Module $02$ Discussion: Stream Ciphers

Introduction

Suppose Alice needs to securely send one textual message to Bob each day, and that they settle on the following method of encryption: As the key Alice uses the string of decimal digits formed by the six winning numbers from that day's Irish National Lottery Daily Million drawing $-$ to illustrate, if the winning numbers were $6,13,14,18,24$ and $34,$ she would form the string $(6,1,3,1,4,1,8,2,4,3,4).$

In this example she would encrypt her textual message by shifting the first character $6$ places ahead in the alphabet, the second $1$ place ahead, the third $3$ ahead, the fourth $1$ ahead, and so on until shifting the eleventh character $($if there is one$)4$ ahead.

Finally, if there are more characters to encrypt she would go back to the beginning of her key and shift the next character $6$ places ahead, the one following $1$ place ahead, and so on$.$

Initial Post

Assuming that Alice and Bob are able to keep their encryption method secret, discuss the security attributes of this cipher. For example, you could use one or more of the following approaches:

$1.$Describe the difficulties that Oscar would encounter in trying to decrypt encrypted messages he intercepts.

$2.$Compare the security of this cipher to some of the others encountered in our class $($e$.$g$.$ shift$($n$)).$

$3.$Evaluate how effective some of the attacks you have seen in our class would be in this situation.