In reality, because case, word spacing and punctuation in the ciphertext give ad

Question

In reality, because case, word spacing and punctuation in the ciphertext give additional clues about the plaintext, they are usually removed, and the ciphertext is often organized into groups of characters. For example, the ciphertext in the example above would then look like the following:

Of course, then it is the responsibility of the person doing the decoding to reconstruct the original word spacing, etc. Removing such clues from the plaintext before enciphering it makes it quite a lot harder to crack the cipher.

When attempting to decipher a shift substitution ciphertext, if you don't already know the number of characters to shift, of course, you need to figure it out.

Using the “brute force” approach, we can try every possibility, until we find a reasonable looking plaintext.

Question 2: Given the approach described above, for a Shift Substitution Cipher, how many possibilities are there for a shift value? Is this a feasible task?

You can try this Java applet (http://pages.central.edu/emp/LintonT/classes/spring01/cryptography/java/Shift.html) [1] that implements this, if you'd like.

Analyze the letter frequency of the ciphertext, and try to deduce the shift value.

Explanation / Answer

If you have any problem with the answer just let me know in the comments and I'll try to solve it as soon as possible.

There 26 letters in the alphabet so we can shift the word by 25 position till we reach to the original position. Hence there are total 25 shift values which is achievable to crack.

KYZJ VETI PGKZ FEDV KYFU NRJL JVUS PIFD REJZ ETVR JRIJ NRIJ

Most frequent letter in cipher = j and we know that the most frequent letters are - ETAOINSHRDLU (in decreasing order of frequency), so we start by putting j=e, we get shift value of +5 and deciphered text will be :

FTUEQZODKBFUAZYQFTAPIMEGEQPNKDAYMZEUZOQMEMDEIMDE. Which does not make sense.

Now j=t, shift value of +16. Result:

UIJTFODSZQUJPONFUIPEXBTVTFECZSPNBOTJODFBTBSTXBST(not right)

Now j=a, shift value +9. Result :

BPQAMVKZGXBQWVUMBPWLEIACAMLJGZWUIVAQVKMIAIZAEIZA(not right)

Now j=o, shift value = +21. Result :

PDEOAJYNULPEKJIAPDKZSWOQOAZXUNKIWJOEJYAWOWNOSWNO(not right)

Now j=I, shift value = +25. Result :

LZAKWFUJQHLAGFEWLZGVOSKMKWVTQJGESFKAFUWSKSJKOSJK(not right)

Now, j=s, shift value = +17. Result :

THISENCRYPTIONMETHODWASUSEDBYROMANSINCEASARSWARS

The shift value of +17 is the correct key.

The decrypted message is :

THIS ENCRYPTION METHOD WAS USED BY ROMAN SINCE ASARS WARS.

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