Cryptography is the study of secret writing. A Cipher is a way of hiding ordinary text, called plaintext, by transforming it into ciphertext. This process is called enciphering or encryption of the plaintext into ciphertext. The reverse process is known as deciphering or decryption.
Ciphers are divided into two categories: substitution and transposition ciphers. Substitution Ciphers replace letters or larger blocks with substitutes, usually of the same length. The letters in the ciphertext of a transposition cipher are the same letters, with same frequencies, as the letter of the plain text but they are simply rearranged.
Cryptanalysis is the study of attacks on ciphers. Methods of attack may be classified into several general types based on what information is known or not known to a cryptoanalyst. In the next section, we provide an outline of different types of known attacks on ciphertexts or algorithms.
Objectives of Cryptography
The main objectives of cryptography include the following:
- Confidentiality: deals with keeping the message secret. No one other than the intended recipient should be able to view or understand the content of the message;
- Data Integrity: concerned with the prevention of alteration to an original message by any third party after or while it is being sent;
- Authentication: provides the abilities to both verify that the sender of a message is indeed who he or she claims to be and to prevent any third party from sending a message as someone they, in actual fact, are not;
- Non-Repudiation: a message must be owned up by its sender. This means that a sender must not be able to deny that the message came from him or her.