Question

Let’s say you have the ciphertext for the given plaintext.

Plaintext:

it was disclosed yesterday that several informal but direct contacts have been

made with political representatives of the viet cong in Moscow

Ciphertext:

UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZVUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSXEPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ

1. Can you determine the cipher used? If yes, name the cipher. Explain why.

2. Can you determine all/part of the key. If yes, give the key or part of the key.

3. Explain how you deduced it.

Answer 1

Ciphertext:

• UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZVUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSXEPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ

Plaintext:

• it was disclosed yesterday that several informal but direct contacts have been made with political representatives of the viet cong in Moscow

Answer: The cipher used here is Monoalphabetic Cipher, and key is 26 letters long and there can be maximum of 25! keys.

**Please Upvote

Explanation:

• Human languages are redundant .Frequency of each lettter used is different.
• The English letter e is by far the most common letter
• Then T,R,N,I,O,A,S
• Other letters are rarely used like Z,J,K,Q,X

Monoalphabetic substitution ciphers do not change relative letter frequencies therefore by calculating letter frequencies for ciphertext and comparing counts against known values will give back the Plain text.

The most common letters are P & Z and are equivalent to e and t

The most common Digram are ZW is equivalent to th and hence ZWP is the

Proceeding with trial and error finally get:

• it was disclosed yesterday that several informal but direct contacts have been made with political representatives of the viet cong in Moscow

• Since, each plaintext letter maps to a different random cipher text letter hence key is 26 letters long and we can have a total of 26! keys .

Example:

Plain: abcdefghijklmnopqrstuvwxyz

Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN