Abstract
M.Sc. (Computer Science)
The main themes of this thesis are the characteristics of natural language,
cryptographic algorithms to encipher natural language and possible figures of
merit with which to compare different cryptographic algorithms.
In this thesis the characteristics of natural language and the influence this
has on cryptographic algorithms is investigated. The entropy function of
Shannon is used extensively to evaluate the different models that can be
constructed to simulate natural language. Natural language redundancy is
, investigated and quantified by the entropy function. The influence this
redundancy has on the theoretic security of different algorithms is
tabulated. Shannon's unicity distance is used as a measure of security for
this purpose. The unicity distance is already shown at this early stage to
be not a very accurate measure of real (practical) security of cryptographic
ciphers.
The cryptographic algorithms discussed in this thesis are arbitarily divided
into three groups: classical algorithms, public key algorithms and computer
algorithms. In the classical algorithms cryptographic techniques such as
transposition and character substitution are included. Well known ciphers
such as the Playfair and Hill encipherment schemes are also included as
classical cryptographic techniques. A special section is devoted to the use
and cryptanalytic techniques of polyaphabetic ciphers.
The public key ciphers are divided into three main groups: knapsack ciphers,
RSA type ciphers and discrete logarithmic systems. Except for the discrete
logarithmic cipher several examples of the other two groups are given.
Examples of knapsack ciphers are: Merkle Hellman knapsack, Graham-Shamir
knapsack and Shamir's random knapsack.