Abstract Objectives: Computer security is all about the study of cyber attacks with a view to defend against them. Cryptography is considered to be a class of science by using the special art of transforming information in a protected way such that it can overcome the attacks. There is an immense requirement of strong cryptographic algorithms in order to withstand against the various attacks. Methods: The Kerchof’’s Principle states that the encryption and decryption algorithms are always available to anyone. The security of the cipher against any sort of attack should be depends only on the secrecy of the key. There comes the play of cryptanalysis. It is art of breaking the keys by identifying the vulnerabilities existing in the systems. This paper deals with the classical encryption schemes and their cryptanalysis. The cryptanalysis for various encryption schemes difers a lot. Various cryptanalysis like statistical analysis, frequency analysis, trial and error (brute force) are car- ried out in this work. Findings: The classical encryption schemes such as caesar cipher, shift cipher, vigenere cipher, affine cipher and hill cipher are discussed in the paper. A detailed analysis about the security of the above mentioned ciphers are explored. Among the ciphers it is identified that if the key varies for each plaintext to be encrypted provides added secu- rity. But the worst is the management of the huge key space. A modified algorithm is proposed which can provide a better security using simple computations. In this approach lots of keys are used but generated from a single key by using simple shift and EXOR operation. In the main stream only one key need to be exchanged between the communication entities and for that exchange we can make use of the public key cryptosystem. Application: Cryptography is considered to be an ineluctable field in era of communication. Cryptographic algorithms acts as an underpinning for lots of applications such as Anonymous Remailers, Digital Signatures, Secured Money transactions etc. Security Analysis and Modifcation of Classical Encryption Scheme Maya Mohan 1* , M. K. Kavitha Devi 2 and V. Jeevan Prakash 3 1 Department of CS&E, NSSCE, Palakkad, Kerala, India; mayajeevan@gmail.com 2 Department of CSE, TCE, Madurai, Tamilnadu, India 3 Department of Mathematics, NSSCE, Palakkad, Kerala, India 1. Introduction Cryptography 1 plays a vital role in the epoch of communication especially in e-transactions. A wide variety of innovations are emerged in the area of cryptog- raphy to achieve diferent levels of security. Cryptography is classifed based on the keys used for encryption as well as decryption. In order to provide integrity, hash functions are used which need not require any key. In Asymmetric key cryptography a key pair is required for encryption and decryption and in symmetric key cryptography single key is enough for encryption and decryption. Te encryption operations are classifed based on the method used for enciphering such as substitution, transposition / product and the way of processing the plaintext as block or stream. Te cryptanalysis 2 basically work on the above parameters. Based on the information needed by the cryptanalyst attacks can be classifed into four broad categories.  Ciphertext only attack- Te encrypted text is the only known information for the cryptanalyst.  Known plaintext attack- given ciphertext , prior knowledge about ciphertext -plaintext pairs  Chosen plaintext attack- a given ciphertext ,by using encryption algorithm with plaintexts and getting the matching ciphertexts *Author for correspondence Indian Journal of Science and Technology, Vol 8(S8), 542–548, April 2015 ISSN (Print) : 0974-6846 ISSN (Online) : 0974-5645 Keywords: Additive Cipher, Monoalphabetic Cipher, Multiplicative Cipher, Polyalphabetic Cipher DOI: 10.17485/ijst/2015/v8iS8/71506