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Given a keyword, you can encrypt a message using alphabetical substitution like this:

plain: 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
cipher: C I P H E R A B D F G J K L M N O Q S T U V W X Y Z
result: C A K S Y I B L T Z P D M U H F N V E G O W R J Q X

My problem is my professor literally never covered this topic and is putting on our upcoming exam. The example problem he gave us is as follows...

Use your knowledge of Substitution Ciphers to find the keyword:

UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ itwasdisclosedyesterdaythatseveralinformalbut

VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX directcontactshavebeenmadewithpolitical

EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ representativesofthevietconginmoscow

I have tried using frequency analysis of the ciphertext, I found that the most common plaintext letters for the keyword are "e","t", "i", "a", and possible "o" and "c".

Is there something I am missing?

EDIT: Looking online, I learned this is something called a Playfair cipher. Still cannot solve it though :( i don't know why I am having so much trouble with this.

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  • $\begingroup$ You can just find a letter, say t and find it will be substituted by Z. So the letter at the position of t - let's call it k - is Z - t as t + k = Z and therefore k = Z - t. And that's all she wrote. $\endgroup$ – Maarten Bodewes Feb 23 '17 at 11:17
  • $\begingroup$ Playfair works with pairs of letters and is quite different than the monoalphabetic substitution you have here $\endgroup$ – Eugene Styer Feb 23 '17 at 15:15
  • $\begingroup$ @EugeneStyer anyway you can help me out? $\endgroup$ – Roach Feb 23 '17 at 15:32
  • $\begingroup$ Can you please read Wikipedia and explain what parts exactly you don't understand / know to apply? $\endgroup$ – SEJPM Feb 23 '17 at 16:27