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Okay, I have this question. First time taking the cryptography course so I have some confusions. First of all, "A Brilliant Detective" part does it mean anything ? I mean, Does it refer to something ?

Also, as far as I am concern decryption should be using the following argument in order to find how substitution cipher works : P(PlainText) = C(CipherText) - K(Key) Mod26

But how can I find the K in this equation ?

Mark
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As V is the most common letter in the ciphertext, it may well represent E because E is the most common letter in English. That would give $5=22-K, K=17$. Try that $K$ and see if the text comes out readable. If not, try S and X for E. One of them will probably work. The removal of "the" from the text has changed the letter distributions so A or I may be the most common letter.

An easy way to check is a spreadsheet. Put the numeric equivalents of the first fifteen or twenty letters in the A column. Use the next $25$ columns for each of the possible values of K and translate the numbers back to letters. Only one column will read properly.

Ross Millikan
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  • Okay and also is this the only way solving the question ? Are there any others ? Just asking out of curiousty – Mark Oct 05 '20 at 14:51
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    I think it is the easiest for a cipher like this. Another approach is to look at the list of common bigrams in English. See if the spacing between the letters matches the spacing of the ones common in the text. Yet another is to look for letter patterns of common words like THAT. It becomes harder, but still not so hard, if the key can be any permutation of the letters, a substitution cipher. – Ross Millikan Oct 05 '20 at 15:00
  • I see, thank you for the help appreciated. – Mark Oct 05 '20 at 15:01