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,
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
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.
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.
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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