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The word cryptography comes from two Greek words meaning “secret writing” and is the art and science of information hiding. This field is very much associated with mathematics and computer science with application in many fields like computer security, electronic commerce, telecommunication, etc.

So cryptography is a subject that should be of interest to many people, especially because we now live in the Information Age, and our secrets can be transmitted in so many ways – email, cell phone, etc. – and all these channels need to be protected [ simon singh].

Secrecy and Encryption

In the ancient days, cryptography was mostly referred to as encryption – the mechanism to convert the readable plaintext into unreadable (incomprehensible) text i.e. ciphertext, and decryption – the opposite process of encryption i.e. conversion of ciphertext back to the plaintext. Though the consideration of cryptography was on message confidentiality (encryption) in the past, nowadays cryptography considers the study and practices of authentication, digital signatures, integrity checking, and key management, etc. Encryption mostly provides the secrecy of massage being transmitted over the communication network. This is called confidentiality of massage. The only sender knows the keys and can decipher the massage.


Cryptanalysis is the breaking of codes. Cryptanalysis encompasses all of the techniques to recover the plaintext and/or key from the ciphertext.
The combined study of cryptography and cryptanalysis is known as cryptology. Though most of the time we use cryptography and cryptology in the same way.

Objective of cryptography

Encryption and Decryption
Encryption is the process of encoding a message so that its meaning is not obvious i.e. converting information from one form to some other unreadable form using some algorithm called cipher with the help of secret message called key. The converting text is called is plaintext and the converted text is called ciphertext.

Decryption is the reverse process, transforming an encrypted message back into its normal, original form. In decryption process also the use of key is important. Alternatively, the terms encode and decode or encipher and decipher are used instead of encrypt and decrypt. That is, we say that we encode, encrypt, or encipher the original message to hide its meaning. Then, we decode, decrypt, or decipher it to reveal the original message.

Fig: Encryption-Decryption

The use of encryption techniques is being used since very long period as it can be noted from the technique called Caesar’s cipher used by Julius Caesar for information passing to his soldiers. Encryption techniques have also been extensively used in military purposes to conceal the information from the enemy. Nowadays to gain the confidentiality encryption is being used in many areas like communication, internet banking, digital right management, etc.

A key is a parameter or a piece of information used to determine the output of cryptographic algorithm. While doing the encryption, key determines the transformation of plaintext to the cipher text and vice versa. Keys are also used in other cryptographic processes like message authentication codes and digital signatures. Most of the cryptographic systems depend upon the key and thus the secrecy of the key is very important and is one of the difficult problems in practice. Another important issue for the key is its length. Since key is the sole entity that defines the strength of the security (normally algorithm used is public) we need to select the key in a way such that attacker should take long enough to try all possibilities. To prevent the key from being guessed the choice of the key must be random.


A cipher is an algorithm for performing encryption and decryption. The operation of cipher depends upon the special information called key. Without knowledge of the key, it should be difficult, if not nearly impossible, to decrypt the resulting cipher into readable plaintext. There are many types of encryption techniques that have advanced from history, however the distinction of encryption technique can be broadly categorized in terms of number of key used and way of converting plaintext to the ciphertext.

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