Paul C. van Oorschot

Authentication and authenticated key exchanges

We discuss two-party mutual authentication protocols providing authenticated key exchange, focusing on those using asymmetric techniques. A simple, efficient protocol referred to as the station-to-station (STS) protocol is introduced, examined in detail, and considered in relation to existing protocols. The definition of a secure protocol is considered, and desirable characteristics of secure protocols are discussed.

Parallel Collision Search with Cryptanalytic Applications

Abstract. A simple new technique of parallelizing methods for solving search problems which seek collisions in pseudorandom walks is presented. This technique can be adapted to a wide range of cryptanalytic problems which can be reduced to finding collisions. General constructions are given showing how to adapt the technique to finding discrete logarithms in cyclic groups, finding meaningful collisions in hash functions, and performing meet-in-the-middle attacks such as a known-plaintext attack on double encryption. The new technique greatly extends the reach of practical attacks, providing the most cost-effective means known to date for defeating: the small subgroup used in certain schemes based on discrete logarithms such as Schnorr, DSA, and elliptic curve cryptosystems; hash functions such as MD5, RIPEMD, SHA-1, MDC-2, and MDC-4; and double encryption and three-key triple encryption. The practical significance of the technique is illustrated by giving the design for three

A methodology for empirical analysis of permission-based security models and its application to android

10 million custom machines which could be built with current technology: one finds elliptic curve logarithms in GF(2155) thereby defeating a proposed elliptic curve cryptosystem in expected time 32 days, the second finds MD5 collisions in expected time 21 days, and the last recovers a double-DES key from two known plaintexts in expected time 4 years, which is four orders of magnitude faster than the conventional meet-in-the-middle attack on double-DES. Based on this attack, double-DES offers only 17 more bits of security than single-DES.

White-Box Cryptography and an AES Implementation

Permission-based security models provide controlled access to various system resources. The expressiveness of the permission set plays an important role in providing the right level of granularity in access control. In this work, we present a methodology for the empirical analysis of permission-based security models which makes novel use of the Self-Organizing Map (SOM) algorithm of Kohonen (2001). While the proposed methodology may be applicable to a wide range of architectures, we analyze 1,100 Android applications as a case study. Our methodology is of independent interest for visualization of permission-based systems beyond our present Android-specific empirical analysis. We offer some discussion identifying potential points of improvement for the Android permission model attempting to increase expressiveness where needed without increasing the total number of permissions or overall complexity.

MDx-MAC and Building Fast MACs from Hash Functions

Conventional software implementations of cryptographic algorithms are totally insecure where a hostile user may control the execution environment, or where co-located with malicious software. Yet current trends point to increasing usage in environments so threatened. We discuss encrypted-composed-function methods intended to provide a practical degree of protection against white-box (total access) attacks in untrusted execution environments. As an example, we show how AES can be implemented as a series of lookups in key-dependent tables. The intent is to hide the key by a combination of encoding its tables with random bijections representing compositions rather than individual steps, and extending the cryptographic boundary by pushing it out further into the containing application. We partially justify our AES implementation, and motivate its design, by showing how removal of parts of the recommended implementation allows specified attacks, including one utilizing a pattern in the AES SubBytes table.

A White-Box DES Implementation for DRM Applications

We consider the security of message authentication code (MAC) algorithms, and the construction of MACs from fast hash functions. A new forgery attack applicable to all iterated MAC algorithms is described, the first known such attack requiring fewer operations than exhaustive key search. Existing methods for constructing MACs from hash functions, including the secret prefix, secret suffix, and envelope methods, are shown to be unsatisfactory. Motivated by the absence of a secure, fast MAC algorithm not based on encryption, a new generic construction (MDx-MAC) is proposed for transforming any secure hash function of the MD4-family into a secure MAC of equal or smaller bitlength and comparable speed.

Parallel collision search with application to hash functions and discrete logarithms

For digital rights management (drm) software implementations incorporating cryptography, white-box cryptography (cryptographic implementation designed to withstand the white-box attack context) is more appropriate than traditional black-box cryptography. In the white-box context, the attacker has total visibility into software implementation and execution. Our objective is to prevent extraction of secret keys from the program. We present methods to make such key extraction difficult, with focus on symmetric block ciphers implemented by substitution boxes and linear transformations. A des implementation (useful also for triple-des) is presented as a concrete example.

Extending cryptographic logics of belief to key agreement protocols

Current techniques for collision search with feasible memory requirements involve pseudo-random walks through some space where one must wait for the result of the current step before the next step can begin. These techniques are serial in nature, and direct parallelization is inefficient. We present a simple new method of parallelizing collision searches that greatly extends the reach of practical attacks. The new method is illustrated with applications to hash functions and discrete logarithms in cyclic groups. In the case of hash functions, we begin with two messages; the first is a message that we want our target to digitally sign, and the second is a message that the target is willing to sign. Using collision search adapted for hashing collisions, one can find slightly altered versions of these messages such that the two new messages give the same hash result. As a particular example, a

On the security of two MAC algorithms

10 million custom machine for applying parallel collision search to the MD5 hash function could complete an attack with an expected run time of 24 days. This machine would be specific to MD5, but could be used for any pair of messages. For discrete logarithms in cyclic groups, ideas from Pollards rho and lambda methods for index computation are combined to allow efficient parallel implementation using the new method. As a concrete example, we consider an elliptic curve cryptosystem over GF(2155) with the order of the curve having largest prime factor of approximate size 1036. A

Passwords: If We're So Smart, Why Are We Still Using Them?

10 million machine custom built for this finite field could compute a discrete logarithm with an expected run time of 36 days.