Please help!!! \"A permutation cipher is one in which the letters of the message
ID: 2965478 • Letter: P
Question
Please help!!!
"A permutation cipher is one in which the letters of the message are reordered according to some method agreed upon by the correspondents. The scytale is one sort of permutation cipher. Another, more suited to pencil and paper, is what we will call a rotation. Encipherment consists of grouping message letters into blocks and then rotating the letters in each block by some specified number of letters. For example, if the block size is 5 and the rotation 2, then LOVE IS BLIND; FRIENDSHIP CLOSES ITS EYES becomes VEILO LINSB RIEDF SHIND LOSPC ITSES ESEY.
(a) Describe the decipherment algorithm for a rotation with block length b and rotation r.
(b) The message UNITH ESEIV ERDEO SMAOR IFSTO TOESN OMSFA T was enciphered with a block length of 6 and rotation 3. Decipher it.
(c) Suppose IVGTH EPEEP LONEA WWERD ONDAH ETTHY NKIHE THAYE AVEWN ACFT is an encipherment by a rotation. Describe a systematic approach to breaking the cipher and use it to recover the message.
(d) Suppose a cryptanalyst suspects that SECEC SYHRI IRFET SSETE INLST AFNIA FSOAI HFSRT TEATE was obtained by a succession of two rotations with different block lengths and rotation amounts. Compare the amount of work the cryptanalyst is likely to require under this assumption with what she is likely to require under the assumption of a single rotation. Try to do the cryptanalyst's work."
-Please show all work, specifically on part (d). Thank you!!!
Explanation / Answer
(a)
Given block length b and rotation r.
1. Break the message in blocks of size b
2. Rotate each block by ( (b - r) modulo b ).
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(b)
Using the above technique in part (a),
Breaking in blocks of size 6:
UNITHE SEIVER DEOSMA ORIFST OTOESN OMSFAT
Rotating each block by ((6-3) modulo 3) = 3:
THEUNI VERSEI SMADEO FSTORI ESNOTO FATOMS
After deciphering we get (applying spaces at appropriate places)
THE UNIVERSE IS MADE OF STORIES NOT OF ATOMS
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(c)
A systematic approach.
1. We vary the block length (B) from => 2 to L - 1
2. We vary the Rotation (R) from => 1 to B - 1
3. Then for each such (block length, Rotation) pair, we apply the decipher algorithm in part (a).
4. All answers except 1 will give gibberish results. The one with meaningful answer will be our final answer.
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(d)
A systematic approach.
1. We vary the block length (B1) from => 2 to L - 1
2. We vary the Rotation (R1) from => 1 to B - 1
3. Then for each such (block length, Rotation) pair, we apply the decipher algorithm in part (a).
4. Now for each such deciphered answer we again apply the same decipherment (for 2nd rotation) i.e
4.1. We vary the block length (B2) from => 2 to L - 1
4.2. We vary the Rotation (R2) from => 1 to B - 1
5. Now out of all such results we choose the one and only meaningful (non-gibberish) answer
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Basically in part (d) , apply algorithm in part (c) 2 times back to back, and choose the meaningful (non-gibberish) answer in the end
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