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

Question

This problem explores the use of a one-time pad version of the Vigenère cipher. In this
scheme, the key is a stream of random numbers between 0 and 26. For example, if the key is 3
19 5 . . . , then the first letter of plaintext is encrypted with a shift of 3 letters, the second with a
shift of 19 letters, the third with a shift of 5 letters, and so on.

a) Encrypt the plaintext sendmoremoney with the key stream
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

Take a as the 0th index and z as the 25th index.

send shift with key 9017 will give

s=b

e=e

n=o

d=k.

s shift 9 places lands the letter b.

Similarly for moremoney, the result is jdmsxzpmh.

the ciphertext is "beokjdmsxzpmh"

b)

take the first 4 ciphertext

"beok".

Calculate the shift from b to s which is 17 shifts. thus beok can be decrypted with key 17 0 24 19 to get "send"

thus the resulting key is 17 0 24 19 3 11 5 12 15 14 2 8 17

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