International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Andre Scedrov

Affiliation: University of Pennsylvania

Publications

Year
Venue
Title
2019
JOFC
Automated Analysis of Cryptographic Assumptions in Generic Group Models
We initiate the study of principled, automated methods for analyzing hardness assumptions in generic group models, following the approach of symbolic cryptography. We start by defining a broad class of generic and symbolic group models for different settings—symmetric or asymmetric (leveled) k -linear groups—and by proving “computational soundness” theorems for the symbolic models. Based on this result, we formulate a very general master theorem that formally relates the hardness of a (possibly interactive) assumption in these models to solving problems in polynomial algebra. Then, we systematically analyze these problems. We identify different classes of assumptions and obtain decidability and undecidability results. Next, we develop and implement automated procedures for verifying the conditions of master theorems, and thus the validity of hardness assumptions in generic group models. The concrete outcome of this work is an automated tool which takes as input the statement of an assumption and outputs either a proof of its generic hardness or shows an algebraic attack against the assumption.
2015
EPRINT
2015
PKC
2014
CRYPTO
2014
EPRINT
2006
TCC
2006
EPRINT
Breaking and Fixing Public-Key Kerberos
We report on a man-in-the-middle attack on PKINIT, the public key extension of the widely deployed Kerberos 5 authentication protocol. This flaw allows an attacker to impersonate Kerberos administrative principals (KDC) and end-servers to a client, hence breaching the authentication guarantees of Kerberos. It also gives the attacker the keys that the KDC would normally generate to encrypt the service requests of this client, hence defeating confidentiality as well. The discovery of this attack caused the IETF to change the specification of PKINIT and Microsoft to release a security update for some Windows operating systems. We discovered this attack as part of an ongoing formal analysis of the Kerberos protocol suite, and we have formally verified several fixes to PKINIT that prevent our attack.
2006
EPRINT
Cryptographically Sound Security Proofs for Basic and Public-Key Kerberos
We present a computational analysis of basic Kerberos with and without its public-key extension PKINIT in which we consider authentication and key secrecy properties. Our proofs rely on the Dolev--Yao-style model of Backes, Pfitzmann, and Waidner, which allows for mapping results obtained symbolically within this model to cryptographically sound proofs if certain assumptions are met. This work was the first verification at the computational level of such a complex fragment of an industrial protocol. By considering a recently fixed version of PKINIT, we extend symbolic correctness results we previously attained in the Dolev--Yao model to cryptographically sound results in the computational model.
2005
EPRINT
Games and the Impossibility of Realizable Ideal Functionality
A cryptographic primitive or a security mechanism can be specified in a variety of ways, such as a condition involving a game against an attacker, construction of an ideal functionality, or a list of properties that must hold in the face of attack. While game conditions are widely used, an ideal functionality is appealing because a mechanism that is indistinguishable from an ideal functionality is therefore guaranteed secure in any larger system that uses it. We relate ideal functionalities to games by defining the \textit{set} of ideal functionalities associated with a game condition and show that under this definition, which reflects accepted use and known examples, bit commitment, a form of group signatures, and some other cryptographic concepts do not have any realizable ideal functionality.
2005
EPRINT
Key-dependent Message Security under Active Attacks -- BRSIM/UC-Soundness of Symbolic Encryption with Key Cycles
Key-dependent message security, short KDM security, was introduced by Black, Rogaway and Shrimpton to address the case where key cycles occur among encryptions, e.g., a key is encrypted with itself. It was mainly motivated by key cycles in Dolev-Yao models, i.e., symbolic abstractions of cryptography by term algebras, and a corresponding soundness result was later shown by Ad\~{a}o et al. However, both the KDM definition and this soundness result do not allow the general active attacks typical for Dolev-Yao models and for security protocols in general. We extend these definitions so that we can obtain a soundness result under active attacks. We first present a definition AKDM as a KDM equivalent of authenticated symmetric encryption, i.e., it provides chosen-ciphertext security and integrity of ciphertexts even for key cycles. However, this is not yet sufficient for the desired soundness, and thus we give a definition DKDM that additionally allows limited dynamic revelation of keys. We show that this is sufficient for soundness, even in the strong sense of blackbox reactive simulatability (BRSIM)/UC and including joint terms with other operators. We also present constructions of schemes secure under the new definitions, based on current KDM-secure schemes. Moreover, we explore the relations between the new definitions and existing ones for symmetric encryption in detail, in the sense of implications or separating examples for almost all cases.

Program Committees

Crypto 2011