Can Caesar Beat Galois?

Serge Vaudenay, Damian Vizár
2018
Conference paper

Abstract

The Competition for Authenticated Encryption: Security, Applicability and Robustness (CAESAR) has as its official goal to “identify a portfolio of authenticated ciphers that offer advantages over [the Galois-Counter Mode with AES]” and are suitable for widespread adoption.” Each of the 15 candidate schemes competing in the currently ongoing 3rd round of CAESAR must clearly declare its security claims, i.e. whether it can tolerate nonce misuse, and what is the maximal data complexity for which security is guaranteed. These claims appear to be valid for all 15 candidates. Interpreting “Robustness” in CAESAR as the ability to mitigate damage when security guarantees are void, we describe attacks with 64-bit complexity or above, and/or with nonce reuse for each of the 15 candidates. We then classify the candidates depending on how powerful does an attacker need to be to mount (semi-)universal forgeries, decryption attacks, or key recoveries. Rather than invalidating the security claims of any of the candidates, our results provide an additional criterion for evaluating the security that candidates deliver, which can be useful for e.g. breaking ties in the final CAESAR discussions.

Official source

https://infoscience.epfl.ch/record/264823?ln=en

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Related concepts

Related publications

Authenticated Encryption (AE) is a symmetric key cryptographic primitive that ensures confidentiality and authenticity of processed messages at the same time. The research of AE as a primitive in its own right started in 2000. The security goals of AE were captured in formal definitions in the tradition in the tradition of provable security (such as NAE, MRAE, OAE, RAE or the RUP), where the security of a scheme is formally proven assuming the security of an underlying building block. The prevailing syntax moved to nonce-based AE with associated data (which is an additional input that gets authenticated, but not encrypted). Other types of AE schemes appeared as well, e.g. ones that supported stateful sessions. Numerous AE schemes were designed; in the early years, these were almost exclusively blockcipher modes of operation, most notably OCB in 2001, CCM in 2003 and GCM in 2004. At the same time, issues were discovered both with the security and applicability of the most popular AE schemes, and other applications of symmetric key cryptography. As a response, the Competition for Authenticated Encryption: Security, Applicability, and Robustness (CAESAR) was started in 2013. Its goals were to identify a portfolio of new, secure and reliable AE schemes that would satisfy the needs of practical applications, and also to boost the research in the area of AE. Prompted by CAESAR, 57 new schemes were designed, new types of constructions that gained popularity appeared (such as the Sponge-based AE schemes), and new notions of security were proposed (such as RAE). The final portfolio of the CAESAR competition should be announced in 2018. In this thesis, we push the state of the art in the field of AE in several directions. All of them are related to provable security, in one way, or another. We propose OMD, the first provably secure dedicated AE scheme that is based on a compression function. We further modify OMD to achieve nonce misuse-resistant security (MRAE). We also propose another provably secure variant of OMD called pure OMD, which enjoys a great improvement of performance over OMD. Inspired by the modifications that gave rise to pure OMD, we turn to the popular Sponge-based AE schemes and prove that similar measures can also be applied to the keyed Sponge and keyed Duplex (a variant of the Sponge), allowing a substantial increase of performance without an impact on security. We then address definitional aspects of AE. We critically evaluate the security notion of OAE, whose authors claimed that it provides the best possible security for online schemes under nonce reuse. We challenge these claims, and discuss what are the meaningful requirements for online AE schemes. Based on our findings, we formulate a new definition of online AE security under nonce-reuse, and demonstrate its feasibility. We next turn our attention to the security of nonce-based AE schemes under stretch misuse; i.e. when a scheme is used with varying ciphertext expansion under the same key, even though it should not be. We argue that varying the stretch is plausible, and formulate several notions that capture security in presence of variable stretch. We establish their relations to previous notions, and demonstrate the feasibility of security in this setting. We finally depart from provable security, with the intention to complement it. We compose a survey of universal forgeries, decryption attacks and key recovery attacks on 3rd round CAESAR candidates.

EPFL2018

In this dissertation, we study the security of cryptographic protocols and cryptosystems from the mathematical definition of the primitives, up to their physical implementations in the real world. We propose a representation of the chronological design using six layers (cryptographic primitives, cryptographic protocols, implementation, computer insecurity, side channel cryptanalysis and computer human interactions). We do the assumption that these layers should not be studied independently. Indeed, many negligible security weaknesses coming from different layers can be correlated to provide devastating practical attacks on cryptosystems. However, the complexity of a complete security analysis becomes huge and interdisciplinary knowledge is needed. These limitations are probably the reasons of the lack of complete security analysis in practice. We define a novel approach, to combine and study the six layers simultaneously. We propose to follow the data flow of a system and to perform security analysis across the six layers. This technique is applied in practice to the security analysis of computer keyboards, RC4, IEEE 802.11, and e-passports. Thanks to this method, we found 34 additional exploitable correlations in RC4 and we defined the best key recovery attacks on WEP and WPA. We also identified weaknesses in the design and the implementation of e-passports. Therefore, we show that the security risk of every layer seems to be related to its level of complexity. Thus, the implementation layer, the computer insecurity layer, the side channel layer and the computer human interfaces layer are subject to cost-effective attacks in practice. Interestingly, these layers are not intensively studied in cryptography, where research stays usually focused on the two first layers (and some side channel attacks). In this dissertation, we also propose frameworks for computer aided cryptanalysis. Indeed, when the complexity of a system is too important to perform manual analysis, some tools may automatically find weaknesses. Increasing complexity in systems adds new vulnerabilities. Straightforward but automated analysis becomes relevant. Two frameworks have been developed. The first one automatically highlights linear correlation in RC4. The second framework, called Autodafé automatically detects buffer overflows in modern software, using a technique called Fuzzing by Weighting Attacks with Markers.

EPFL2010

, ,

We propose pure OMD (p-OMD) as a new variant of the Offset Merkle-Damgård (OMD) authenticated encryption scheme. Our new scheme inherits all desirable security features of OMD while having a more compact structure and providing higher efficiency. The original OMD scheme, as submitted to the CAESAR competition, couples a single pass of a variant of the Merkle-Damgård (MD) iteration with the counter-based XOR MAC algorithm to provide privacy and authenticity. Our improved p-OMD scheme dispenses with the XOR MAC algorithm and is purely based on the MD iteration; hence, the name ``pure'' OMD. To process a message of

\ell

blocks and associated data of

a

blocks, OMD needs

\ell+a+2

calls to the compression function while p-OMD only requires max{

\ell, a

2

calls. Therefore, for a typical case where

\ell \geq a

, p-OMD makes just

\ell+2

calls to the compression function; that is, associated data is processed almost freely compared to OMD. We prove the security of p-OMD under the same standard assumption (pseudo-randomness of the compression function) as made in OMD; moreover, the security bound for p-OMD is the same as that of OMD, showing that the modifications made to boost the performance are without any loss of security.

Springer2015