International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Thomas Ristenpart

Publications

Year
Venue
Title
2019
CRYPTO
Asymmetric Message Franking: Content Moderation for Metadata-Private End-to-End Encryption 📺
Content moderation is crucial for stopping abusive and harassing messages in online platforms. Existing moderation mechanisms, such as message franking, require platform providers to be able to associate user identifiers to encrypted messages. These mechanisms fail in metadata-private messaging systems, such as Signal, where users can hide their identities from platform providers. The key technical challenge preventing moderation is achieving cryptographic accountability while preserving deniability.In this work, we resolve this tension with a new cryptographic primitive: asymmetric message franking (AMF) schemes. We define strong security notions for AMF schemes, including the first formal treatment of deniability in moderation settings. We then construct, analyze, and implement an AMF scheme that is fast enough to use for content moderation of metadata-private messaging.
2018
CRYPTO
Fast Message Franking: From Invisible Salamanders to Encryptment 📺
Message franking enables cryptographically verifiable reporting of abusive messages in end-to-end encrypted messaging. Grubbs, Lu, and Ristenpart recently formalized the needed underlying primitive, what they call compactly committing authenticated encryption (AE), and analyze security of a number of approaches. But all known secure schemes are still slow compared to the fastest standard AE schemes. For this reason Facebook Messenger uses AES-GCM for franking of attachments such as images or videos.We show how to break Facebook’s attachment franking scheme: a malicious user can send an objectionable image to a recipient but that recipient cannot report it as abuse. The core problem stems from use of fast but non-committing AE, and so we build the fastest compactly committing AE schemes to date. To do so we introduce a new primitive, called encryptment, which captures the essential properties needed. We prove that, unfortunately, schemes with performance profile similar to AES-GCM won’t work. Instead, we show how to efficiently transform Merkle-Damgärd-style hash functions into secure encryptments, and how to efficiently build compactly committing AE from encryptment. Ultimately our main construction allows franking using just a single computation of SHA-256 or SHA-3. Encryptment proves useful for a variety of other applications, such as remotely keyed AE and concealments, and our results imply the first single-pass schemes in these settings as well.
2017
EUROCRYPT
2017
CRYPTO
2017
CRYPTO
2017
CRYPTO
2016
EUROCRYPT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
EUROCRYPT
2014
EUROCRYPT
2014
EPRINT
2014
FSE
2013
CRYPTO
2013
EUROCRYPT
2012
TCC
2012
CRYPTO
2012
CRYPTO
2011
EUROCRYPT
2011
ASIACRYPT
2010
ASIACRYPT
2009
ASIACRYPT
2009
EUROCRYPT
2009
EUROCRYPT
2009
EPRINT
Simulation without the Artificial Abort: Simplified Proof and Improved Concrete Security for Waters' IBE Scheme
Mihir Bellare Thomas Ristenpart
Waters' variant of the Boneh-Boyen IBE scheme is attractive because of its efficency, applications, and security attributes,but suffers from a relatively complex proof with poor concrete security. This is due in part to the proof's ``artificial abort'' step, which has then been inherited by numerous derivative works. It has often been asked whether this step is necessary. We show that it is not, providing a new proof that eliminates this step. The new proof is not only simpler than the original one but offers better concrete security for important ranges of the parameters. As a result, one can securely use smaller groups, resulting in significant efficiency improvements.
2008
EPRINT
How to Build a Hash Function from any Collision-Resistant Function
Thomas Ristenpart Thomas Shrimpton
Recent collision-finding attacks against hash functions such as MD5 and SHA-1 motivate the use of provably collision-resistant (CR) functions in their place. Finding a collision in a provably CR function implies the ability to solve some hard problem (e.g., factoring). Unfortunately, existing provably CR functions make poor replacements for hash functions as they fail to deliver behaviors demanded by practical use. In particular, they are easily distinguished from a random oracle. We initiate an investigation into building hhash functions from provably CR functions. As a method for achieving this, we present the Mix-Compress-Mix (MCM) construction; it envelopes any provably CR function H (with suitable regularity properties) between two injective ``mixing'' stages. The MCM construction simultaneously enjoys (1) provable collision-resistance in the standard model, and (2) indifferentiability from a monolithic random oracle when the mixing stages themselves are indifferentiable from a random oracle that observes injectivity. We instantiate our new design approach by specifying a blockcipher-based construction that appropriately realizes the mixing stages.
2008
EPRINT
Deterministic Encryption: Definitional Equivalences and Constructions without Random Oracles
We strengthen the foundations of deterministic public-key encryption via definitional equivalences and standard-model constructs based on general assumptions. Specifically we consider seven notions of privacy for deterministic encryption, including six forms of semantic security and an indistinguishability notion, and show them all equivalent. We then present a deterministic scheme for the secure encryption of uniformly and independently distributed messages based solely on the existence of trapdoor one-way permutations. We show a generalization of the construction that allows secure deterministic encryption of independent high-entropy messages. Finally we show relations between deterministic and standard (randomized) encryption.
2008
CRYPTO
2007
ASIACRYPT
2007
EUROCRYPT
2007
FSE
2007
EPRINT
How to Enrich the Message Space of a Cipher
Thomas Ristenpart Phillip Rogaway
Given (deterministic) ciphers $\calE$ and~$E$ that can encipher messages of $\el$ and $n$ bits, respectively, we construct a cipher~$\calE^*=XLS[\calE,E]$ that can encipher messages of $\el+s$ bits for any $s<n$. Enciphering such a string will take one call to~$\calE$ and two calls to~$E$. We prove that~$\calE^*$ is a strong pseudorandom permutation as long as~$\calE$ and~$E$ are. Our construction works even in the tweakable and VIL (variable-input-length) settings. It makes use of a multipermutation (a pair of orthogonal Latin squares), a combinatorial object not previously used to get a provable-security result.
2007
EPRINT
The Power of Proofs-of-Possession: Securing Multiparty Signatures against Rogue-Key Attacks
Thomas Ristenpart Scott Yilek
Multiparty signature protocols need protection against rogue-key attacks, made possible whenever an adversary can choose its public key(s) arbitrarily. For many schemes, provable security has only been established under the knowledge of secret key (KOSK) assumption where the adversary is required to reveal the secret keys it utilizes. In practice, certifying authorities rarely require the strong proofs of knowledge of secret keys required to substantiate the KOSK assumption. Instead, proofs of possession (POPs) are required and can be as simple as just a signature over the certificate request message. We propose a general registered key model, within which we can model both the KOSK assumption and in-use POP protocols. We show that simple POP protocols yield provable security of Boldyreva's multisignature scheme [11], the LOSSW multisignature scheme [28], and a 2-user ring signature scheme due to Bender, Katz, and Morselli [10]. Our results are the first to provide formal evidence that POPs can stop rogue-key attacks.
2007
EPRINT
Hash Functions in the Dedicated-Key Setting: Design Choices and MPP Transforms
Mihir Bellare Thomas Ristenpart
In the dedicated-key setting, one starts with a compression function f:{0,1}^k x {0,1}^{n+d} -> {0,1}^n and builds a family of hash functions H^f:K x M -> {0,1}^n indexed by a key space K. This is different from the more traditional design approach used to build hash functions such as MD5 or SHA-1, in which compression functions and hash functions do not have dedicated key inputs. We explore the benefits and drawbacks of building hash functions in the dedicated-key setting (as compared to the more traditional approach), highlighting several unique features of the former. Should one choose to build hash functions in the dedicated-key setting, we suggest utilizing multi-property-preserving (MPP) domain extension transforms. We analyze seven existing dedicated-key transforms with regard to the MPP goal and propose two simple new MPP transforms.
2006
ASIACRYPT
2006
EPRINT
Multi-Property-Preserving Hash Domain Extension and the EMD Transform
Mihir Bellare Thomas Ristenpart
We point out that the seemingly strong pseudorandom oracle preserving (PRO-Pr) property of hash function domain-extension transforms defined and implemented by Coron et. al. [12] can actually weaken our guarantees on the hash function, in particular producing a hash function that fails to be even collision-resistant (CR) even though the compression function to which the transform is applied is CR. Not only is this true in general, but we show that all the transforms presented in [12] have this weakness. We suggest that the appropriate goal of a domain extension transform for the next generation of hash functions is to be multi-property preserving, namely that one should have a single transform that is simultaneously at least collision-resistance preserving, pseudorandom function preserving and PRO-Pr. We present an efficient new transform that is proven to be multi-property preserving in this sense.

Program Committees

Crypto 2020 (Program chair)
Eurocrypt 2018
Eurocrypt 2016
Eurocrypt 2014
Crypto 2013
Eurocrypt 2012
FSE 2010
FSE 2009