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Publication# Chaum's Designated Confirmer Signature Revisited

Résumé

This article revisits the original designated confirmer signature scheme of Chaum. Following the same spirit we naturally extend the Chaum's construction in a more general setting and analyze its security in a formal way. We prove its security in the random oracle model by using a random hash function and a random permutation. We notably consider the confirmer as an attacker against the existential forgery under an adaptive chosen-message attack. This security property is shown to rely on the hardness of forging signatures in a universal way of a classical existentially forgeable signature scheme. Furthermore, we show that the invisibility of the signatures under a non-adaptive chosen-message (aka lunchtime) attack relies on some invisibility properties of an existentially forgeable undeniable signature scheme. The existence of this cryptographic primitive is shown equivalent to the existence of public-key cryptosystems. It is also interesting to see that this article confirms the security of Chaum's scheme since our construction is a natural generalization of this scheme.

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Related key attacks (RKAs) are powerful cryptanalytic attacks where an adversary can change the secret key and observe the effect of such changes at the output. The state of the art in RKA security protects against an a-priori unbounded number of certain algebraic induced key relations, e.g., affine functions or polynomials of bounded degree. In this work, we show that it is possible to go beyond the algebraic barrier and achieve security against arbitrary key relations, by restricting the number of tampering queries the adversary is allowed to ask for. The latter restriction is necessary in case of arbitrary key relations, as otherwise a generic attack of Gennaro et al. (TCC 2004) shows how to recover the key of almost any cryptographic primitive. We describe our contributions in more detail below. 1. We show that standard ID and signature schemes constructed from a large class of -protocols (including the Okamoto scheme, for instance) are secure even if the adversary can arbitrarily tamper with the prover's state a bounded number of times and obtain some bounded amount of leakage. Interestingly, for the Okamoto scheme we can allow also independent tampering with the public parameters. 2. We show a bounded tamper and leakage resilient CCA secure public key cryptosystem based on the DDH assumption. We first define a weaker CPA-like security notion that we can instantiate based on DDH, and then we give a general compiler that yields CCA-security with tamper and leakage resilience. This requires a public tamperproof common reference string. 3. Finally, we explain how to boost bounded tampering and leakage resilience (as in 1. and 2. above) to continuous tampering and leakage resilience, in the so-called floppy model where each user has a personal hardware token (containing leak- and tamper-free information) which can be used to refresh the secret key. We believe that bounded tampering is a meaningful and interesting alternative to avoid known impossibility results and can provide important insights into the security of existing standard cryptographic schemes.

2013Our main motivation is to design more user-friendly security protocols. Indeed, if the use of the protocol is tedious, most users will not behave correctly and, consequently, security issues occur. An example is the actual behavior of a user in front of an SSH certificate validation: while this task is of utmost importance, about 99% of SSH users accept the received certificate without checking it. Designing more user-friendly protocols may be difficult since the security should not decrease at the same time. Interestingly, insecure channels coexist with channels ensuring authentication. In practice, these latters may be used for a string comparison or a string copy, e.g., by voice over IP spelling. The shorter the authenticated string is, the less human interaction the protocol requires, and the more user-friendly the protocol is. This leads to the notion of SAS-based cryptography, where SAS stands for Short Authenticated String. In the first part of this thesis, we analyze and propose optimal SAS-based message authentication protocols. By using these protocols, we show how to construct optimal SAS-based authenticated key agreements. Such a protocol enables any group of users to agree on a shared secret key. SAS-based cryptography requires no pre-shared key, no trusted third party, and no public-key infrastructure. However, it requires the user to exchange a short SAS, e.g., five decimal digits. By using the just agreed secret key, the group can now achieve a secure communication based on symmetric cryptography. SAS-based authentication protocols are often used to authenticate the protocol messages of a key agreement. Hence, each new secure communication requires the interaction of the users to agree on the SAS. A solution to reduce the user interaction is to use digital signature schemes. Indeed, in a setup phase, the users can use a SAS-based authentication protocol to exchange long-term verification keys. Then, using digital signatures, users are able to run several key agreements and the authentication of protocol messages is done by digital signatures. In the case where no authenticated channel is available, but a public-key infrastructure is in place, the SAS-based setup phase is avoided since verification keys are already authenticated by the infrastructure. In the second part of this thesis, we also study two problems related to digital signatures: (1) the insecurity of digital signature schemes which use weak hash functions and (2) the privacy issues from signed documents. Digital signatures are often proven to be secure in the random oracle model. The role of random oracles is to model ideal hash functions. However, real hash functions deviate more and more from this idealization. Indeed, weaknesses on hash functions have already been discovered and we are expecting new ones. A question is how to fix the existing signature constructions based on these weak hash functions. In this thesis, we first try to find a better way to model weak hash function. Then, we propose a (randomized) pre-processing to the input message which transforms any weak signature implementation into a strong signature scheme. There remains one drawback due to the randomization. Indeed, the random coins must be sent and thus the signature enlarges. We also propose a method to avoid the increase in signature length by reusing signing coins. Digital signatures may also lead to privacy issues. Indeed, given a message and its signature, anyone can publish the pair which will confirm the authenticity of the message. In certain applications, like in electronic passports (e-passports), publishing the authenticated data leads to serious privacy issues. In this thesis, we define the required security properties in order to protect the data privacy, especially in the case of e-passport verification. The main idea consists for the e-passport to keep the signature secret. The e-passport should only prove that it knows a valid signature instead of revealing it. We propose a new primitive, called Offline Non-Transferable Authentication Protocol (ONTAP), as well as efficient implementations that are compatible with the e-passport standard signature schemes.

Attempting to reach a minimal number of moves in cryptographic protocols is a quite classical issue. Besides the theoretical interests, minimizing the number of moves can clearly facilitate practical implementations in environments with communication constraints. In this paper, we offer a solution to this problem in the context of undeniable signatures with interactive verification protocols by proposing a way to achieve these protocols in 2 moves. To this goal, we review a scheme we proposed at Asiacrypt 2004 whose property is the full scalability of the signature length against security. We slightly modify (to make it non-transferable) a 2-move version of this scheme which was mentioned in the original article without a proof of security. In the random oracle model, we prove the security of the modified version against an active adversary and precisely assess the security in terms of the signature length. To the best of our knowledge, this scheme is the first 2-move undeniable signature scheme with a security proof.

2006