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Lower and Upper Bounds for Deniable Public-Key Encryption.

Rikke Bendlin, Jesper Buus Nielsen, Peter Sebastian Nordholt, Claudio Orlandi

Published Jan 1, 2011• 0 citations
OpenAlex

Abstract

Abstract. A deniable cryptosystem allows a sender and a receiver to communicate over an insecure channel in such a way that the communication is still secure even if the adversary can threaten the parties into revealing their internal states after the execution of the protocol. This is done by allowing the parties to change their internal state to make it look like a given ciphertext decrypts to a message di erent from what it really decrypts to. Deniable encryption was in this way introduced to allow to deny a message exchange and hence combat coercion. Depending on which parties can be coerced, the security level, the avor and the number of rounds of the cryptosystem, it is possible to de ne a number of notions of deniable encryption. In this paper we prove that there does not exist any non-interactive receiver-deniable cryptosystem with better than polynomial security. This also shows that it is impossible to construct a non-interactive bi-deniable public-key encryption scheme with better than polynomial security. Speci cally, we give an explicit bound relating the security of the scheme to how e cient the scheme is in terms of key size. Our impossibility result establishes a lower bound on the security. As a nal contribution we give constructions of deniable public-key encryption schemes which establishes upper bounds on the security in terms of key length. There is a gap between our lower and upper bounds, which leaves the interesting open problem of nding the tight bounds. 1

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Abstract. The primitive of deniable encryption was first introduced by Canetti et al. (CRYPTO, 1997). Deniable encryption is a regular public key encryption scheme with the added feature that after running the protocol honestly and transmitting a message m, both Sender and Receiver may produce random coins showing that the transmitted ciphertext was an encryption of any message m ′ in the message space. Deniable encryption is a key tool for constructing incoercible protocols, since it allows a party to send one message and later provide apparent evidence to a coercer that a different message was sent. In addition, deniable encryption may be used to obtain adaptively-secure multiparty computation (MPC) protocols and is secure under selective-opening attacks. Different flavors such as sender-deniable and receiver-deniable encryption, where only the Sender or Receiver can produce fake random coins, have been considered. Recently, several open questions regarding the feasibility of deniable encryption have been resolved (c.f. (O’Neill et al., CRYPTO, 2011), (Bendlin et al., ASIACRYPT, 2011)). A fundamental remaining open question is whether it is possible to construct sender-deniable Encryption Schemes with super-polynomial security, where an adversary has negligible advantage in distinguishing real and fake openings. The primitive of simulatable public key encryption (PKE), introduced by Damg˚ard and Nielsen (CRYPTO, 2000), is a public key encryption scheme with additional properties that allow oblivious sampling of public keys and

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•Jan 1, 2013•0 citations
Abstract. A deniable cryptosystem allows a sender and a receiver to communicate over an insecure channel in such a way that the communication is still secure even if the adversary can threaten the parties into revealing their internal states after the execution of the protocol. This is done by allowing the parties to change their internal state to make it look like a given ciphertext decrypts to a message di erent from what it really decrypts to. Deniable encryption was in this way introduced to allow to deny a message exchange and hence combat coercion. Depending on which parties can be coerced, the security level, the avor and the number of rounds of the cryptosystem, it is possible to de ne a number of notions of deniable encryption. In this paper we prove that there does not exist any non-interactive receiver-deniable cryptosystem with better than polynomial security. This also shows that it is impossible to construct a non-interactive bi-deniable public-key encryption scheme with better than polynomial security. Speci cally, we give an explicit bound relating the security of the scheme to how e cient the scheme is in terms of key size. Our impossibility result establishes a lower bound on the security. As a nal contribution we give constructions of deniable public-key encryption schemes which establishes upper bounds on the security in terms of key length. There is a gap between our lower and upper bounds, which leaves the interesting open problem of nding the tight bounds. 1
Abstract. The primitive of deniable encryption was first introduced by Canetti et al. (CRYPTO, 1997). Deniable encryption is a regular public key encryption scheme with the added feature that after running the protocol honestly and transmitting a message m, both Sender and Receiver may produce random coins showing that the transmitted ciphertext was an encryption of any message m ′ in the message space. Deniable encryption is a key tool for constructing incoercible protocols, since it allows a party to send one message and later provide apparent evidence to a coercer that a different message was sent. In addition, deniable encryption may be used to obtain adaptively-secure multiparty computation (MPC) protocols and is secure under selective-opening attacks. Different flavors such as sender-deniable and receiver-deniable encryption, where only the Sender or Receiver can produce fake random coins, have been considered. Recently, several open questions regarding the feasibility of deniable encryption have been resolved (c.f. (O’Neill et al., CRYPTO, 2011), (Bendlin et al., ASIACRYPT, 2011)). A fundamental remaining open question is whether it is possible to construct sender-deniable Encryption Schemes with super-polynomial security, where an adversary has negligible advantage in distinguishing real and fake openings. The primitive of simulatable public key encryption (PKE), introduced by Damg˚ard and Nielsen (CRYPTO, 2000), is a public key encryption scheme with additional properties that allow oblivious sampling of public keys and
Abstract. Adaptively-secure encryption schemes ensure secrecy even in the presence of an adversary who can corrupt parties in an adaptive manner based on public keys, ciphertexts, and secret data of alreadycorrupted parties. Ideally, an adaptively-secure encryption scheme should, like standard public-key encryption, allow arbitrarily-many parties to use a single encryption key to securely encrypt arbitrarily-many messages to a given receiver who maintains only a single short decryption key. However, it is known that these requirements are impossible to achieve: no non-interactive encryption scheme that supports encryption of an unbounded number of messages and uses a single, unchanging decryption key can be adaptively secure. Impossibility holds even if secure data erasure is possible. We show that this limitation can be overcome by updating the decryption key over time and making some mild assumptions about the frequency of communication between parties. Using this approach, we construct adaptively-secure, completely non-interactive encryption schemes supporting secure encryption of arbitrarily-many messages from arbitrarilymany senders. Our schemes additionally provide forward security and security against chosen-ciphertext attacks. 1
The notion of deniable authentication protocol was introduced in 1998 by Dwork et al. [11] and Aumann and Rabin [1, 2] independently. As a new cryptographic authentication protocol, a deniable authentication protocol enables an intended receiver to identify the source of a given message without being able to prove the identity of the sender to a third party. Over the past years, many deniable authentication protocols have been proposed. In this paper, we would like to present a universal encrypted deniable authentication protocol from any public key encryption scheme. Provided that the public key encryption scheme is secure, a secure deniable authentication protocol can be directly derived.
To ensure resistance to attacks with coercion to disclose a secret key by an active adversary, the protocol of deniable encryption includes a procedure for mutual authentication of the sender and receiver of a message with their long-term public keys, which is combined with a hidden exchange of single-use public keys used to generate a single-use shared key, on which the secret message is encrypted. Longterm public keys are used to generate a shared secret key, on which a fake message is encrypted. The produced intermediate ciphertexts are converted into a single ciphertext, which is computationally indistinguishable from the ciphertext obtained by probabilistic encryption of a fake message. This approach allows us to build an encryption protocol that is resistant to bi-sided attacks with coercion, since the disclosure of users' private keys gives access only to the fake message and cannot be used to prove the possibility of alternative decryption of the ciphertext transmitted over a public communication channel. The authentication of the single-use public keys is also used to implement protocols for secure encryption using short shared keys.
Abstract. In this paper, we propose a new notion of timed encryption, in which the encryption is secure within time t while it is totally insecure after some time T> t. We are interested in the case where t and T are both polynomial. We propose a concrete construction that is provably secure in the random oracle model. We show that it can be generically (although inefficient) constructed from a timed commitment of Boneh and Naor (CRYPTO’00). Finally, we apply this primitive to construct a deniable secure key exchange protocol, where the deniability and secrecy both hold adaptively and the adversary can conduct session state reveal attacks and eavesdropping attacks in the non-eraser model. Our protocol is the first to achieve each of the following properties: adaptive deniability admitting eavesdropping attacks and deniability admitting session state reveal attacks in the non-eraser model. Our protocol is constructed using a timing restriction (inherited from the timed encryption). However, the requirement is rather weak. It essentially asks a user to respond to a ciphertext as soon as possible and hence does not artificially cause any delay. Our usage of timed encryption for the deniability is to use the forceful decryption to obtain the plaintext and hence does not use any random oracle assumption (even if the secrecy proof needs this).
Abstract. A receiver-anonymous channel allows a sender to send a message to a receiver without an adversary learning for whom the message is intended. Wireless broadcast channels naturally provide receiver anonymity, as does multi-casting one message to a receiver population containing the intended receiver. While anonymity and confidentiality appear to be orthogonal properties, making anonymous communication confidential is more involved than one might expect, since the ciphertext might reveal which public key has been used to encrypt. To address this problem, public-key cryptosystems with enhanced security properties have been proposed. We investigate constructions as well as limitations for preserving receiver anonymity when using public-key encryption (PKE). We use the constructive cryptography approach by Maurer and Renner and interpret cryptographic schemes as constructions of a certain ideal resource (e.g. a confidential anonymous channel) from given real resources (e.g. a broadcast channel). We define appropriate anonymous communication resources and show that a very natural resource can be constructed by using a PKE scheme which fulfills three properties that appear in cryptographic literature (IND-CCA, key-privacy, weak robustness). We also show that a desirable stronger variant, preventing the adversary from selective “trial-deliveries ” of messages, is unfortunately unachievable by any PKE scheme, no matter how strong. The constructive approach makes the guarantees achieved by applying a cryptographic scheme explicit in the constructed (ideal) resource; this specifies the exact requirements for the applicability of a cryptographic scheme in a given context. It also allows to decide which of the existing security properties of such a cryptographic scheme are adequate for the considered scenario, and which are too weak or too strong. Here, we show that weak robustness is necessary but that so-called strong robustness is unnecessarily strong in that it does not construct a (natural) stronger resource.