Another version of the columnar transposition cipher is the cipher usi

Question

Another version of the columnar transposition cipher is the cipher using a key word. In this cipher, we encrypt as follows: Given a key word, we remove all the duplicate characters in the key word. For example, if the key word is ‘SECRET’, we remove the second ‘E’ and use ‘SECRT’ as the key word. To encrypt, we form a table as follows: In the first row, we write down the key word. In the following rows, we write the plaintext. Suppose we want to encrypt the text ‘ATTACKATDAWN’. We make a table as follows:

S	E	C	R	T
A	T	T	A	C
K	A	T	D	A
W	N	X	X	X

Then we read off the columns in alphabetical order. We first read the column under ‘C’, followed by the columns under ‘E’, ‘R’, ‘S’ and ‘T’. We get the cipher text TTX TAN ADX AKW CAX. To decrypt, we reverse the process. Note that, since we know the length of the keyword, we can find the length of the columns by dividing the length of the message by the length of the keyword.

Given the ciphertext ‘HNDWUEOESSRORUTXLARFASUXTINOOGFNEGASTORX’ and the key word ‘LANCE’, find the plaintext.

05 Feb 2025

Answer :

Word Count : 783

The given ciphertext is: ``` HNDWUEOESSRORUTXLARFASUXTINOOGFNEGASTORX ``` The key word is LANCE. First, remove duplicates (none here), so the key remains LANCE. Step 1: Determine the column order Alphabetically, the letters in the key LANCE are: ``` A C E L N ``` So the column order is: 1. A → 2nd column 2. C → 3rd column 3. E → 5th column 4. L → 1st column 5. N → 4th column Step 2: Determine column lengths Ciphertext length = 42 Keyword length = 5 Rows = ceil(42 / 5) = 9 (since 5 × 8 = 40 < 42, so we need 9 rows) So the table has 9 rows × 5 columns = 45 cells. Ciphertext has 42 letters, so last 3 cells are padding (X or empty). Step 3: Determine number of letters per column Columns corresponding to the alphabetical order: * Columns A, C, E, L, N → assign letters evenly from ciphertext. Let’s distribute 42 letters into 5 columns: 42 ÷ 5 = 8 full letters each + 2 extra letters in the first 2 columns. So columns length: * Column A → 9 letters * Column C → ____ ______ ______ _________ _____ _____ ________ _____.
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