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2021

Efficient Adaptively-Secure IB-KEMs and VRFs via Near-Collision Resistance

T. Jager, R. Kurek, D. Niehues, in: Public-Key Cryptography – PKC 2021, 2021

We construct more efficient cryptosystems with provable security against adaptive attacks, based on simple and natural hardness assumptions in the standard model. Concretely, we describe: – An adaptively-secure variant of the efficient, selectively-secure LWE- based identity-based encryption (IBE) scheme of Agrawal, Boneh, and Boyen (EUROCRYPT 2010). In comparison to the previously most efficient such scheme by Yamada (CRYPTO 2017) we achieve smaller lattice parameters and shorter public keys of size O(log λ), where λ is the security parameter. – Adaptively-secure variants of two efficient selectively-secure pairing- based IBEs of Boneh and Boyen (EUROCRYPT 2004). One is based on the DBDH assumption, has the same ciphertext size as the cor- responding BB04 scheme, and achieves full adaptive security with public parameters of size only O(log λ). The other is based on a q- type assumption and has public key size O(λ), but a ciphertext is only a single group element and the security reduction is quadrat- ically tighter than the corresponding scheme by Jager and Kurek (ASIACRYPT 2018). – A very efficient adaptively-secure verifiable random function where proofs, public keys, and secret keys have size O(log λ). As a technical contribution we introduce blockwise partitioning, which leverages the assumption that a cryptographic hash function is weak near-collision resistant to prove full adaptive security of cryptosystems.


    Verifiable Random Functions with Optimal Tightness

    D. Niehues, in: Public-Key Cryptography – PKC 2021, 2021

    Verifiable random functions (VRFs), introduced by Micali, Rabin and Vadhan (FOCS’99), are the public-key equivalent of pseudo- random functions. A public verification key and proofs accompanying the output enable all parties to verify the correctness of the output. How- ever, all known standard model VRFs have a reduction loss that is much worse than what one would expect from known optimal constructions of closely related primitives like unique signatures. We show that: 1. Every security proof for a VRF that relies on a non-interactive assumption has to lose a factor of Q, where Q is the number of adver- sarial queries. To that end, we extend the meta-reduction technique of Bader et al. (EUROCRYPT’16) to also cover VRFs. 2. This raises the question: Is this bound optimal? We answer this ques- tion in the affirmative by presenting the first VRF with a reduction from the non-interactive qDBDHI assumption to the security of VRF that achieves this optimal loss. We thus paint a complete picture of the achievability of tight verifiable random functions: We show that a security loss of Q is unavoidable and present the first construction that achieves this bound.


      2020

      On the Real-World Instantiability of Admissible Hash Functions and Efficient Verifiable Random Functions

      T. Jager, D. Niehues, in: Lecture Notes in Computer Science, 2020

      Verifiable random functions (VRFs) are essentially digital signatures with additional properties, namely verifiable uniqueness and pseudorandomness, which make VRFs a useful tool, e.g., to prevent enumeration in DNSSEC Authenticated Denial of Existence and the CONIKS key management system, or in the random committee selection of the Algorand blockchain. Most standard-model VRFs rely on admissible hash functions (AHFs) to achieve security against adaptive attacks in the standard model. Known AHF constructions are based on error-correcting codes, which yield asymptotically efficient constructions. However, previous works do not clarify how the code should be instantiated concretely in the real world. The rate and the minimal distance of the selected code have significant impact on the efficiency of the resulting cryptosystem, therefore it is unclear if and how the aforementioned constructions can be used in practice. First, we explain inherent limitations of code-based AHFs. Concretely, we assume that even if we were given codes that achieve the well-known Gilbert-Varshamov or McEliece-Rodemich-Rumsey-Welch bounds, existing AHF-based constructions of verifiable random functions (VRFs) can only be instantiated quite inefficiently. Then we introduce and construct computational AHFs (cAHFs). While classical AHFs are information-theoretic, and therefore work even in presence of computationally unbounded adversaries, cAHFs provide only security against computationally bounded adversaries. However, we show that cAHFs can be instantiated significantly more efficiently. Finally, we use our cAHF to construct the currently most efficient verifiable random function with full adaptive security in the standard model.


      2018

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

      D. Derler, T. Jager, D. Slamanig, C. Striecks, in: Advances in Cryptology - EUROCRYPT 2018 - 37th Annual International Conference on the Theory and Applications of Cryptographic Techniques, Tel Aviv, Israel, April 29 - May 3, 2018 Proceedings, Part III, 2018, pp. 425--455


      On the Security of the PKCS#1 v1.5 Signature Scheme

      T. Jager, S. Kakvi, A. May, in: Proceedings of the 2018 ACM SIGSAC Conference on Computer and Communications Security - CCS '18, ACM Press, 2018


      Short Digital Signatures and ID-KEMs via Truncation Collision Resistance

      T. Jager, R. Kurek, in: Advances in Cryptology - ASIACRYPT 2018 - 24th International Conference on the Theory and Application of Cryptology and Information Security, Brisbane, QLD, Australia, December 2-6, 2018, Proceedings, Part II, 2018, pp. 221-250


      Simple and More Efficient PRFs with Tight Security from LWE and Matrix-DDH

      T. Jager, R. Kurek, J. Pan, in: Advances in Cryptology - ASIACRYPT 2018 - 24th International Conference on the Theory and Application of Cryptology and Information Security, Brisbane, QLD, Australia, December 2-6, 2018, Proceedings, Part III, 2018, pp. 490-518


      Practical and Tightly-Secure Digital Signatures and Authenticated Key Exchange

      K. Gjøsteen, T. Jager, in: Advances in Cryptology - CRYPTO 2018 - 38th Annual International Cryptology Conference, Santa Barbara, CA, USA, August 19-23, 2018, Proceedings, Part II, 2018


      How to build time-lock encryption

      J. Liu, T. Jager, S. Kakvi, B. Warinschi, Designs, Codes and Cryptography (2018), pp. 2549-2586


      2017

      Authenticated Confidential Channel Establishment and the Security of TLS-DHE

      T. Jager, F. Kohlar, S. Schäge, J. Schwenk, J. Cryptology (2017)(4), pp. 1276--1324


      0-RTT Key Exchange with Full Forward Secrecy

      F. Günther, B. Hale, T. Jager, S. Lauer, in: Advances in Cryptology - EUROCRYPT 2017 - 36th Annual International Conference on the Theory and Applications of Cryptographic Techniques, Paris, France, April 30 - May 4, 2017, Proceedings, Part III, 2017, pp. 519--548


      Simple Security Definitions for and Constructions of 0-RTT Key Exchange

      B. Hale, T. Jager, S. Lauer, J. Schwenk, in: Applied Cryptography and Network Security - 15th International Conference, ACNS 2017, Kanazawa, Japan, July 10-12, 2017, Proceedings, 2017, pp. 20--38


      Multi-key Authenticated Encryption with Corruptions: Reductions Are Lossy

      T. Jager, M. Stam, R. Stanley-Oakes, B. Warinschi, in: Theory of Cryptography - 15th International Conference, TCC 2017, Baltimore, MD, USA, November 12-15, 2017, Proceedings, Part I, 2017, pp. 409--441


      Optimal Security Proofs for Full Domain Hash, Revisited

      S. Kakvi, E. Kiltz, Journal of Cryptology (2017), pp. 276-306


      2016

      Tightly secure signatures and public-key encryption

      D. Hofheinz, T. Jager, Des. Codes Cryptography (2016)(1), pp. 29--61


      On the Impossibility of Tight Cryptographic Reductions

      C. Bader, T. Jager, Y. Li, S. Schäge, in: Advances in Cryptology - EUROCRYPT 2016 - 35th Annual International Conference on the Theory and Applications of Cryptographic Techniques, Vienna, Austria, May 8-12, 2016, Proceedings, Part II, 2016, pp. 273--304


      Selective opening security of practical public-key encryption schemes

      F. Heuer, T. Jager, S. Schäge, E. Kiltz, IET Information Security (2016)(6), pp. 304--318


      Public-Key Encryption with Simulation-Based Selective-Opening Security and Compact Ciphertexts

      D. Hofheinz, T. Jager, A. Rupp, in: Theory of Cryptography - 14th International Conference, TCC 2016-B, Beijing, China, October 31 - November 3, 2016, Proceedings, Part II, 2016, pp. 146--168


      Black-Box Accumulation: Collecting Incentives in a Privacy-Preserving Way

      T. Jager, A. Rupp, PoPETs (2016)(3), pp. 62--82


      Verifiable Random Functions from Standard Assumptions

      D. Hofheinz, T. Jager, in: Theory of Cryptography - 13th International Conference, TCC 2016-A, Tel Aviv, Israel, January 10-13, 2016, Proceedings, Part I, 2016, pp. 336--362


      How to Generate and Use Universal Samplers

      D. Hofheinz, T. Jager, D. Khurana, A. Sahai, B. Waters, M. Zhandry, in: Advances in Cryptology - ASIACRYPT 2016 - 22nd International Conference on the Theory and Application of Cryptology and Information Security, Hanoi, Vietnam, December 4-8, 2016, Proceedings, Part II, 2016, pp. 715--744


      Breaking PPTP VPNs via RADIUS Encryption

      M. Horst, M. Grothe, T. Jager, J. Schwenk, in: Cryptology and Network Security - 15th International Conference, CANS 2016, Milan, Italy, November 14-16, 2016, Proceedings, 2016, pp. 159--175


      Black-Box Accumulation: Collecting Incentives in a Privacy-Preserving Way

      T. Jager, A. Rupp, Proceedings on Privacy Enhancing Technologies (2016), 2016(3), pp. 62-82


      2015

      One-Round Key Exchange with Strong Security: An Efficient and Generic Construction in the Standard Model

      F. Bergsma, T. Jager, J. Schwenk, in: Public-Key Cryptography - PKC 2015 - 18th IACR International Conference on Practice and Theory in Public-Key Cryptography, Gaithersburg, MD, USA, March 30 - April 1, 2015, Proceedings, 2015, pp. 477--494


      Confined Guessing: New Signatures From Standard Assumptions

      F. Böhl, D. Hofheinz, T. Jager, J. Koch, C. Striecks, J. Cryptology (2015)(1), pp. 176--208


      Verifiable Random Functions from Weaker Assumptions

      T. Jager, in: Theory of Cryptography - 12th Theory of Cryptography Conference, TCC 2015, Warsaw, Poland, March 23-25, 2015, Proceedings, Part {II}, 2015, pp. 121--143


      Tightly-Secure Signatures from Chameleon Hash Functions

      O. Blazy, S. Kakvi, E. Kiltz, J. Pan, in: Lecture Notes in Computer Science, Springer Berlin Heidelberg, 2015, pp. 256-279


      On the Security of TLS 1.3 and QUIC Against Weaknesses in PKCS#1 v1.5 Encryption

      T. Jager, J. Schwenk, J. Somorovsky, in: Proceedings of the 22nd ACM SIGSAC Conference on Computer and Communications Security, Denver, CO, USA, October 12-16, 2015, 2015, pp. 1185--1196


      Tightly-Secure Authenticated Key Exchange

      C. Bader, D. Hofheinz, T. Jager, E. Kiltz, Y. Li, in: Theory of Cryptography - 12th Theory of Cryptography Conference, TCC 2015, Warsaw, Poland, March 23-25, 2015, Proceedings, Part I, 2015, pp. 629--658


      Practical Invalid Curve Attacks on TLS-ECDH

      T. Jager, J. Schwenk, J. Somorovsky, in: Computer Security - ESORICS 2015 - 20th European Symposium on Research in Computer Security, Vienna, Austria, September 21-25, 2015, Proceedings, Part I, 2015, pp. 407--425


      On the Selective Opening Security of Practical Public-Key Encryption Schemes

      F. Heuer, T. Jager, E. Kiltz, S. Schäge, in: Public-Key Cryptography - PKC 2015 - 18th IACR International Conference on Practice and Theory in Public-Key Cryptography, Gaithersburg, MD, USA, March 30 - April 1, 2015, Proceedings, 2015, pp. 27--51


      2014

      Die Zukunft der Kryptographie

      T. Jager, Datenschutz und Datensicherheit (2014)(7), pp. 445--451


      On Tight Security Proofs for Schnorr Signatures

      N. Fleischhacker, T. Jager, D. Schröder, in: Advances in Cryptology - ASIACRYPT 2014 - 20th International Conference on the Theory and Application of Cryptology and Information Security, Kaoshiung, Taiwan, R.O.C., December 7-11, 2014. Proceedings, Part I, 2014, pp. 512--531


      2013

      On the Analysis of Cryptographic Assumptions in the Generic Ring Model

      T. Jager, J. Schwenk, J. Cryptology (2013)(2), pp. 225--245


      One Bad Apple: Backwards Compatibility Attacks on State-of-the-Art Cryptography

      T. Jager, K.G. Paterson, J. Somorovsky, in: 20th Annual Network and Distributed System Security Symposium, NDSS 2013, San Diego, California, USA, February 24-27, 2013, 2013


      Practical Signatures from Standard Assumptions

      F. Böhl, D. Hofheinz, T. Jager, J. Koch, J. Hong Seo, C. Striecks, in: Advances in Cryptology - EUROCRYPT 2013, 32nd Annual International Conference on the Theory and Applications of Cryptographic Techniques, Athens, Greece, May 26-30, 2013. Proceedings, 2013, pp. 461--485


      2012

      Waters Signatures with Optimal Security Reduction

      D. Hofheinz, T. Jager, E. Knapp, in: Public Key Cryptography - PKC 2012 - 15th International Conference on Practice and Theory in Public Key Cryptography, Darmstadt, Germany, May 21-23, 2012. Proceedings, 2012, pp. 66--83


      Certifying RSA

      S. Kakvi, E. Kiltz, A. May, in: Advances in Cryptology – ASIACRYPT 2012, Springer Berlin Heidelberg, 2012, pp. 404-414


      On black-box models of computation in cryptology

      T. Jager, Ruhr University Bochum, 2012


      Optimal Security Proofs for Full Domain Hash, Revisited

      S. Kakvi, E. Kiltz, in: Advances in Cryptology – EUROCRYPT 2012, Springer Berlin Heidelberg, 2012, pp. 537-553


      On the Security of TLS-DHE in the Standard Model

      T. Jager, F. Kohlar, S. Schäge, J. Schwenk, in: Advances in Cryptology - CRYPTO 2012 - 32nd Annual Cryptology Conference, Santa Barbara, CA, USA, August 19-23, 2012. Proceedings, 2012, pp. 273--293


      Tightly Secure Signatures and Public-Key Encryption

      D. Hofheinz, T. Jager, in: Advances in Cryptology - CRYPTO 2012 - 32nd Annual Cryptology Conference, Santa Barbara, CA, USA, August 19-23, 2012. Proceedings, 2012, pp. 590--607


      Bleichenbacher's Attack Strikes again: Breaking PKCS#1 v1.5 in XML Encryption

      T. Jager, S. Schinzel, J. Somorovsky, in: Computer Security - ESORICS 2012 - 17th European Symposium on Research in Computer Security, Pisa, Italy, September 10-12, 2012. Proceedings, 2012, pp. 752--769


      2011

      Short Signatures from Weaker Assumptions

      D. Hofheinz, T. Jager, E. Kiltz, in: Advances in Cryptology - ASIACRYPT 2011 - 17th International Conference on the Theory and Application of Cryptology and Information Security, Seoul, South Korea, December 4-8, 2011. Proceedings, 2011, pp. 647--666


      How to break XML encryption

      T. Jager, J. Somorovsky, in: Proceedings of the 18th ACM Conference on Computer and Communications Security, CCS 2011, Chicago, Illinois, USA, October 17-21, 2011, 2011, pp. 413--422


      2010

      The Semi-Generic Group Model and Applications to Pairing-Based Cryptography

      T. Jager, A. Rupp, in: Advances in Cryptology - ASIACRYPT 2010 - 16th International Conference on the Theory and Application of Cryptology and Information Security, Singapore, December 5-9, 2010. Proceedings, 2010, pp. 539--556


      Generic Compilers for Authenticated Key Exchange

      T. Jager, F. Kohlar, S. Schäge, J. Schwenk, in: Advances in Cryptology - ASIACRYPT 2010 - 16th International Conference on the Theory and Application of Cryptology and Information Security, Singapore, December 5-9, 2010. Proceedings, 2010, pp. 232--249


      Simple and Efficient Public-Key Encryption from Computational Diffie-Hellman in the Standard Model

      K. Haralambiev, T. Jager, E. Kiltz, V. Shoup, in: Public Key Cryptography - PKC 2010, 13th International Conference on Practice and Theory in Public Key Cryptography, Paris, France, May 26-28, 2010. Proceedings, 2010, pp. 1--18


      2009

      Reinforcement Learning for Blackjack

      S. Kakvi, in: Lecture Notes in Computer Science, Springer Berlin Heidelberg, 2009, pp. 300-301


      Reinforcement Learning for Blackjack

      S. Kakvi, in: Lecture Notes in Computer Science, 2009


      On the Analysis of Cryptographic Assumptions in the Generic Ring Model

      T. Jager, J. Schwenk, in: Advances in Cryptology - ASIACRYPT 2009, 15th International Conference on the Theory and Application of Cryptology and Information Security, Tokyo, Japan, December 6-10, 2009. Proceedings, 2009, pp. 399--416


      2008

      A Browser-Based Kerberos Authentication Scheme

      S. Gajek, T. Jager, M. Manulis, J. Schwenk, in: Computer Security - ESORICS 2008, 13th European Symposium on Research in Computer Security, Málaga, Spain, October 6-8, 2008. Proceedings, 2008, pp. 115--129


      On Black-Box Ring Extraction and Integer Factorization

      K. Altmann, T. Jager, A. Rupp, in: Automata, Languages and Programming, 35th International Colloquium, ICALP 2008, Reykjavik, Iceland, July 7-11, 2008, Proceedings, Part II - Track B: Logic, Semantics, and Theory of Programming & Track C: Security and Cryptography Foundations, 2008, pp. 437--448


      On the Equivalence of Generic Group Models

      T. Jager, J. Schwenk, in: Provable Security, Second International Conference, ProvSec 2008, Shanghai, China, October 30 - November 1, 2008. Proceedings, 2008, pp. 200--209


      Nutzung von selbstsignierten Client-Zertifikaten zur Authentifikation bei SSL/TLS

      T. Jager, H. Jäkel, J. Schwenk, in: Sicherheit 2008: Sicherheit, Schutz und Zuverlässigkeit. Konferenzband der 4. Jahrestagung des Fachbereichs Sicherheit der Gesellschaft für Informatik e.V. (GI), 2.-4. April 2008 im Saarbrücker Schloss., 2008, pp. 127--136


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