Padding oracle attack | EPFL Graph Search

In cryptography, a padding oracle attack is an attack which uses the padding validation of a cryptographic message to decrypt the ciphertext. In cryptography, variable-length plaintext messages often have to be padded (expanded) to be compatible with the underlying cryptographic primitive. The attack relies on having a "padding oracle" who freely responds to queries about whether a message is correctly padded or not. Padding oracle attacks are mostly associated with CBC mode decryption used within block ciphers. Padding modes for asymmetric algorithms such as OAEP may also be vulnerable to padding oracle attacks. Symmetric cryptography In symmetric cryptography, the padding oracle attack can be applied to the CBC mode of operation, where the "oracle" (usually a server) leaks data about whether the padding of an encrypted message is correct or not. Such data can allow attackers to decrypt (and sometimes encrypt) messages through the oracle using the oracle's key, without know

Misuse Attacks on Post-quantum Cryptosystems

Ciprian Baetu, Fatma Betül Durak, Loïs Evan Huguenin-Dumittan, Abdullah Talayhan, Serge Vaudenay

Many post-quantum cryptosystems which have been proposed in the National Institute of Standards and Technology (NISI) standardization process follow the same meta-algorithm, but in different algebras or different encoding methods. They usually propose two constructions, one being weaker and the other requiring a random oracle. We focus on the weak version of nine submissions to NISI. Submitters claim no security when the secret key is used several times. In this paper, we analyze how easy it is to run a key recovery under multiple key reuse. We mount a classical key recovery under plaintext checking attacks (i.e., with a plaintext checking oracle saying if a given ciphertext decrypts well to a given plaintext) and a quantum key recovery under chosen ciphertext attacks. In the latter case, we assume quantum access to the decryption oracle.

SPRINGER INTERNATIONAL PUBLISHING AG2019

Provably Secure Authenticated Encryption

Damian Vizár

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

Cryptanalysis of Enhanced MORE

Serge Vaudenay, Damian Vizár

Fully homomorphic encryption (FHE) has been among the most popular research topics of the last decade. While the bootstrapping-based, public key cryptosystems that follow Gentry's original design are getting more and more efficient, their performance is still far from being practical. This lead to several attempts to construct symmetric FHE schemes that would not be as inefficient as their public key counterparts. Unfortunately, most such schemes were also based on (randomized) linear transformations, and shown completely insecure. One such broken scheme was the Matrix Operation for Randomization and Encryption (MORE). In a recent paper, Hariss, Noura and Samhat propose Enhanced MORE, which is supposed to improve over MORE's weaknesses. We analyze Enhanced MORE, discuss why it does not improve over MORE, and show that it is even less secure by presenting a highly efficient ciphertext-only decryption attack. We implement the attack and confirm its correctness.

2018

Provably Secure Authenticated Encryption

Damian Vizár

EPFL2018