This problem explores the use of a one-time pad version of the Vigenere cipher.

Question

This problem explores the use of a one-time pad version of the Vigenere cipher. In this scheme, the key is a stream of random numbers between O and 26. For example, if the key is 3 19 5 ... , then the first letter of plaintext is encrypted with a shift of 3 lettters, the second with a shift of 19 letters, the third with a shift of 5 letters, and so on.

1. a) Encrypt the plaintext sendmoremoney with the key stream 2 9 0 1 7 23 15 21 14 11 11 2 8 9

b) Using the ciphertext produced in part (a), find a key so that the cipher text decrypts to the plaintext cashnotneeded.

Explanation / Answer

As per given instructions following be cipher text for given plain text:

Position 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Alphabets A B C D E F G H I J K L M N O P Q R S T U V W X Y Z KeyStream 2 9 0 1 7 23 15 21 14 11 11 2 8 9 PlainText S e n d m o r e m o n e y Position for Cipher test using keyStream S=19 and key = 2, 19+2 = 21 e= 5, key = 9, 5+9 = 14 n= 14, key =0, 14+0= 14 d=4, key=1,4+1=5 m=13, key=7, 13+7=20 O=15,key=23, 15+23=38 r=18, key = 15, 16+15=31 e=5, key = 21, 5+21= 26 m=13, key=14, 13+14= 27 O=15,key=11, 15+11=26 N= 14, key=11, 14+11=25 e=5, key = 2, 5+2=7 y=25, key = 8, 25+8= 33 Cipher Text V N N E T O(out of range) r(Out of range) Z m(out of range) Z Y G y (out of range)

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