The Moral Character of Cryptographic Work

Cryptography rearranges power: it configures who can do what, from what. This makes cryptography an inherently \textit{political} tool, and it confers on the field an intrinsically \textit{moral} dimension. The Snowden revelations motivate a reassessment of the political and moral positioning of cryptography. They lead one to ask if our inability to effectively address mass surveillance constitutes a failure of our field. I believe that it does. I call for a community-wide effort to develop more effective means to resist mass surveillance. I plea for a reinvention of our disciplinary culture to attend not only to puzzles and math, but, also, to the societal implications of our work

09031 Abstracts Collection -- Symmetric Cryptography

From 11.01.09 to 16.01.09, the Seminar 09031 in ``Symmetric Cryptography \u27\u27 was held in Schloss Dagstuhl~--~Leibniz Center for Informatics. During the seminar, several participants presented their current research, and ongoing work and open problems were discussed. Abstracts of the presentations given during the seminar as well as abstracts of seminar results and ideas are put together in this paper. The first section describes the seminar topics and goals in general. Links to extended abstracts or full papers are provided, if available

Code-Based Game-Playing Proofs and the Security of Triple Encryption

The game-playing technique is a powerful tool for analyzing cryptographic constructions. We illustrate this by using games as the central tool for proving security of three-key triple-encryption, a long-standing open problem. Our result, which is in the ideal-cipher model, demonstrates that for DES parameters (56-bit keys and 64-bit plaintexts) an adversary\u27s maximal advantage is small until it asks about $2^{78}$ queries. Beyond this application, we develop the foundations for game playing, formalizing a general framework for game-playing proofs and discussing techniques used within such proofs. To further exercise the game-playing framework we show how to use games to get simple proofs for the PRP/PRF Switching Lemma, the security of the basic CBC~MAC, and the chosen-plaintext-attack security of OAEP

On Committing Authenticated Encryption

We provide a strong definition for committing authenticated-encryption (cAE), as well as a framework that encompasses earlier and weaker definitions. The framework attends not only to what is committed but also the extent to which the adversary knows or controls keys. We slot into our framework strengthened cAE-attacks on GCM and OCB. Our main result is a simple and efficient construction, CTX, that makes a nonce-based AE (nAE) scheme committing. The transformed scheme achieves the strongest security notion in our framework. Just the same, the added computational cost (on top of the nAE scheme\u27s cost) is a single hash over a short string, a cost independent of the plaintext\u27s length. And there is no increase in ciphertext length compared to the base nAE scheme. That such a thing is possible, let alone easy, upends the (incorrect) intuition that you can\u27t commit to a plaintext or ciphertext without hashing one or the other. And it motivates a simple and practical tweak to AE-schemes to make them committing

Anonymous AE

The customary formulation of authenticated encryption (AE) requires the decrypting party to supply the correct nonce with each ciphertext it decrypts. To enable this, the nonce is often sent in the clear alongside the ciphertext. But doing this can forfeit anonymity and degrade usability. Anonymity can also be lost by transmitting associated data (AD) or a session-ID (used to identify the operative key). To address these issues, we introduce anonymous AE, wherein ciphertexts must conceal their origin even when they are understood to encompass everything needed to decrypt (apart from the receiver\u27s secret state). We formalize a type of anonymous AE we call anAE, anonymous nonce-based AE, which generalizes and strengthens conventional nonce-based AE, nAE. We provide an efficient construction for anAE, NonceWrap, from an nAE scheme and a blockcipher. We prove NonceWrap secure. While anAE does not address privacy loss through traffic-flow analysis, it does ensure that ciphertexts, now more expansively construed, do not by themselves compromise privacy

On generalized Feistel networks

We prove beyond-birthday-bound security for the well-known types of generalized Feistel networks, including: (1) unbalanced Feistel networks, where the $n$-bit to $m$-bit round functions may have $n\ne m$; (2) alternating Feistel networks, where the round functions alternate between contracting and expanding; (3) type-1, type-2, and type-3 Feistel networks, where $n$-bit to $n$-bit round functions are used to encipher $kn$-bit strings for some $k\ge2$; and (4) numeric variants of any of the above, where one enciphers numbers in some given range rather than strings of some given size. Using a unified analytic framework we show that, in any of these settings, for any $\varepsilon>0$, with enough rounds, the subject scheme can tolerate CCA attacks of up to $q\sim N^{1-\varepsilon}$ adversarial queries, where $N$ is the size of the round functions\u27 domain (the size of the larger domain for alternating Feistel). This is asymptotically optimal. Prior analyses for generalized Feistel networks established security to only $q\sim N^{0.5}$ adversarial queries

Simplifying Game-Based Definitions: Indistinguishability up to Correctness and Its Application to Stateful AE

Often the simplest way of specifying game-based cryptographic definitions is apparently barred because the adversary would have some trivial win. Disallowing or invalidating these wins can lead to complex or unconvincing definitions. We suggest a generic way around this difficulty. We call it indistinguishability up to correctness, or IND|C. Given games G and H and a correctness condition C we define an advantage measure Adv_{G,H,C}^indc wherein G/H distinguishing attacks are effaced to the extent that they are inevitable due to C. We formalize this in the language of oracle silencing, an alternative to exclusion-style and penalty-style definitions. We apply our ideas to a domain where game-based definitions have been cumbersome: stateful authenticated-encryption (sAE). We rework existing sAE notions and encompass new ones, like replay-free AE permitting a specified degree of out-of-order message delivery

Security of Symmetric Encryption against Mass Surveillance

Motivated by revelations concerning population-wide surveillance of encrypted communications, we formalize and investigate the resistance of symmetric encryption schemes to mass surveillance. The focus is on algorithm-substitution attacks (ASAs), where a subverted encryption algorithm replaces the real one. We assume that the goal of ``big~brother\u27\u27 is undetectable subversion, meaning that ciphertexts produced by the subverted encryption algorithm should reveal plaintexts to big~brother yet be indistinguishable to users from those produced by the real encryption scheme. We formalize security notions to capture this goal and then offer both attacks and defenses. In the first category we show that successful (from the point of view of big brother) ASAs may be mounted on a large class of common symmetric encryption schemes. In the second category we show how to design symmetric encryption schemes that avoid such attacks and meet our notion of security. The lesson that emerges is the danger of choice: randomized, stateless schemes are subject to attack while deterministic, stateful ones are not

Big-Key Symmetric Encryption: Resisting Key Exfiltration

This paper aims to move research in the bounded retrieval model (BRM) from theory to practice by considering symmetric (rather than public-key) encryption, giving efficient schemes, and providing security analyses with sharp, concrete bounds. The threat addressed is malware that aims to exfiltrate a user\u27s key. Our schemes aim to thwart this by using an enormously long key, yet paying for this almost exclusively in storage cost, not speed. Our main result is a general-purpose lemma, the subkey prediction lemma, that gives a very good bound on an adversary\u27s ability to guess a (modest length) subkey of a big-key, the subkey consisting of the bits of the big-key found at random, specified locations, after the adversary has exfiltrated partial information about the big key (e.g., half as many bits as the big-key is long). We then use this to design a new kind of key encapsulation mechanism, and, finally, a symmetric encryption scheme. Both are in the random-oracle model. We also give a less efficient standard-model scheme that is based on universal computational extractors (UCE). Finally, we define and achieve hedged BRM symmetric encryption, which provides authenticity in the absence of leakage

Robust Authenticated-Encryption: AEZ and the Problem that it Solves

With a scheme for \textit{robust} authenticated-encryption a user can select an arbitrary value $\lambda \ge 0$ and then encrypt a plaintext of any length into a ciphertext that\u27s $\lambda$ characters longer. The scheme must provide all the privacy and authenticity possible for the requested~$\lambda$. We formalize and investigate this idea, and construct a well-optimized solution, AEZ, from the AES round function. Our scheme encrypts strings at almost the same rate as OCB-AES or CTR-AES (on Haswell, AEZ has a peak speed of about 0.7 cpb). To accomplish this we employ an approach we call \textit{prove-then-prune}: prove security and then instantiate with a \textit{scaled-down} primitive (e.g., reducing rounds for blockcipher calls)