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 Kerchoff"s Principle states that the encryption and decryption algorithms are always available to

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... yone. 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 differs a lot. Various cryptanalysis like statistical analysis, frequency analysis, trial and error (brute force) are carried 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 security. 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.