Various published authentication protocols that employ symmetric cryptographic algorithms are examined. A number of misconceptions found in the specification, design and implementation of these protocols are revealed. Some misconceptions are considered responsible for definite security flaws, while others are shown to cause weaknesses which may help in attacks on the cryptographic mechanisms. We identify an underlying problem and attempt a remedy by developing a methodology for the development of secure and strong authentication protocols.

In this paper modes of operation for block ciphers are considered. The focus is on a special class of modes, called the block chaining modes. We discuss modes with finite and infinite error propagation and propose the Accumulated Block Chaining (ABC) mode of the latter type. ABC is proposed for application for the Advanced Encryption Standard.

In the past few years the International Organisation for Standardization (ISO) and International ElectroTechnical Commission (IEC) have been jointly developing several standards for entity authentication and/or encryption key exchange. Some of the mechanisms proposed use a symmetric block encryption algorithm, in a particular mode of operation, to achieve security. In this paper a general property of the algorithm is explained. It is shown that some of the protocols in the ISO/IEC draft documents for standardisation are vulnerable to an attack which exploits this property.

In many published authentication protocols, the cryptographic services are coarsely specified in that, whenever needed, a uniform notation is used to denote them while the exact nature of protection required is left unclear. In this paper we reason that such a coarse treatment not only forms a foundation why authentication protocols are error prone, but is also responsible for a typical feature of misusing redundancy which causes many protocols to be unnecessarily weak. We propose new notations to refine protocol specifications. The refinement leads to a methodology for the development of secure and strong authentication protocols.