L7 Stream cipher systems

3.1 RUNNING-KEY CIPHER:
If we choose a key with its length as long as the plaintext message, then the cipher is called running-key cipher. A text of a book can be used as a key sequence in a substitution cipher based on shifted alphabets (i.e. non-periodic vigenere). Running key might be expected to be unbreakable because it has non-periodic key;
but the key has redundancy due to non-randomness of its characters.
Example:
Giving a key K selected from a textbook and a message M:
M: T H E T R E A S U R E I S B U R I E D....
K: T H E S E C O N D C I P H E R I S A N....
EK (M): M O I L V G O F X T M X Z F L A A E Q....
If the key has redundancy, the cipher may be breakable using friend Nan s method.Fried Nan s approach is based on the observation that a large proportion of letters in the cipher text will correspond to encipherments , where both, the plaintext and key letters fall in the high frequency category.

3.2 VERNAM CIPHER:
The implementation of one-time ??d cipher is in vented by Gilbert Vernam in 1917; he designed a cryptographic device based on 32-character Baudot code for new teletype writer developed at AT&T. Each character is represented as a combination of five marks and spaces corresponding to 1 and 0. A Vemam idea is to run a second synchronized non repeating random sequence of key character punched on paper tape during transmission which would cause each Baudot (0 or 1) on the message tape to be Xored with its corresponding value on the key tape. Each plain text is added (mod 2) to the key stream, and generates a cipher text bit stream.
C = Ek (m) =C1 , C2 ,C3, ….. Where Ci = (mi +kj) mod 2
Vernam cipher is efficiently implemented in microelectronics by taking the (exlusive_or) (XOR).