Attacks based on Conditional Correlations against the Nonlinear Filter Generator

In this paper we extend the conditional correlation attack ([LCPP96]) against the nonlinear filter generator (NLFG) by introducing new conditions and generalisations and present two known-plaintext attacks, called hybrid correlation attack and concentration attack. The NLFG is a well known LFSR-based keystream generator which could be used as a basic building block in a synchronous stream cipher system. Both new attacks use methods from the conditional correlation attack and additional from fast correlation attacks to derive the unknown initial state of the LFSR of the NLFG. The basic principle of iteratively cumulating and updating conditional correlations for the NLFG was proposed in [Loh01] and for general combiners with memory in [GBM02]. With the hybrid correlation attack it is possible to successfully attack the NLFG by applying a fast correlation attack, even if the filter function $f$ of the NLFG is highly nonlinear, e.g. the normalised nonlinearity $p_{e,f}$ is $\ge 0.45$. The concentration attack maps all computed conditional correlations to $D-B$ unknown LFSR bits, where $D \ge k$ and $1 \le B \le k$ are parameters which can be chosen by the attacker, and $k$ is the length of the LFSR of the NLFG. Even with low values of conditional correlations, it is possible to mount the hybrid correlation attack and the concentration attack successfully. This is not the case for the originally version of the conditional correlation attack ([LCPP96]) in a time lower than a full search over all possible initial states

The Conditional Correlation Attack: A Practical Attack on Bluetooth Encryption

Abstract. Motivated by the security of the nonlinear filter generator, the concept of correlation was previously extended to the conditional correlation, that studied the linear correlation of the inputs conditioned on a given (short) output pattern of some specific nonlinear function. Based on the conditional correlations, conditional correlation attacks were shown to be successful and efficient against the nonlinear filter generator. In this paper, we further generalize the concept of conditional correlations by assigning it with a different meaning, i.e. the correlation of the output of an arbitrary function conditioned on the unknown (partial) input which is uniformly distributed. Based on this generalized conditional correlation, a general statistical model is studied for dedicated key-recovery distinguishers. It is shown that the generalized conditional correlation is no smaller than the unconditional correlation. Consequently, our distinguisher improves on the traditional one (in the worst case it degrades into the traditional one). In particular, the distinguisher may be successful even if no ordinary correlation exists. As an application, a conditional correlation attack is developed and optimized against Bluetooth two-level E0. The attack is based on a recently detected flaw in the resynchronization of E0, as well as the investigation of conditional correlations in the Finite State Machine (FSM) governing the keystream output of E0. Our best attack finds the original encryption key for two-level E0 using the first 24 bits of 2 23.8 frames and with 2 38 computations. This is clearly the fastest and only practical known-plaintext attack on Bluetooth encryption compared with all existing attacks. Current experiments confirm our analysis

The conditional correlation attack: a practical attack on Bluetooth encryption

Motivated by the security of the nonlinear filter generator, the concept of correlation was previously extended to the conditional correlation, that studied the linear correlation of the inputs conditioned on a given (short) output pattern of some specific nonlinear function. Based on the conditional correlations, conditional correlation attacks were shown to be successful and efficient against the nonlinear filter generator. In this paper, we further generalize the concept of conditional correlations by assigning it with a different meaning, i.e. the correlation of the output of an arbitrary function conditioned on the unknown (partial) input which is uniformly distributed. Based on this generalized conditional correlation, a general statistical model is studied for dedicated key-recovery distinguishers. It is shown that the generalized conditional correlation is no smaller than the unconditional correlation. Consequently, our distinguisher improves on the traditional one (in the worst case it degrades into the traditional one). In particular, the distinguisher may be successful even if no ordinary correlation exists. As an application, a conditional correlation attack is developed and optimized against Bluetooth two-level E0. The attack is based on a recently detected flaw in the resynchronization of EO, as well as the investigation of conditional correlations in the finite state machine (FSM) governing the keystream output of E0. Our best attack finds the original encryption key for two-level E0 using the first 24 bits of 223.8 frames and with 238 computations. This is clearly the fastest and only practical known-plaintext attack on Bluetooth encryption compared with all existing attacks. Current experiments confirm our analysi

An Automatic, Time-Based, Secure Pairing Protocol for Passive RFID

This paper introduces the Adopted-Pet (AP) protocol, an automatic (i.e. requiring no human interaction) secure pairing protocol, adequate for the pairing between a passive RFID tag and a reader. Most pairing protocols rely for their security on a certain advantage that the legitimate devices have over any malicious users. Such advantages include proximity (employing near-field communication) or secret keys that are either produced with the assistance of, or verified by, the legitimate user. The advantage exploited by our novel AP protocol is the amount of uninterrupted time spent by the two devices in the proximity (although not requiring near-field communication) of each-other. We discuss several implementation configurations, all based on pseudo-random bit generators, employing short-length LFSRs, and requiring no more than 2000 transistors. This makes the protocol ideally suited for low-cost passive RFID tags. For each configuration we show that the AP protocol is highly secure against occasional malicious entities

Attacks based on Conditional Correlations against the Nonlinear Filter Generator

Abstract. In this paper we extend the conditional correlation attack ([LCPP96]) against the nonlinear filter generator (NLFG) by introducing new conditions and generalisations and present two known-plaintext attacks, called hybrid correlation attack and concentration attack. The NLFG is a well known LFSR-based keystream generator which could be used as a basic building block in a synchronous stream cipher system. Both new attacks use methods from the conditional correlation attack and additional from fast correlation attacks to derive the unknown initial state of the LFSR of the NLFG. The basic principle of iteratively cumulating and updating conditional correlations for the NLFG was proposed in [Löh01] and for general combiners with memory in [GBM02]. With the hybrid correlation attack it is possible to successfully attack the NLFG by applying a fast correlation attack, even if the filter function f of the NLFG is highly nonlinear, e.g. the normalised nonlinearity pe,f is ≥ 0.45. The concentration attack maps all computed conditional correlations to D − B unknown LFSR bits, where D ≥ k and 1 ≤ B ≤ k are parameters which can be chosen by the attacker, and k is the length of the LFSR of the NLFG. Even with low values of conditional correlations, it is possible to mount the hybrid correlation attack and the concentration attack successfully. This is not the case for the originally version of the conditional correlation attack ([LCPP96]) in a time lower than a full search over all possible initial states. Key words: stream ciphers, keystream generator, nonlinear filter generator, NLFG, conditional correlation attack, fast correlation attacks

Методи оцінювання та обґрунтування стійкості потокових шифрів відносно статистичних атак на основі алгебраїчно вироджених наближень булевих функцій

У дисертації розв’язано актуальну наукову задачу розробки методів по-будови науково обґрунтованих оцінок стійкості синхронних потокових шиф-рів (СПШ) відносно статистичних атак на основі алгебраїчно вироджених наближень булевих функцій. Отримані нові результати дозволяють на прак-тиці оцінювати і обґрунтовувати стійкість сучасних СПШ, що, зрештою, на-дає можливість суттєво скоротити час проведення експертних досліджень алгоритмів потокового шифрування, призначених для захисту державних інформаційних ресурсів України