Problem 2: Vigener Cipher Consider the following Vigener encryption. Is it possi

Question

Problem 2: Vigener Cipher

Consider the following Vigener encryption. Is it possible to guess the key length of this cipher with only ciphertext? (HINT: The distance between the repetitions of CSASTP is 16. What does this suggest about the key length?)

a) Key:         ABCDABCDABCDABCDABCDABCDABCD

Plaintext: CRYPTOISSHORTFORCRYPTOGRAPHY

Ciphertext:CSASTPKVSIQUTGQUCSASTPIUAQJB

b) The following ciphertext has two segments that are repeated: Write the possible key length(s) for this cipher.

Ciphertext: VHVSSPQUCEMRVBVBBBVHVSURQGIBDUGRNICJQUCERVUAXSSR

Explanation / Answer

Hey. Here is your solution.

Note : There is no perfect relation between repeatating letter sequence and key length.

a} In given case, "ABCD" is a key having length of 4 (BY OBSERVATION)

But the distance between the repetitions of CSASTP is 16. We dont have more cipher data hence key length can be assumed as 16.

b)

-There is repeatation of two letter sequence "VHVS" and "QUCE".

-Letter sequence "VHVS" repeated at a distance of 18

- Letter sequence "QUCE" repeated at a distance of 30

- In this case we can take HCF of distance to arrive at probable key length.

i.e. HCF(18,30) = 6

- Hence possible key length for this cipher is 6

Note : Reason for repeatation of letter sequence in cipher text is coincidence of Plain Text letter segment and Cipher key letter segment.

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