New collision attacks on SHA-1 based on optimal joint local-collision analysis

The main contributions of this paper are two-fold. Firstly, we present a novel direction in the cryptanalysis of the cryptographic hash function {\SHA}. Our work builds on previous cryptanalytic efforts on {\SHA} based on combinations of local collisions. Due to dependencies, previous approaches used heuristic corrections when combining the success probabilities and message conditions of the individual local collisions. Although this leads to success probabilities that are seemingly sufficient for feasible collision attacks, this approach most often does not lead to the maximum success probability possible as desired. We introduce novel techniques that enable us to determine the theoretical maximum success probability for a given set of (dependent) local collisions, as well as the smallest set of message conditions that attains this probability. We apply our new techniques and present an implemented open-source near-collision attack on {\SHA} with a complexity equivalent to $2^{57.5}$ {\SHA} compressions. Secondly, we present an identical-prefix collision attack and a chosen-prefix collision attack on {\SHA} with complexities equivalent to approximately $2^{61}$ and $2^{77.1}$ {\SHA} compressions, respectively

Efficient Doubling on Genus Two Curves over Binary Fields

In most algorithms involving elliptic and hyperelliptic curves, the costliest part consists in computing multiples of ideal classes. This paper investigates how to compute faster doubling over fields of characteristic two. We derive explicit doubling formulae making strong use of the defining equation of the curve. We analyze how many field operations are needed depending on the curve making clear how much generality one loses by the respective choices. Note, that none of the proposed types is known to be weak – one only could be suspicious because of the more special types. Our results allow to choose curves from a large enough variety which have extremely fast doubling needing only half the time of an addition. Combined with a sliding window method this leads to fast computation of scalar multiples. We also speed up the general case

On immutability of blockchains

Recently we presented a single-party cryptographic timestamping mechanism based on proof-of-sequential-work, which we proved secure in the universal composability framework. This paper describes this construction and its security claims and uses it to construct a multi-party permissioned blockchain protocol and show that it achieves an immutability notion.Finally we discuss applications of this protocol, including unpermissioned blockchains, and how these may benefi

Reverse-Engineering of the Cryptanalytic Attack Used in the Flame Super-Malware

In May 2012, a highly advanced malware for espionage dubbed Flame was found targeting the Middle-East. As it turned out, it used a forged signature to infect Windows machines by MITM-ing Windows Update. Using counter-cryptanalysis, Stevens found that the forged signature was made possible by a chosen-prefix attack on MD5 \cite{DBLP:conf/crypto/Stevens13}. He uncovered some details that prove that this attack differs from collision attacks in the public literature, yet many questions about techniques and complexity remained unanswered. In this paper, we demonstrate that significantly more information can be deduced from the example collision. Namely, that these details are actually sufficient to reconstruct the collision attack to a great extent using some weak logical assumptions. In particular, we contribute an analysis of the differential path family for each of the four near-collision blocks, the chaining value differences elimination procedure and a complexity analysis of the near-collision block attacks and the associated birthday search for various parameter choices. Furthermore, we were able to prove a lower-bound for the attack's complexity. This reverse-engineering of a non-academic cryptanalytic attack exploited in the real world seems to be without precedent. As it allegedly was developed by some nation-state(s) \cite{WashingtonPost_Flame,kaspersky_flame,crysis_flame}, we discuss potential insights to their cryptanalytic knowledge and capabilities

Reverse-Engineering of the Cryptanalytic Attack Used in the Flame Super-Malware

In May 2012, a highly advanced malware for espionage dubbed Flame was found targeting the Middle-East. As it turned out, it used a forged signature to infect Windows machines by MITM-ing Windows Update. Using counter-cryptanalysis, Stevens found that the forged signature was made possible by a chosen-prefix attack on MD5 \cite{DBLP:conf/crypto/Stevens13}. He uncovered some details that prove that this attack differs from collision attacks in the public literature, yet many questions about techniques and complexity remained unanswered. In this paper, we demonstrate that significantly more information can be deduced from the example collision. Namely, that these details are actually sufficient to reconstruct the collision attack to a great extent using some weak logical assumptions. In particular, we contribute an analysis of the differential path family for each of the four near-collision blocks, the chaining value differences elimination procedure and a complexity analysis of the near-collision block attacks and the associated birthday search for various parameter choices. Furthermore, we were able to prove a lower-bound for the attack's complexity. This reverse-engineering of a non-academic cryptanalytic attack exploited in the real world seems to be without precedent. As it allegedly was developed by some nation-state(s), we discuss potential insights to their cryptanalytic knowledge and capabilities