1.1 Background of the Study

The security of data transmission is an important problem in communication networks. A communication system is reliable as long as it maintains the integrity, availability, and privacy of data. Data integrity is the protection of data from unauthorized modification, resistance to penetration and undetected modification. Therefore, it is important to secure cryptosystem which provides encryption and decryption to the data. To achieve a secure cryptosystem, Cryptology is essential. Cryptology is the study of cryptography and cryptanalysis.

Cryptography is the study of information hiding and verification. It includes the protocols, algorithms and strategies to securely and consistently prevent or delay unauthorized access to sensitive information and enable verifiability of every component in a communication (Saini and Mandal, 2015).

Cryptanalysis is the art of ‘attacking’ cryptosystems in order to ‘crack’ them or at least discover their weaknesses (Stallings, 2011). When cryptanalysis reveals weaknesses in cryptosystems, cryptographers create more secure cryptosystems. Conversely, as cryptosystems become stronger, cryptanalysts try to discover more powerful methods of attacking them. Thus, cryptography and cryptanalysis are complementary.

Cryptography is divided into three main branches (Chirstof and Pelzi 2010) which are:

  • Symmetric Algorithms: it refers to encryption and decryption methods in which both the sender and receiver share a secret key. All cryptography from ancient times until 1976 was exclusively based on symmetric methods. Symmetric ciphers are still in widespread use, especially for data encryption and integrity check of messages
  • Asymmetric (or Public-Key) Algorithms: In public-key cryptography, a user possesses a secret key as in symmetric cryptography and a public key that may be freely distributed. Asymmetric algorithms can be used for applications such as digital signatures and key establishment, and also for classical data encryption
    1. Cryptographic Protocols: crypto protocols deal with the application of cryptographic algorithms. Symmetric and asymmetric algorithms can be viewed as building blocks with which applications such as secure Internet communication can be realized. The Transport Layer Security (TLS) scheme,


    which is used in every Web browser, is an example of a cryptographic protocol. Symmetric cryptography addresses the problem of secrecy protection by using the shared secret key to transform the message in such a way that it cannot be recovered anymore without this key. This process is called symmetric encryption. Based on the paradigm used to process the message, these ciphers are typically categorized into one of two classes: block ciphers and stream ciphers. The security of symmetric encryption algorithms can in general not be proved (the notable exception being the one-time pad). Instead, the trust in a cipher is merely based on the fact that no weaknesses have been found after a long and thorough evaluation phase (Canniere, 2007). Two types of ciphers are used in Symmetric Key Cryptography: Transposition cipher and Substitution cipher (Stallings, 2006).


    In transposition cipher the characters in the plaintext are swapped to get the cipher text i.e. the characters retain their plaintext form but their position is changed. The plaintext is organized into two dimensional table and columns are interchanged according to a