{"id":29,"date":"2018-07-11T09:22:02","date_gmt":"2018-07-11T09:22:02","guid":{"rendered":"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/?post_type=chapter&#038;p=29"},"modified":"2019-05-13T11:28:47","modified_gmt":"2019-05-13T11:28:47","slug":"symmetric-cipher-model-substitution-techniques-ceasar-cipher-monoalphabetic-ciphers-playfair-cipher","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/itp4\/chapter\/symmetric-cipher-model-substitution-techniques-ceasar-cipher-monoalphabetic-ciphers-playfair-cipher\/","title":{"rendered":"Symmetric cipher model, substitution techniques-Ceasar cipher, Monoalphabetic ciphers, playfair cipher"},"content":{"raw":"<div><span style=\"float: right;\"><a href=\"https:\/\/youtu.be\/gOaawm0LlOc\" target=\"_blank\" rel=\"noopener\"><img src=\"http:\/\/epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/2018\/11\/download.png\" alt=\"epgp books\" width=\"75px\" height=\"75px;\" \/><\/a>\r\n<\/span><\/div>\r\n\r\n<strong>Symmetric Cipher Model \u2013 Key Terms<\/strong>\r\n\r\n&nbsp;\r\n<ul>\r\n \t<li>Plaintext \u2013 Original message or Data that is input to the algorithm.<\/li>\r\n \t<li>Encryption Algorithm \u2013 The algorithm that generates text by performing substitution or permutation.<\/li>\r\n \t<li>Secret Key \u2013 Key that decides substitution and transformation that is to be applied to the algorithm.<\/li>\r\n \t<li>Ciphertext \u2013 The substituted or permuted message which is produced by inputting plaintext and key to algorithm.<\/li>\r\n \t<li>Decryption Algorithm \u2013The algorithm takes ciphertext and key as input and generates plaintext.<\/li>\r\n<\/ul>\r\nFor secure communication using encryption following requirements must be satisfied:\r\n<ul>\r\n \t<li>Strong encryption algorithm - Given the algorithm and one or more ciphertexts attacker can neither decrypt the ciphertext nor find key.<\/li>\r\n \t<li>The key must be exchanged in secret manner by communicating entities.<\/li>\r\n<\/ul>\r\n<strong>Symmetric Encryption Model :<\/strong>\r\n\r\n<img class=\"size-full wp-image-30 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-6.png\" alt=\"\" width=\"838\" height=\"360\" \/>\r\n\r\n<strong>Symmetric Encryption Scheme :<\/strong>\r\n<ul>\r\n \t<li>Input is, Message P and the encryption key K - Algorithm forms the Ciphertext C denoted as C=E(K,P) .<\/li>\r\n \t<li>The receiver has key K. P = D(K,C)<\/li>\r\n<\/ul>\r\n<img class=\"size-full wp-image-32 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-8.png\" alt=\"\" width=\"863\" height=\"339\" \/>\r\n\r\n<strong>Three dimensions specify characteristics of Cryptographic systems:<\/strong>\r\n<ul>\r\n \t<li>The form of operations used for converting plaintext to ciphertext<\/li>\r\n \t<li>The number of key used<\/li>\r\n \t<li>The method used to process plaintext \u2013 block cipher, stream cipher<\/li>\r\n<\/ul>\r\n<img class=\"size-full wp-image-33 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-9.png\" alt=\"\" width=\"677\" height=\"178\" \/>\r\n\r\n<strong>Substitution Technique:<\/strong>\r\n<ul>\r\n \t<li>Each letter of the plaintext is replaced by other letter or by number or by symbol.<\/li>\r\n \t<li>Plaintext is bit sequence, ciphertext is also bit sequence<\/li>\r\n<\/ul>\r\n<img class=\"size-full wp-image-34 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-10.png\" alt=\"\" width=\"832\" height=\"775\" \/>\r\n\r\n&nbsp;\r\n<ul>\r\n \t<li>C = E(3, P)<\/li>\r\n<\/ul>\r\n=(p + 3) mod 26\r\n<ul>\r\n \t<li>General form,<\/li>\r\n<\/ul>\r\nC = E(K,P) = (P+K) mod 26 P = D(K,C) = (C-K) mod 26\r\n\r\n&nbsp;\r\n\r\n<img class=\"size-full wp-image-35 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-11.png\" alt=\"\" width=\"969\" height=\"273\" \/>\r\n<div>\r\n\r\n<strong>Permutation:<\/strong>\r\n\r\n&nbsp;\r\n\r\n\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Each element of set with finite elements can be arranged in the other controlled of all the elements of S where the element appears only once.\r\n\r\n\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 n elements can have n! permutations.\r\n\r\n\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 In Caesar cipher, if permutation of 26 alphabetic characters is done , then 26! Keys are possible.\r\n\r\n&nbsp;\r\n\r\n<strong>Monoalphabetic Substitution Cipher:<\/strong>\r\n\r\n&nbsp;\r\n\r\n\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 If per message different substitutions are done.\r\n\r\n\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Single plaintext alphabet to ciphertext alphabet mapping is done.\r\n\r\n\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 This approach is monoalphabetic substitution cipher.\r\n\r\n\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 This would make brute force cryptanalysis difficult.\r\n\r\n&nbsp;\r\n\r\n<strong>Is monoalphabetic substitution secure?<\/strong>\r\n\r\n&nbsp;\r\n\r\n\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Frequency of letters can be checked in the ciphertext and can be compared to a standard frequency distribution in English language\r\n<p style=\"text-align: justify\">\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 In the ciphertext, if S occurs for highest times, it is equivalent to e. That way letters with high frequency can be mapped with letters mapped to the set {a,I,o,n}. The letters having lower frequencies can be mapped with letters from the set {b,q,v,x}<\/p>\r\n&nbsp;\r\n\r\n<strong>Frequency analysis monoalphabetic substitution :<\/strong>\r\n<ul>\r\n \t<li>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Use two letter combinations \u2013 known as digrams.<\/li>\r\n \t<li>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 The most common digrams in English language is \u201cth\u201d \u2013 so if in ciphertext \u201cSY\u201d appears maximum time , it can be mapped to \u201cSY\u201d.<\/li>\r\n \t<li>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 If \u201cSYR\u201d occurs maximum time, it can be mapped to \u201cthe\u201d.<\/li>\r\n<\/ul>\r\n<strong>To avoid frequency analysis \u2013 Two methods are used:<\/strong>\r\n\r\n&nbsp;\r\n\r\n1.\u00a0\u00a0\u00a0\u00a0\u00a0 Encrypt multiple letters of plaintext. \u2013 Playfair cipher\r\n\r\n2.\u00a0\u00a0\u00a0\u00a0\u00a0 Use multiple cipher alphabets. \u2013 Hill cipher\r\n\r\n&nbsp;\r\n\r\n<strong>Playfair cipher<\/strong>\r\n\r\n&nbsp;\r\n\r\n\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 It works on digrams of plaintext and substitutes these by digrams pertaining to ciphertext.\r\n\r\n\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 The playfair algorithm works with 5x5 matrix which is constructed using a keyword.\r\n\r\n<\/div>\r\n<img class=\"size-full wp-image-36 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-12.png\" alt=\"\" width=\"862\" height=\"725\" \/>\r\n<ul>\r\n \t<li><strong>Rules for the Playfair cipher<\/strong><\/li>\r\n<\/ul>\r\n<ol>\r\n \t<li>Two letters of plaintext are encrypted at the same time. Group of two letters are formed.<\/li>\r\n \t<li>If pair contains same letters, they can be separated by any filler letter. For example, tree would be trexex<\/li>\r\n<\/ol>\r\n<img class=\"size-full wp-image-37 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-13.png\" alt=\"\" width=\"645\" height=\"182\" \/>\r\n\r\n<img class=\"size-full wp-image-38 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-14.png\" alt=\"\" width=\"804\" height=\"504\" \/>\r\n\r\n<img class=\"size-full wp-image-39 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-15.png\" alt=\"\" width=\"738\" height=\"488\" \/>\r\n\r\nRW is substituted by WI.\r\n<ol start=\"5\">\r\n \t<li>If not 3,4 then from pair, each pair of plaintext is replaced by the letter at intersection point of its own row and column pertaining to the other letter in the plaintext. WD is replaced by ZI.<\/li>\r\n<\/ol>\r\n<strong>Strength of Playfair cipher:<\/strong>\r\n<ul>\r\n \t<li>Frequency analysis reveals structure that can be worked upon by cryptanalyst.<\/li>\r\n<\/ul>\r\n\r\n<table>\r\n<tbody>\r\n<tr>\r\n<td><strong>you can view video on Symmetric cipher model, substitution techniques-Ceasar cipher, Monoalphabetic ciphers, playfair cipher<\/strong><\/td>\r\n<td><a href=\"https:\/\/youtu.be\/gOaawm0LlOc\" target=\"_blank\" rel=\"noopener\"><img class=\"alignnone wp-image-120\" src=\"http:\/\/epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/2018\/11\/download.png\" alt=\"\" width=\"36\" height=\"36\" \/><\/a><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n\r\n<strong>Suggested Reading:<\/strong>\r\n\r\n&nbsp;\r\n<ol>\r\n \t<li>Cryptography and Network Security Principles and Practice by William Stallings, sixth Edition, PEARSON.<\/li>\r\n \t<li>Security in Computing by Charles Pfleeger &amp; Shari Lawrence Pfleeger, fourth Edition, PEARSON.<\/li>\r\n \t<li>Network Security by Charlie Kaufman, Radia Perlman, Mike Speciner, second Edition, PHI.<\/li>\r\n \t<li>The Complete Reference \u2013 Network Security by Roberta Bragg, Mark Rhodes-Ousley &amp; Keith Strassberg, Tata McGraw Hill<\/li>\r\n \t<li>Network Security Bible by Eric Cole, Ronald Krutz, James Conley, Wiley<\/li>\r\n \t<li>Hacking 6 Exposed by Stuart McClure, Joel Scambray &amp; George Kurtz , Tata McGraw Hill .<\/li>\r\n \t<li><a href=\"http:\/\/www.snort.org\/\">www.snort.org<\/a><\/li>\r\n \t<li><a href=\"https:\/\/nmap.org\/\">https:\/\/nmap.org<\/a><\/li>\r\n<\/ol>","rendered":"<div><span style=\"float: right;\"><a href=\"https:\/\/youtu.be\/gOaawm0LlOc\" target=\"_blank\" rel=\"noopener\"><img decoding=\"async\" src=\"http:\/\/epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/2018\/11\/download.png\" alt=\"epgp books\" width=\"75px\" height=\"75px;\" \/><\/a><br \/>\n<\/span><\/div>\n<p><strong>Symmetric Cipher Model \u2013 Key Terms<\/strong><\/p>\n<p>&nbsp;<\/p>\n<ul>\n<li>Plaintext \u2013 Original message or Data that is input to the algorithm.<\/li>\n<li>Encryption Algorithm \u2013 The algorithm that generates text by performing substitution or permutation.<\/li>\n<li>Secret Key \u2013 Key that decides substitution and transformation that is to be applied to the algorithm.<\/li>\n<li>Ciphertext \u2013 The substituted or permuted message which is produced by inputting plaintext and key to algorithm.<\/li>\n<li>Decryption Algorithm \u2013The algorithm takes ciphertext and key as input and generates plaintext.<\/li>\n<\/ul>\n<p>For secure communication using encryption following requirements must be satisfied:<\/p>\n<ul>\n<li>Strong encryption algorithm &#8211; Given the algorithm and one or more ciphertexts attacker can neither decrypt the ciphertext nor find key.<\/li>\n<li>The key must be exchanged in secret manner by communicating entities.<\/li>\n<\/ul>\n<p><strong>Symmetric Encryption Model :<\/strong><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-30 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-6.png\" alt=\"\" width=\"838\" height=\"360\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-6.png 838w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-6-300x129.png 300w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-6-768x330.png 768w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-6-65x28.png 65w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-6-225x97.png 225w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-6-350x150.png 350w\" sizes=\"auto, (max-width: 838px) 100vw, 838px\" \/><\/p>\n<p><strong>Symmetric Encryption Scheme :<\/strong><\/p>\n<ul>\n<li>Input is, Message P and the encryption key K &#8211; Algorithm forms the Ciphertext C denoted as C=E(K,P) .<\/li>\n<li>The receiver has key K. P = D(K,C)<\/li>\n<\/ul>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-32 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-8.png\" alt=\"\" width=\"863\" height=\"339\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-8.png 863w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-8-300x118.png 300w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-8-768x302.png 768w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-8-65x26.png 65w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-8-225x88.png 225w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-8-350x137.png 350w\" sizes=\"auto, (max-width: 863px) 100vw, 863px\" \/><\/p>\n<p><strong>Three dimensions specify characteristics of Cryptographic systems:<\/strong><\/p>\n<ul>\n<li>The form of operations used for converting plaintext to ciphertext<\/li>\n<li>The number of key used<\/li>\n<li>The method used to process plaintext \u2013 block cipher, stream cipher<\/li>\n<\/ul>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-33 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-9.png\" alt=\"\" width=\"677\" height=\"178\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-9.png 677w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-9-300x79.png 300w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-9-65x17.png 65w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-9-225x59.png 225w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-9-350x92.png 350w\" sizes=\"auto, (max-width: 677px) 100vw, 677px\" \/><\/p>\n<p><strong>Substitution Technique:<\/strong><\/p>\n<ul>\n<li>Each letter of the plaintext is replaced by other letter or by number or by symbol.<\/li>\n<li>Plaintext is bit sequence, ciphertext is also bit sequence<\/li>\n<\/ul>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-34 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-10.png\" alt=\"\" width=\"832\" height=\"775\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-10.png 832w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-10-300x279.png 300w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-10-768x715.png 768w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-10-65x61.png 65w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-10-225x210.png 225w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-10-350x326.png 350w\" sizes=\"auto, (max-width: 832px) 100vw, 832px\" \/><\/p>\n<p>&nbsp;<\/p>\n<ul>\n<li>C = E(3, P)<\/li>\n<\/ul>\n<p>=(p + 3) mod 26<\/p>\n<ul>\n<li>General form,<\/li>\n<\/ul>\n<p>C = E(K,P) = (P+K) mod 26 P = D(K,C) = (C-K) mod 26<\/p>\n<p>&nbsp;<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-35 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-11.png\" alt=\"\" width=\"969\" height=\"273\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-11.png 969w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-11-300x85.png 300w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-11-768x216.png 768w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-11-65x18.png 65w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-11-225x63.png 225w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-11-350x99.png 350w\" sizes=\"auto, (max-width: 969px) 100vw, 969px\" \/><\/p>\n<div>\n<p><strong>Permutation:<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Each element of set with finite elements can be arranged in the other controlled of all the elements of S where the element appears only once.<\/p>\n<p>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 n elements can have n! permutations.<\/p>\n<p>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 In Caesar cipher, if permutation of 26 alphabetic characters is done , then 26! Keys are possible.<\/p>\n<p>&nbsp;<\/p>\n<p><strong>Monoalphabetic Substitution Cipher:<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 If per message different substitutions are done.<\/p>\n<p>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Single plaintext alphabet to ciphertext alphabet mapping is done.<\/p>\n<p>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 This approach is monoalphabetic substitution cipher.<\/p>\n<p>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 This would make brute force cryptanalysis difficult.<\/p>\n<p>&nbsp;<\/p>\n<p><strong>Is monoalphabetic substitution secure?<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Frequency of letters can be checked in the ciphertext and can be compared to a standard frequency distribution in English language<\/p>\n<p style=\"text-align: justify\">\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 In the ciphertext, if S occurs for highest times, it is equivalent to e. That way letters with high frequency can be mapped with letters mapped to the set {a,I,o,n}. The letters having lower frequencies can be mapped with letters from the set {b,q,v,x}<\/p>\n<p>&nbsp;<\/p>\n<p><strong>Frequency analysis monoalphabetic substitution :<\/strong><\/p>\n<ul>\n<li>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Use two letter combinations \u2013 known as digrams.<\/li>\n<li>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 The most common digrams in English language is \u201cth\u201d \u2013 so if in ciphertext \u201cSY\u201d appears maximum time , it can be mapped to \u201cSY\u201d.<\/li>\n<li>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 If \u201cSYR\u201d occurs maximum time, it can be mapped to \u201cthe\u201d.<\/li>\n<\/ul>\n<p><strong>To avoid frequency analysis \u2013 Two methods are used:<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p>1.\u00a0\u00a0\u00a0\u00a0\u00a0 Encrypt multiple letters of plaintext. \u2013 Playfair cipher<\/p>\n<p>2.\u00a0\u00a0\u00a0\u00a0\u00a0 Use multiple cipher alphabets. \u2013 Hill cipher<\/p>\n<p>&nbsp;<\/p>\n<p><strong>Playfair cipher<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 It works on digrams of plaintext and substitutes these by digrams pertaining to ciphertext.<\/p>\n<p>\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 The playfair algorithm works with 5&#215;5 matrix which is constructed using a keyword.<\/p>\n<\/div>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-36 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-12.png\" alt=\"\" width=\"862\" height=\"725\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-12.png 862w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-12-300x252.png 300w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-12-768x646.png 768w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-12-65x55.png 65w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-12-225x189.png 225w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-12-350x294.png 350w\" sizes=\"auto, (max-width: 862px) 100vw, 862px\" \/><\/p>\n<ul>\n<li><strong>Rules for the Playfair cipher<\/strong><\/li>\n<\/ul>\n<ol>\n<li>Two letters of plaintext are encrypted at the same time. Group of two letters are formed.<\/li>\n<li>If pair contains same letters, they can be separated by any filler letter. For example, tree would be trexex<\/li>\n<\/ol>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-37 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-13.png\" alt=\"\" width=\"645\" height=\"182\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-13.png 645w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-13-300x85.png 300w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-13-65x18.png 65w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-13-225x63.png 225w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-13-350x99.png 350w\" sizes=\"auto, (max-width: 645px) 100vw, 645px\" \/><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-38 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-14.png\" alt=\"\" width=\"804\" height=\"504\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-14.png 804w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-14-300x188.png 300w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-14-768x481.png 768w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-14-65x41.png 65w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-14-225x141.png 225w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-14-350x219.png 350w\" sizes=\"auto, (max-width: 804px) 100vw, 804px\" \/><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-39 aligncenter\" src=\"http:\/\/itp4.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-15.png\" alt=\"\" width=\"738\" height=\"488\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-15.png 738w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-15-300x198.png 300w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-15-65x43.png 65w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-15-225x149.png 225w, https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-content\/uploads\/sites\/25\/2018\/07\/Untitled-15-350x231.png 350w\" sizes=\"auto, (max-width: 738px) 100vw, 738px\" \/><\/p>\n<p>RW is substituted by WI.<\/p>\n<ol start=\"5\">\n<li>If not 3,4 then from pair, each pair of plaintext is replaced by the letter at intersection point of its own row and column pertaining to the other letter in the plaintext. WD is replaced by ZI.<\/li>\n<\/ol>\n<p><strong>Strength of Playfair cipher:<\/strong><\/p>\n<ul>\n<li>Frequency analysis reveals structure that can be worked upon by cryptanalyst.<\/li>\n<\/ul>\n<table>\n<tbody>\n<tr>\n<td><strong>you can view video on Symmetric cipher model, substitution techniques-Ceasar cipher, Monoalphabetic ciphers, playfair cipher<\/strong><\/td>\n<td><a href=\"https:\/\/youtu.be\/gOaawm0LlOc\" target=\"_blank\" rel=\"noopener\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-120\" src=\"http:\/\/epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/2018\/11\/download.png\" alt=\"\" width=\"36\" height=\"36\" \/><\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><strong>Suggested Reading:<\/strong><\/p>\n<p>&nbsp;<\/p>\n<ol>\n<li>Cryptography and Network Security Principles and Practice by William Stallings, sixth Edition, PEARSON.<\/li>\n<li>Security in Computing by Charles Pfleeger &amp; Shari Lawrence Pfleeger, fourth Edition, PEARSON.<\/li>\n<li>Network Security by Charlie Kaufman, Radia Perlman, Mike Speciner, second Edition, PHI.<\/li>\n<li>The Complete Reference \u2013 Network Security by Roberta Bragg, Mark Rhodes-Ousley &amp; Keith Strassberg, Tata McGraw Hill<\/li>\n<li>Network Security Bible by Eric Cole, Ronald Krutz, James Conley, Wiley<\/li>\n<li>Hacking 6 Exposed by Stuart McClure, Joel Scambray &amp; George Kurtz , Tata McGraw Hill .<\/li>\n<li><a href=\"http:\/\/www.snort.org\/\">www.snort.org<\/a><\/li>\n<li><a href=\"https:\/\/nmap.org\/\">https:\/\/nmap.org<\/a><\/li>\n<\/ol>\n","protected":false},"author":4,"menu_order":2,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":["miss-hiteishi-diwanji"],"pb_section_license":""},"chapter-type":[],"contributor":[58],"license":[],"class_list":["post-29","chapter","type-chapter","status-publish","hentry","contributor-miss-hiteishi-diwanji"],"part":3,"_links":{"self":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-json\/pressbooks\/v2\/chapters\/29","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-json\/wp\/v2\/users\/4"}],"version-history":[{"count":6,"href":"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-json\/pressbooks\/v2\/chapters\/29\/revisions"}],"predecessor-version":[{"id":423,"href":"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-json\/pressbooks\/v2\/chapters\/29\/revisions\/423"}],"part":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-json\/pressbooks\/v2\/parts\/3"}],"metadata":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-json\/pressbooks\/v2\/chapters\/29\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-json\/wp\/v2\/media?parent=29"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-json\/pressbooks\/v2\/chapter-type?post=29"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-json\/wp\/v2\/contributor?post=29"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/itp4\/wp-json\/wp\/v2\/license?post=29"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}