# Cryptography

Some terms related to cryptography :
Cryptography :
It is the study of the mathematics techniques for all aspects of the information security. Cryptanalysis is the complementary science with the methods to defeat these techniques. The study of cryptography and cryptanalysis is called Cryptology.
Encryption :
It is the process of disguising the message in order to hide all the information that it contains.. This process includes encoding and enciphering.
Protocol :
It is an algorithm that is defined by a sequence of steps, precisely specifying the actions of multiple parties in order to acheive the objective.
Plain text :
The message that is transmitted or stored.
Ciphertext :
The disguised message that is in encrypted form.
Encode :
Encode means to convert the message into a representaion in a standard alphabet, such as to the alphabet {a,……z} or to numerical alphabet.
Decode :
Decode means to convert the encoded message into its original form. The process of encoding a message is not an obscure process, and the result that we get after decoding can be considered as equivalent to the plaintext message.
Cipher :
A map from a space of plaintext to a space of ciphertext.
Encipher :
To convert the plaintext to ciphertext.
Decipher :
To convert the ciphertext back to the plaintext.
Stream cipher :
A cipher which acts on the plaintext one symbol at a time.
Block cipher :
A cipher which acts on the plaintext in blocks of symbols.
Substitution cipher :
A stream cipher which acts on plaintext by making a substitution of the characters with the elements of a new alphabets or by permutation of the characters in the plaintext alphabets.
Transposition cipher :
A block cipher which acts on the plaintext by permuting the positions of the characters in the plaintext.