A disadvantage of the general monoalphabetic cipher is that bo sender and receiv

Question

A disadvantage of the general monoalphabetic cipher is that bo sender and receiver must commit the permuted cipher sequence to memory. A common technique for avoiding this is to use a keyword from which the cipher sequence can be generated. For example, using the keyword CIPHER , write out the keyword followed by unused letters in normal order and match this against the plaintext letters:

The question is to determine the Key word.

plain: abcdefghjk1mnoparstuvwxyz If it is felt that this process does not produce sufficient mixing, write the remaining letters on successive lines and then generate the sequence by reading down the columns: CIPHER A B D F G J K L M N O Q S T U V W x Y Z This yields the sequence: CAK SYIBLTZ PDMUHFNVEGOWRJQ X Such a system is used in the example in Section 2.2 (the one that begins "it was disclosed yesterday", see below if you don't have the book). Determine the keyword. cipher: UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX EPYEPOPDZSZUFPOMBZWP FUP ZHMDJUDTMOHMQ plain: it was disclosed yesterday that several informal but direct contacts have been made with political representatives of the viet cong in moscow

Explanation / Answer

Plain Text : ITWASDISCLOSEDYESTERDAYTHATSEVERALINFORMALBUTDIRECTCONTACTSHAVEBEENMADEWITHPOLITICALREPRESENTATIVESOFTHEVIETCONGINMOSCOW

Cipher Text : UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZVUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSXEPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ

Solution :

The keyword is 'SPUTNIK'.

Explanation :

We should first find the cipher sequence in order to find the cipher key word.

In order to find the cipher sequence we have to compare the both text's given.

I->U

T->Z

W->Q

A->S

S->O

D->V

.

.

.

.

.

.

.

.

W->Q

By comparing the two sets I came to conclusion about the series as :

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

now writing the sequence into columns :

S P U T N I K

A B C D E F G

H J L M O Q R

V W X Y Z

Hence from the above columns we can take out the first column and it is the cipher word :

' SPUTNIK '

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