One of the most important aims in cryptography is to make information confidential by applying encryption. The goal is to transform the message by encrypting it, so that only those who have the key can decrypt the message and read the related information. There are two major classes of encryption systems, namely - symmetric encryption systems and – asymmetric encryption systems also known as public-key systems. The sender and receiver of a message m share a secret key k for encryption and decryption in symmetric systems. Symmetric systems can be divided further into block ciphers and stream ciphers. In block ciphers the message is divided into blocks which are encrypted by the same key, whereas in stream ciphers the secret key is expanded into a long sequence of pseudo-random bits which are added to the stream of message bits.
An additive stream cipher is a function f which generates a keystream sequence z of length l>n from the key k of length n bit. Encryption is done by carrying out the binary XOR operation to the message m=m1, m2, ... ml and the keystream sequence z. The result of the encryption is the ciphertext sequence c. The receiver of the message generates the same keystream sequence z using the function f and the key k, which means that in order to recover the message m, the receiver only has to apply the binary XOR operation to the ciphertext sequence c and the keystream sequence z. The keystream sequence z is a pseudorandom sequence and it should fullfil well defined statistical and cryptographic requirements.
At the chair of Communication Systems stream ciphers were dealt with. The main research topics were:
|Prof. Firoz Kaderali|