Modification and implementation of Data Encryption Standard

Abstract

This thesis describes some aspects of cryptography i.e. encryption and decryption of a plaintext to a ciphertext using some key that must be kept secret by some means of protection , the thesis is intended to study the concepts of cryptography that can be programmed into an application program using the C programming language. Cryptography can be defined as the study of mathematical techniques related to aspects of information security such as confidentiality, data integrity, entity authentication, and data origin authentication, these terms has been defined and presented clearly along the thesis. The thesis at the beginning describes some cryptographic fundamentals, some related mathematical concepts to cryptography, the information security and how they are related to the cryptography, the cryptographic goals and systems, how the systems can be complex and the degree of their complexity It examines some encryption and decryption block cipher algorithms their dependencies on the work of Feistel at IBM and his structure of confusion and diffusion and how by these two terms a complex algorithms can be constructed that they cannot be broken unless using a brute force attack, a term that is defined at the thesis that means it needs huge resource of computer power and long time to break a ciphertext. The thesis concentrates mainly on the block cipher algorithms and studies many block ciphers, presents them briefly, then the thesis get into more deep study on four of the block ciphers algorithms, the four are Twofish, Mars, Frog and RC6 ,,and states the results of the comparing these four block cipher algorithms according to some evaluation criteria, such as the speed of the execution of the code that implements the algorithm, i.e. how fast is the code can be executed The thesis comes to the main goal of it by studying in detail the DES (Data Encryption Standard) encryption technique, modifies it by increasing the length of the key and implements it in a C language application program.