Abstract: It was recently shown by Michael Rabin that a sequence of random 0-1 values, prepared and distributed by a trusted "dealer," can be used to achieve Byzantine agreement in constant expected time in a network of processors. A natural question is whether it is possible to generate these values uniformly at random within the network. In this paper we present a cryptography based protocol for agreernent on a 0-1 randona value, if less than half of the processors are faulty. In fact the protocol allows uniform sampling from any finite set, and thus solves the problem of choosing a network leader uniformly at random. The protocol is usable both when all the communication is via "broadcast," in which case it needs three rounds of information exchange, and when each pair of processors communicate on a private line, in which case it needs 3t + 3 rounds, where t is the number of faulty proccssors. The protocol remains valid even if passive eavesdropping is allowed. On the other hand we show that no (probabilistic) protocol can achieve agreement on a fair coin in fewer phases then necessary for Byzantine agreement, and hence the "pre-dealt" nature of the random sequence required for Rabin's algorithm is crucial.

Abstract: Investigating the capabilities of public key and related cryptographic techniques has recently become an important area of cryptographic research. In this paper we present some new algorithms and cryptographic protocols (Cryptoprotocols) which enlarge the range of applications of public key systems and enable us to perform certain transactions in communication networks. The basic cryptographic tools used are Rabin’s Oblivious Transfer Protocol and an algorithm we developed for Number Embedding which is provably hard to invert.

Abstract: Recently considerable attention has been given to designing provably secure cryptographic protocols Three basic problems that have been studied are: exchange of secrets, contract signing, and certified mail Several protocols have been proposed for these problems These solutions are very diverse in nature: they are secure under different assumptions, have different probabilities of cheating, and require different number of message exchanges Consequently, there is a need to investigate the underlying relationships between these problems In this paper, we study reducibilities among these problems, ie how a protocol for one problem can be transformed into a protocol for another problem, preserving (roughly) the operating conditions

Abstract: Investigating the capabilities of public key and related cryptographic techniques has recently become an important area of cryptographic research. In this paper we present some new algorithms and cryptographic protocols (Cryptoprotocols) which enlarge the range of applications of public key systems and enable us to perform certain transactions in communication networks. The basic cryptographic tools used are Rabin’s Oblivious Transfer Protocol and an algorithm we developed for Number Embedding which is provably hard to invert.

Abstract: x i i CHAPTER 1 INTRODUCTION 1 1•1 General 1 1.2 Thesis Organization 2 CHAPTER 2 CRYPTOGRAPHIC CONCEPTS 4 2ml Cryptographic Systems 4 2.2 Cryptosystem S e c u r i t y and Complexity 7 Theory 2.3 Cryptographic Techniques 10 2.3.1 Block Cipher 10 2.3.2 Stream Cipher 12 CHAPTER 3 DATA ENCRYPTION ALGORITHMS 16 3.1 General 16 3.1.1 Transposition Cipher 16 3.1.2 S u b s t i t u t i o n Cipher 16 3.1.3 Product Cipher 17 3.2 Data Encryption Standard 18 3.2.1 DES Algorithm An Overview 18 3.2.2 The Key Schedule Procedure 22 3.2.3 DES Encryption and Decryption 24 3.3 Software DES Implementation 27 3.3.1 A Pbssible Advantage of DES 28 Software 3.4 Some C h a r a c t e r i s t i c s of DES Algorithm 30 3.4.1 Avalanche E f f e c t 30 3.4.2 Complementary Property 31 3.5 Design C r i t e r i a 38 3.5.1 S-Boxes 38 3.5.2 I n i t i a l and F i n a l Permutations 39 3.5.3 P-P^rmutat ion 39 3.6 C r i t i c i s m and Weaknesses of DES 40 3.6.1 The Key Length 40 3.6.2 Unpublished Design P r i n c i p l e s 41 3.6.3 Number of Rounds 42 3.6.4 Key Schedule Algorithm42 Weak and Semiweak Keys

Abstract: Recent cooperation between European telecommunications administrations has highlighted the need for wide-ranging guidelines on the secure design and operation of cryptographic equipment. This Code of Practice is an attempt to meet the need with a generalised approach, applicable in a wide range of applications. The guidelines are intentionally flexible where possible to permit cost-effective implementations. Decisions on which security features are to be incorporated are left to the system designer, along with responsibility for those decisions. Secure design is considered at system, logical, and physical levels. Operational aspects such as the structure, generation, loading, storage, and utilisation of key-variables are all treated, together with cryptographic equipment controls and indicators. Special consideration is given to cryptographic equipment utilising multiple key-variables. The principles given do not in general compel particular constructional techniques, software formats, or electrical practices, but constitute a set of recommendations to assist in secure implementation.

Abstract: The 1984 CCITT recommendations on message handling provide for encrypted data but do not propose any standard method Since use of both data encryption and electronic mail is increasing, the need for encryption standards within the message handling system will grow The following note examines the different encryption requirements and proposes protocol extensions to handle them

Abstract: The authors discuss the ground rules of cryptographic systems and show how they are evolving to protect information from the 'computer burglar'. The legal and commercial framework surrounding the application of cryptographic functions is also discussed

Abstract: A network-optimised approach to electronic funds transfer at point-of-sale is described which gives excellent system flexibility, growth capacity and cryptographic operation. End-to-end cryptographic security techniques are examplified for illustrative message protocols and PIN verification strategies.

Abstract: We present a new protocol for establishing secure communications over an insecure communications charmel in the absence of trusted third parties or authenticated keys. The protocol is an improvement over the simpler protocol in which the communicating parties exchanged their public encryption keys and used them to encrypt messages. It forces a potential eavesdropper--if he wants to understand the messages--to reveal his existence by modifying and seriously garbling the communication.

Abstract: A new type of cryptosystem is presented that may find application in broadcast communication networks and database systems. To date, all proposals for such systems have either been shown to be weak under cryptanalytic attack or to possess undesirable characteristics such as data expansion. The new scheme is a layered system that is a concatenation of SP-networks and Hill broadcast encryption and is referred to as an SP-Hill cryptosystem. Tests carried out on it indicate that the scheme performs well cryptographically.

Abstract: In the paper some of the cryptographic techniques, algorithms and applications in computer networks are reviewed. Symmetric and asymmetric cryptosystems, possible cryptanalytical attacks, cryptosecurity and the relevance of complexity theory to the security of ciphers are described. The major threats and counter-measures in computer-network communications are outlined. Finally, cryptographic applications and limitations in computer networks are examined.