Instant messaging applications like WhatsApp and iMessage have been widely used to exchange public and private information for both individuals and organizations. At the same time, the asynchronicity nature of these applications also introduces various security issues. To this end, considerable efforts have been made to strengthen the security and privacy of these applications, where a major goal is to capture the forward secrecy of asynchronous messaging without sacrificing other reasonable security properties. In this paper, we mainly focus on forward-secure zero round-trip time (0-RTT) key exchange protocols. Such a protocol enables a user to send the encrypted application data along with the first protocol message, and thus is especially suitable for securing asynchronous messaging. Concretely, we introduce a new cryptographic primitive dubbed as puncturable identity-based matchmaking key encapsulation mechanism (PIB-MKEM). It captures the authenticity of ciphertexts, and also allo
Introduced by von Ahn et al. (STOC’05), covert two-party computation is an appealing cryptographic primitive that allows Alice and Bob to securely evaluate a function on their secret inputs in a steganographic manner, i.e., even the existence of a computation is oblivious to each party - unless the output of the function is favourable to both. A prominent form of covert computation is covert authentication, where Alice and Bob want to authenticate each other based on their credentials, in a way such that the party who does not hold the appropriate credentials cannot pass the authentication and is even unable to distinguish a protocol instance from random noise. Jarecki (PKC’14) put forward a blueprint for designing covert authentication protocols, relying on a covert conditional key-encapsulation mechanism, an identity escrow scheme, a covert commitment scheme and a Σ -protocol satisfying several specific properties. He also proposed an instantiation based on the Strong RSA, the D
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