Christoph Striecks

selected publications

1. Bloom Filter Encryption and Applications to Efficient Forward-Secret 0-RTT Key Exchange

   With David Derler, Kai Gellert, Tibor Jager, and Daniel Slamanig.

   Journal of Cryptology, Volume 34, 2021.

   Abstract URL PDF ePrint

   Forward secrecy is considered an essential design goal of modern key establishment (KE) protocols, such as TLS 1.3, for example. Furthermore, efficiency considerations such as zero round-trip time (0-RTT), where a client is able to send cryptographically protected payload data along with the very first KE message, are motivated by the practical demand for secure low-latency communication. For a long time, it was unclear whether protocols that simultaneously achieve 0-RTT and full forward secrecy exist. Only recently, the first forward-secret 0-RTT protocol was described by Günther et al. (EUROCRYPT 2017). It is based on Puncturable Encryption. Forward secrecy is achieved by “puncturing” the secret key after each decryption operation, such that a given ciphertext can only be decrypted once (cf. also Green and Miers, S&P 2015). Unfortunately, their scheme is completely impractical, since one puncturing operation takes between 30 seconds and several minutes for reasonable security and deployment parameters, such that this solution is only a first feasibility result, but not efficient enough to be deployed in practice. In this paper, we introduce a new primitive that we term Bloom Filter Encryption (BFE), which is derived from the probabilistic Bloom filter data structure. We describe different constructions of BFE schemes, and show how these yield new puncturable encryption mechanisms with extremely efficient puncturing. Most importantly, a puncturing operation only involves a small number of very efficient computations, plus the deletion of certain parts of the secret key, which outperforms previous constructions by orders of magnitude. This gives rise to the first forward-secret 0-RTT protocols that are efficient enough to be deployed in practice. We believe that BFE will find applications beyond forward-secret 0-RTT protocols.

2. Fine-Grained and Controlled Rewriting in Blockchains: Chameleon-Hashing Gone Attribute-Based

   With David Derler, Kai Samelin, and Daniel Slamanig.

   NDSS 2019, San Diego, USA, February 24-27. (Invited to ACM Advances in Financial Technologies (AFT 2019) as highlights-track presentation.)

   Abstract URL PDF ePrint Talk Slides

   Blockchain technologies recently received a considerable amount of attention. While the initial focus was mainly on the use of blockchains in the context of cryptocurrencies such as Bitcoin, application scenarios now go far beyond this. Most blockchains have the property that once some object, e.g., a block or a transaction, has been registered to be included into the blockchain, it is persisted and there are no means to modify it again. While this is an essential feature of most blockchain scenarios, it is still often desirable—at times it may be even legally required–to allow for breaking this immutability in a controlled way. Only recently, Ateniese et al. (EuroS&P 2017) proposed an elegant solution to this problem on the block level. Thereby, the authors replace standard hash functions with so-called chameleon-hashes (Krawczyk and Rabin, NDSS 2000). While their work seems to offer a suited solution to the problem of controlled re-writing of blockchains, their approach is too coarse-grained in that it only offers an all-or-nothing solution. We revisit this idea and introduce the novel concept of policy-based chameleon-hashes (PBCH). PBCHs generalize the notion of chameleon-hashes by giving the party computing a hash the ability to associate access policies to the generated hashes. Anyone who possesses enough privileges to satisfy the policy can then find arbitrary collisions for a given hash. We then apply this concept to transaction-level rewriting within blockchains, and thus support fine-grained and controlled modifiability of blockchain objects. Besides modeling PBCHs, we present a generic construction of PBCHs (using a strengthened version of chameleon-hashes with ephemeral trapdoors which we also introduce), rigorously prove its security, and instantiate it with efficient building blocks. We report first implementation results.

3. Identity-Based Encryption with (Almost) Tight Security in the Multi-instance, Multi-ciphertext Setting

   With Dennis Hofheinz, and Jessica Koch.

   PKC 2015, Gaithersburg, MD, USA, March 30 - April 1. (Invited to the Journal of Cryptology.)

   Abstract URL PDF ePrint

   We construct an identity-based encryption (IBE) scheme that is tightly secure in a very strong sense. Specifically, we consider a setting with many instances of the scheme and many encryptions per instance. In this setting, we reduce the security of our scheme to a variant of a simple assumption used for a similar purpose by Chen and Wee (Crypto 2013). The security loss of our reduction is O(k)(where k is the security parameter). Our scheme is the first IBE scheme to achieve this strong flavor of tightness under a simple assumption. Technically, our scheme is a variation of the IBE scheme by Chen and Wee. However, in order to "lift" their results to the multi-instance, multi-ciphertext case, we need to develop new ideas. In particular, while we build on (and extend) their high-level proof strategy, we deviate significantly in the low-level proof steps.

4. Programmable Hash Functions in the Multilinear Setting

   With Eduarda S. V. Freire, Dennis Hofheinz, and Kenneth G. Paterson.

   CRYPTO 2013, Santa Barbara, USA, August 18-22.

   Abstract URL PDF ePrint

   We adapt the concept of a programmable hash function (PHF, Crypto 2008) to a setting in which a multilinear map is available. This enables new PHFs with previously unachieved parameters. To demonstrate their usefulness, we show how our (standard-model) PHFs can replace random oracles in several well-known cryptographic constructions. Namely, we obtain standard-model versions of the Boneh-Franklin identity-based encryption scheme, the Boneh-Lynn-Shacham signature scheme, and the Sakai-Ohgishi-Kasahara identity-based non- interactive key exchange (ID-NIKE) scheme. The ID-NIKE scheme is the first scheme of its kind in the standard model. Our abstraction also allows to derive hierarchical versions of the above schemes in settings with multilinear maps. This in particular yields simple and efficient hierarchical generalizations of the BF, BLS, and SOK schemes. In the case of hierarchical ID-NIKE, ours is the first such scheme with full security, in either the random oracle model or the standard model. While our constructions are formulated with respect to a generic multilinear map, we also outline the necessary adaptations required for the recent “noisy” multilinear map candidate due to Garg, Gentry, and Halevi.

selected talks

1. Puncturable Encryption - A Fine-Grained Approach to Forward Security and More

   Real World Crypto Symposium 2022, Amsterdam, April 13-15.

   Abstract URL Video Slides

   Forward security is an essential design goal of modern cryptographic protocols with a long body of literature in several application domains such as interactive key-establishment protocols (prominently, in TLS 1.3), digital signatures, search on encrypted data, updatable cryptography, mobile Cloud backups, decentralized contact tracing, new approaches to Tor, and even novel distributed protocols such as Dfinity’s Internet Computer, among others. The well-known benefit of forward security is the mitigation of key leakage by evolving secret keys over epochs and thereby revoking access to prior-epoch ciphertexts. Such a strong security guarantee is highly recognized by industry to be included into security products (e.g., by companies such as Google, Apple, Meta, Microsoft, and Cloudflare), particularly resulting in over 99% of Internet sites surveyed by Qualys SSL Labs (https://www.ssllabs.com/ssl-pulse) support at least some form of forward security at the time of writing. Green and Miers (S&P 2015) initiated the studies of puncturable encryption (PE) as a new cryptographic primitive towards the strong form of non-interactive forward-secure encryption (in particular, without the need of any pre-shared key material). Already several follow-up works showed the versatility of such a concept yielding a rich abstraction of forward security investigated in a variety of (data-in-transit and data-at-rest) application domains such as 0-RTT key exchange with replay protection for TLS (Eurocrypt’17, Eurocrypt’18, Asiacrypt’20, JoC’21), Google’s QUIC (Cans’20), Searchable Encryption (CCS’17, CCS’18, NDSS’21), mobile Cloud backups (OSDI’20), Content Distribution Networks (Financial Crypto’21), Tor (PoPETS’20), and Updatable Encryption (ePrint’21). Loosely speaking, PE is a promising variant of public-key encryption that allows realizing the property of fine-grained and non-interactive forward security with several useful applications. This talk provides an exhausting overview to the concept of PE with motivation, presents state-of-the-art research on PE schemes, and discusses applications (such as 0-RTT key exchange with forward security and replay protection, and forward security for Content Delivery Networks). The overall goal is to make PE more accessible to the general audience and industry in a developer-friendly way, and also presenting new insights and results. The presentation builds on an existing blog post with the same title (https://profet.at/blog/pe).