob (confidentiality) and can use signatures to assure Bob she sent these messages (authenticity). While one typically considers stateless security guarantees---for example a channel that Alice can use to send messages securely to Bob---one can also consider stateful ones---e.g. an interactive conversation between Alice, Bob and their friends where...

In this paper, we initiate the study of multi-designated detector watermarking (MDDW) for large language models (LLMs). This technique allows model providers to generate watermarked outputs from LLMs with two key properties: (i) only specific, possibly multiple, designated detectors can identify the watermarks, and (ii) there is no perceptible degradation in the output quality for ordinary users. We formalize the security definitions for MDDW and present a framework for constructing MDDW...

Deniable Authentication is a highly desirable property for secure messaging protocols: it allows a sender Alice to authentically transmit messages to a designated receiver Bob in such a way that only Bob gets convinced that Alice indeed sent these messages. In particular, it guarantees that even if Bob tries to convince a (non-designated) party Judy that Alice sent some message, and even if Bob gives Judy his own secret key, Judy will not be convinced: as far as Judy knows, Bob could be...

This paper introduces a new type of public-key encryption scheme, called Multi-Designated Receiver Signed Public Key Encryption (MDRS-PKE), which allows a sender to select a set of designated receivers and both encrypt and sign a message that only these receivers will be able to read and authenticate (confidentiality and authenticity). An MDRS-PKE scheme provides several additional security properties which allow for a fundamentally new type of communication not considered before. Namely, it...

Off-the-Record (OTR) messaging is a two-party message authentication protocol that also provides plausible deniability: there is no record that can later convince a third party what messages were actually sent. To extend OTR to group messaging we need to consider issues that are not present in the 2-party case. In group OTR (as in two-party OTR), the sender should be able to authenticate (or sign) his messages so that group members can verify who sent a message (that is, signatures should be...

Deniable encryption (Canetti et al., Crypto 1996) enhances secret communication over public channels, providing the additional guarantee that the secrecy of communication is protected even if the parties are later coerced (or willingly bribed) to expose their entire internal states: plaintexts, keys and randomness. To date, constructions of deniable encryption --- and more generally, interactive deniable communication --- only address restricted cases where only one party is compromised...

Deniable messaging protocols allow two parties to have 'off-the-record' conversations without leaving any record that can convince external verifiers about what either of them said during the conversation. Recent events like the Podesta email dump underscore the importance of deniable messaging to politicians, whistleblowers, dissidents and many others. Consequently, messaging protocols like Signal and OTR are designed with cryptographic mechanisms to ensure deniable communication,...

We aim to understand, formalize and provably achieve the goals underlying the core key-ratcheting technique of Borisov, Goldberg and Brewer, extensions of which are now used in secure messaging systems. We give syntax and security definitions for ratcheted encryption and key-exchange. We give a proven-secure protocol for ratcheted key exchange. We then show how to generically obtain ratcheted encryption from ratcheted key-exchange and standard encryption.

We reformalize and recast dual-receiver encryption (DRE) proposed in CCS '04, a public-key encryption (PKE) scheme for encrypting to two independent recipients in one shot. We start by defining the crucial soundness property for DRE, which ensures that two recipients will get the same decryption result. While conceptually simple, DRE with soundness turns out to be a powerful primitive for various goals for PKE, such as complete non-malleability (CNM) and plaintext-awareness (PA). We then...