Cryptography

Cryptography – secret writing.

Plain text – The original message is known as the plaintext. A message in plain text can be understood by anybody, as the message is not coded in any manner.

Cipher Text – The coded message is called the cipher text when a plain text message is codified using any suitable scheme, the resulting message is called as cipher text.

Encryption – The process of converting from plain text to cipher text is known as Encryption or Enciphering.

Decryption – Restoring Plaintext from the cipher text is called Decryption or Deciphering.

Cryptography – The schemes used for enciphering constitute the area of study known as Cryptography. Such a scheme is known as a Cipher.

Cryptanalysis – Techniques used for deciphering a message without any knowledge of the enciphering details constitute the area of Cryptanalysis. Cryptanalysis means “breaking the code”.

Cryptology – The areas of cryptography and cryptanalysis together are called cryptology.

Encryption methods divided into two categories: Substitution ciphers and transposition ciphers.

Substitution Ciphers:

Here the letters of plaintext are replaced by other letters or by numbers or symbols. If the plain text is viewed as a sequence of bits then substitution involves replacing the plaintext bit patterns with ciphertext  bit patterns.

Caesar Cipher:

The Caesar cipher is attributed to Julius Caesar. It was the first example of substitution cipher.

Eg: Plain:a b c d

Cipher:d e f g

Mono alphabetic cipher:

In a plain text message each A can be replaced by any other alphabet (B through Z), each B can also be replaced by any other random alphabet (A or C through Z) and so on. There is no relation between the replacement of B and replacement of A.

Transposition Ciphers:

Transposition techniques do not simply replace one alphabet with another. They also

Perform some permutation over plain text alphabets.