Abstract The block cipher DESX is defined by DESX k:k1:k2 (x) = k2 \Phi DES k (k1 \Phi x), where \Phi denotes bitwise exclusive-or. This construction was first suggested by Rivest as a computationallycheap way to protect DES against exhaustive key-search attacks. This paper proves, in a formal model, that the DESX construction is sound. We show that, when F is an idealized block cipher, FX

. 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. 1 Introduction Hash functions play a fundamental role in modern cryptography. One main application is their use in conjunction with digital signature schemes; another is in conventional techniques for message authentication. In the latter, it is preferable that a hash function take as a d...

Twofish is a 128-bit block cipher that accepts a variable-length key up to 256 bits. The cipher is a 16-round Feistel network with a bijective F function made up of four key-dependent 8-by-8-bit S-boxes, a fixed 4-by-4 maximum distance separable matrix over GF(2 8 ), a pseudo-Hadamard transform, bitwise rotations, and a carefully designed key schedule. A fully optimized implementation of Twofish encrypts on a Pentium Pro at 17.8 clock cycles per byte, and an 8-bit smart card implementation encrypts at 1660 clock cycles per byte. Twofish can be implemented in hardware in 14000 gates. The design of both the round function and the key schedule permits a wide variety of tradeoffs between speed, software size, key setup time, gate count, and memory. We have extensively cryptanalyzed Twofish; our best attack breaks 5 rounds with 2 22.5 chosen plaintexts and 2 51 effort.

this paper we consider the so called plaintext-ciphertext attack. Here the cryptanalyst obtains a plaintext and its corresponding ciphertext and wishes to determine the key used to perform the encryption. The most naive approach to this problem is to try all 2

Cryptanalysis of symmetric and asymmetric ciphers is computationally extremely demanding. Since the security parameters (in particular the key length) of almost all practical crypto algorithms are chosen such that attacks with conventional computers are computationally infeasible, the only promising way to tackle existing ciphers (assuming no mathematical breakthrough) is to build special-purpose hardware. Dedicating those machines to the task of cryptanalysis holds the promise of a dramatically improved cost-performance ratio so that breaking of commercial ciphers comes within reach. This contribution presents the design and realization of the COPACOBANA (Cost-Optimized Parallel Code Breaker) machine, which is optimized for running cryptanalytical algorithms and can be realized for less than US $ 10,000. It will be shown that, depending on the actual algorithm, the architecture can outperform conventional computers by several orders in magnitude. COPA-COBANA hosts 120 low-cost FPGAs and is able to, e.g., perform an exhaustive key search of the Data Encryption Standard (DES) in less than nine days on average. As a real-world application, our architecture can be used to attack machine readable travel documents (ePass). COPACOBANA is intended, but not necessarily restricted to solving problems related to cryptanalysis. The hardware architecture is suitable for computational problems which are parallelizable and have low communication requirements. The hardware can be used, e.g., to attack elliptic curve cryptosystems and to factor numbers. Even though breaking full-size RSA (1024 bit or more) or elliptic curves (ECC with 160 bit or more) is out of reach with COPACOBANA, it can be used to analyze cryptosystems with a (deliberately chosen) small bitlength to provide reliable security estimates of RSA and ECC by extrapolation.

The Internet Engineering Task Force (IETF) is in the process of adopting standards for IP-layer encryption and authentication (IPSEC). We describe how "probable plaintext" can be used to aid in cryptanalytic attacks, and analyze the protocol to show how much probable plaintext is available. We also show how traffic analysis is a powerful aid to the cryptanalyst. We conclude by outlining some likely changes to the underlying protocols that may strengthen them against these attacks.

. Most modern security protocols and security applications are defined to be algorithm independent, that is, they allow a choice from a set of cryptographic algorithms for the same function. Therefore a key-search machine which is also defined to be algorithm independent might be interesting. We researched the feasibility of a universal key-search machine using the Data Encryption Standard (DES) as an example algorithm. Field Programmable Gate Arrays (FPGA) provide an ideal match for an algorithm independent cracker as they can switch algorithms on-the-fly and run much faster than software. We designed, implemented and compared various architecture options of DES with strong emphasis on high-speed performance. Techniques like pipelining and loop unrolling were used and their effectiveness for DES on FPGAs investigated. The most interesting result is that we could achieve data rates of up to 403 Mbit/s using a standard Xilinx FPGA. This result is by a factor 31 faster than software imp...

We present Keyed HIP (KHIP), a secure, hierarchical multicast routing protocol. We show that other shared-tree multicast routing protocols are subject to attacks against the multicast routing infrastructure that can isolate receivers or domains or introduce loops into the structure of the multicast routing tree. KHIP changes the multicast routing model so that only trusted members are able to join the multicast tree. This protects the multicast routing against attacks that could form branches to unauthorized receivers, prevents replay attacks and limits the effects of flooding attacks. Untrusted routers that are present on the path between trusted routers cannot change the routing and can mount no denialof -service attack stronger than simply dropping control messages. KHIP also provides a simple mechanism for distributing data encryption keys while adding little overhead to the protocol. 1 Introduction A multicast routing protocol provides efficient many-tomany delivery across a net...

This paper describes the authors' experiences attacking the IBM 4758 CCA, used in retail banking to protect the ATM infrastructure.

We introduce the notion of key stretching, a mechanism to convert short s-bit keys into longer keys, such that the complexity required to brute-force search a s + t-bit keyspace is the same as the time required to brute-force search a s-bit key stretched by t bits.