Johannes Mittmann

One-Way Encryption and Message Authentication

Abstract

In modern society the protection of the authenticity of information has become as important as the protection of its confidentiality. This means that there is a need for data origin authentication as well as for verification of data integrity. Hash functions are versatile cryptographic building blocks that are used in this context, but also in conjunction with digital signature schemes and many other applications such as password protection or pseudo-random numbers generation. A hash function is an algorithm that takes inputs of arbitrary length and returns a short string of bits, the message digest. However, for cryptographic hash functions to be secure, additional properties are required. For instance, it should be hard to find two distinct messages that hash to the same value. Hash functions that depend on a secret key are called message authentication codes (MACs). This paper gives definitions of the basic terms of cryptographic hash functions, following the description in [Sti02]. First, we discuss generic attacks that can be applied to arbitrary hash functions and give a comparison of security criteria. Second, we describe design principles of iterated hash functions in general, and the Secure Hash Algorithm (SHA-1) in particular. Finally we introduce message authentication codes and show their construction from other cryptographic primitives.