Analysis Design & Applications of Cryptographic Building Blocks

This thesis deals with the basic design and rigorous analysis of cryptographic schemes and primitives, especially of authenticated encryption schemes, hash functions, and password-hashing schemes. In the last decade, security issues such as the PS3 jailbreak demonstrate that common security notions are rather restrictive, and it seems that they do not model the real world adequately. As a result, in the first part of this work, we introduce a less restrictive security model that is closer to reality. In this model it turned out that existing (on-line) authenticated encryption schemes cannot longer beconsidered secure, i.e. they can guarantee neither data privacy nor data integrity. Therefore, we present two novel authenticated encryption scheme, namely COFFE and McOE, which are not only secure in the standard model but also reasonably secure in our generalized security model, i.e. both preserve full data inegrity. In addition, McOE preserves a resonable level of data privacy. The second part of this thesis starts with proposing the hash function Twister-Pi, a revised version of the accepted SHA-3 candidate Twister. We not only fixed all known security issues of Twister, but also increased the overall soundness of our hash-function design. Furthermore, we present some fundamental groundwork in the area of password-hashing schemes. This research was mainly inspired by the medial omnipresence of password-leakage incidences. We show that the password-hashing scheme scrypt is vulnerable against cache-timing attacks due to the existence of a password-dependent memory-access pattern. Finally, we introduce Catena the first password-hashing scheme that is both memory-consuming and resistant against cache-timing attacks

General Classification of the Authenticated Encryption Schemes for the CAESAR Competition

An Authenticated encryption scheme is a scheme which provides privacy and integrity by using a secret key. In 2013, CAESAR (the ``Competition for Authenticated Encryption: Security, Applicability, and Robustness\u27\u27) was co-founded by NIST and Dan Bernstein with the aim of finding authenticated encryption schemes that offer advantages over AES-GCM and are suitable for widespread adoption. The first round started with 57 candidates in March 2014; and nine of these first-round candidates where broken and withdrawn from the competition. The remaining 48 candidates went through an intense process of review, analysis and comparison. While the cryptographic community benefits greatly from the manifold different submission designs, their sheer number implies a challenging amount of study. This paper provides an easy-to-grasp overview over functional aspects, security parameters, and robustness offerings by the CAESAR candidates, clustered by their underlying designs (block-cipher-, stream-cipher-, permutation-/sponge-, compression-function-based, dedicated). After intensive review and analysis of all 48 candidates by the community, the CAESAR committee selected only 30 candidates for the second round. The announcement for the third round candidates was made on 15th August 2016 and 15 candidates were chosen for the third round

Why Do Developers Get Password Storage Wrong? A Qualitative Usability Study

Passwords are still a mainstay of various security systems, as well as the cause of many usability issues. For end-users, many of these issues have been studied extensively, highlighting problems and informing design decisions for better policies and motivating research into alternatives. However, end-users are not the only ones who have usability problems with passwords! Developers who are tasked with writing the code by which passwords are stored must do so securely. Yet history has shown that this complex task often fails due to human error with catastrophic results. While an end-user who selects a bad password can have dire consequences, the consequences of a developer who forgets to hash and salt a password database can lead to far larger problems. In this paper we present a first qualitative usability study with 20 computer science students to discover how developers deal with password storage and to inform research into aiding developers in the creation of secure password systems

Passphone: Outsourcing Phone-based Web Authentication while Protecting User Privacy

This work introduces PassPhone, a new smartphone-based authentication scheme that outsources user verification to a trusted third party without sacrificing privacy: neither can the trusted third party learn the relation between users and service providers, nor can service providers learn those of their users to others. When employed as a second factor in conjunction with, for instance, passwords as a first factor, our scheme maximizes the deployability of two-factor authentication for service providers while maintaining user privacy. We conduct a twofold formal analysis of our scheme, the first regarding its general security, and the second regarding anonymity and unlinkability of its users. Moreover, we provide an automatic analysis using AVISPA, a comparative evaluation to existing schemes under Bonneau et al.\u27s framework, and an evaluation of a prototypical implementation

The Collision Security of MDC-4

There are four somewhat classical double length block cipher based compression functions known: MDC-2, MDC-4, Abreast-DM, and Tandem-DM. They all have been developed over 20 years ago. In recent years, cryptographic research has put a focus on block cipher based hashing and found collision security results for three of them (MDC-2, Abreast-DM, Tandem-DM). In this paper, we add MDC-4, which is part of the IBM CLiC cryptographic module (FIPS 140-2 Security Policy for IBM CrytoLite in C, October 2003), to that list by showing that - \u27instantiated\u27 using an ideal block cipher with 128 bit key/plaintext/ciphertext size - no adversary asking less than $2^{74.76}$ queries can find a collision with probability greater than $1/2$. This is the first result on the collision security of the hash function MDC-4. The compression function MDC-4 is created by interconnecting two MDC-2 compression functions but only hashing one message block with them instead of two. The developers aim for MDC-4 was to offer a higher security margin, when compared to MEDC-2, but still being fast enough for practical purposes. The MDC-2 collision security proof of Steinberger (EUROCRYPT 2007) cannot be directly applied to MDC-4 due to the structural differences. Although sharing many commonalities, our proof for MDC-4 is much shorter and we claim that our presentation is also easier to grasp

Overview of the Candidates for the Password Hashing Competition - And Their Resistance Against Garbage-Collector Attacks

In this work we provide an overview of the candidates of the Password Hashing Competition (PHC) regarding to their functionality, e.g., client-independent update and server relief, their security, e.g., memory-hardness and side-channel resistance, and its general proper- ties, e.g., memory usage and flexibility of the underlying primitives. Furthermore, we formally introduce two kinds of attacks, called Garbage- Collector and Weak Garbage-Collector Attack, exploiting the memory management of a candidate. Note that we consider all candidates which are not yet withdrawn from the competition

Efficient Beyond-Birthday-Bound-Secure Deterministic Authenticated Encryption with Minimal Stretch

Block-cipher-based authenticated encryption has obtained considerable attention from the ongoing CAESAR competition. While the focus of CAESAR resides primarily on nonce-based authenticated encryption, Deterministic Authenticated Encryption (DAE) is used in domains such as key wrap, where the available message entropy motivates to omit the overhead for nonces. Since the highest possible security is desirable when protecting keys, beyond-birthday-bound (BBB) security is a valuable goal for DAE. In the past, significant efforts had to be invested into designing BBB-secure AE schemes from conventional block ciphers, with the consequences of losing efficiency and sophisticating security proofs. This work proposes Deterministic Counter in Tweak (DCT), a BBB-secure DAE scheme inspired by the Counter-in-Tweak encryption scheme by Peyrin and Seurin. Our design combines a fast $\epsilon$-almost-XOR-universal family of hash functions, for $\epsilon$ close to $2^{-2n}$, with a single call to a $2n$-bit SPRP, and a BBB-secure encryption scheme. First, we describe our construction generically with three independent keys, one for each component. Next, we present an efficient instantiation which (1) requires only a single key, (2) provides software efficiency by encrypting at less than two cycles per byte on current x64 processors, and (3) produces only the minimal $\tau$-bit stretch for $\tau$ bit authenticity. We leave open two minor aspects for future work: our current generic construction is defined for messages of at least $2n-\tau$ bits, and the verification algorithm requires the inverse of the used $2n$-bit SPRP and the encryption scheme

Biclique Cryptanalysis Of PRESENT, LED, And KLEIN

In this paper, we analyze the resistance of the lightweight ciphers PRESENT, LED, and KLEIN to biclique attacks. Primarily, we describe attacks on the full-round versions PRESENT-80, PRESENT-128, LED-64, LED-128, KLEIN-80, and KLEIN-96. Our attacks have time complexities of $2^{79.49}$, $2^{127.32}$, $2^{63.58}$, $2^{127.42}$, $2^{79.00}$, and $2^{95.18}$ encryptions, respectively. In addition, we consider attacks on round-reduced versions of PRESENT and LED, to show the security margin for which an adversary can obtain an advantage of at least a factor of two compared to exhaustive search

Pipelineable On-Line Encryption

Correct authenticated decryption requires the receiver to buffer the decrypted message until the authenticity check has been performed. In high-speed networks, which must handle large message frames at low latency, this behavior becomes practically infeasible. This paper proposes CCA-secure on-line ciphers as a practical alternative to AE schemes since the former provide some defense against malicious message modifications. Unfortunately, all published on-line ciphers so far are either inherently sequential, or lack a CCA-security proof. This paper introduces POE, a family of on-line ciphers that combines provable security against chosen-ciphertext attacks with pipelineability to support efficient implementations. POE combines a block cipher and an e-AXU family of hash functions. Different instantiations of POE are given, based on different universal hash functions and suitable for different platforms. Moreover, this paper introduces POET, a provably secure on-line AE scheme, which inherits pipelineability and chosen-ciphertext-security from POE and provides additional resistance against nonce-misuse attacks

Proof of Space from Stacked Expanders

Recently, proof of space (PoS) has been suggested as a more egalitarian alternative to the traditional hash-based proof of work. In PoS, a prover proves to a verifier that it has dedicated some specified amount of space. A closely related notion is memory-hard functions (MHF), functions that require a lot of memory/space to compute. While making promising progress, existing PoS and MHF have several problems. First, there are large gaps between the desired space-hardness and what can be proven. Second, it has been pointed out that PoS and MHF should require a lot of space not just at some point, but throughout the entire computation/protocol; few proposals considered this issue. Third, the two existing PoS constructions are both based on a class of graphs called superconcentrators, which are either hard to construct or add a logarithmic factor overhead to efficiency. In this paper, we construct PoS from stacked expander graphs. Our constructions are simpler, more efficient and have tighter provable space-hardness than prior works. Our results also apply to a recent MHF called Balloon hash. We show Balloon hash has tighter space-hardness than previously believed and consistent space-hardness throughout its computation