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Introduction to Data Security
Basic Terminology
Suppose that someone wants to send a message to a receiver, and wants to be sure
that no-one else can read the message. However, there is the possibility that
someone else opens the letter or hears the electronic communication.
In cryptographic terminology, the message is called plaintext or cleartext.
Encoding the contents of the message in such a way that hides its contents from
outsiders is called encryption. The encrypted message is called the ciphertext.
The process of retrieving the plaintext from the ciphertext is called decryption.
Encryption and decryption usually make use of a key, and the coding method is
such that decryption can be performed only by knowing the proper key.
Cryptography is the art or science of keeping messages secret. Cryptanalysis is
the art of breaking ciphers, i.e. retrieving the plaintext without knowing the proper
key. People who do cryptography are cryptographers, and practitioners of
cryptanalysis are cryptanalysts.
Cryptography deals with all aspects of secure messaging, authentication, digital
signatures, electronic money, and other applications. Cryptology is the branch of
mathematics that studies the mathematical foundations of cryptographic methods.
Basic Cryptographic Algorithms
A method of encryption and decryption is called a cipher. Some cryptographic
methods rely on the secrecy of the algorithms; such algorithms are only of
historical interest and are not adequate for real-world needs. All modern algorithms
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use a key to control encryption and decryption; a message can be decrypted only if
the key matches the encryption key. The key used for decryption can be different
from the encryption key, but for most algorithms they are the same.
There are two classes of key-based algorithms, symmetric (or secret-key) and
asymmetric (or public-key) algorithms. The difference is that symmetric
algorithms use the same key for encryption and decryption (or the decryption key
is easily derived from the encryption key), whereas asymmetric algorithms use a
different key for encryption and decryption, and the decryption key cannot be
derived from the encryption key.