Data Protection and Security

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Introduction To Cryptography

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Cryptography and Its Applications

Generic Setting

The animation on the right shows the general setting of (secret-key) cryptography for secrecy (confidentiality).

Encryption is an algorithmic way to transform the message so that the output form of the message does not provide any useful information about the message to third parties who do not have the key which is a parameter of the transformation. Decryption is the process of transforming the encrypted message back into its original form using the same key. Encryption is not a random way of transformation but it is an operation which should have a related reverse operation (decryption) so that the original message can be recovered. In cryptographic terminology the message itself is called the plaintext, the output of the encryption process is the ciphertext and the cipher is the cryptographic method (or the hardware) used in the transformation. Here the cryptographic algorithm or the method is not kept secret, it is publicized. On the other hand the key is the secret known only by two parties communicating.

It is generally useful to have a notation for relating plaintext, ciphertext and keys. We will use C = EK(P) to mean that the encryption of the plaintext P using the key K gives the ciphertext C. Similarly, P = DK(C) represents the decryption of C to get the plaintext again. It then follows that

DK(EK(P)) = P

Animation I.I-I

Animation depicting general setting of cryptography for secrecy.
[click to enlarge]