{"id":1335,"date":"2020-04-11T12:57:09","date_gmt":"2020-04-11T07:27:09","guid":{"rendered":"https:\/\/binaryterms.com\/?p=1335"},"modified":"2022-12-20T10:51:52","modified_gmt":"2022-12-20T05:21:52","slug":"substitution-technique-in-cryptography","status":"publish","type":"post","link":"https:\/\/binaryterms.com\/substitution-technique-in-cryptography.html","title":{"rendered":"Substitution Technique in Cryptography"},"content":{"rendered":"

Substitution technique<\/strong> is a classical encryption technique where the characters present in the original message<\/strong> are replaced<\/strong> by the other characters or numbers or by symbols.<\/strong> If the plain text (original message) is considered as the string of bits, then the substitution technique would replace bit pattern of plain text with the bit pattern of cipher text.<\/p>\n

We will discuss some of the substitution techniques which will help us to understand the procedure of converting plain text o cipher text.\u00a0 In this section, we will study the following substitution techniques:<\/p>\n

Substitution Technique:<\/h2>\n
    \n
  1. Caesar Cipher<\/a><\/li>\n
  2. Monoalphabetic Cipher<\/a><\/li>\n
  3. Playfair Cipher<\/a><\/li>\n
  4. Hill Cipher<\/a><\/li>\n
  5. Polyalphabetic Cipher<\/a><\/li>\n
  6. One-Time Pad<\/a><\/li>\n<\/ol>\n

    <\/a><\/p>\n

    Caesar Cipher<\/h3>\n

    This the simplest substitution cipher by Julius Caesar. In this substitution technique, to encrypt the plain text, each alphabet of the plain text is replaced by the alphabet three places further it. And to decrypt the cipher text each alphabet of cipher text is replaced by the alphabet three places before it.<\/p>\n

    Let us take a simple example:<\/p>\n

    Plain Text:<\/strong> meet me tomorrow<\/p>\n

    Cipher Text:<\/strong> phhw ph wrpruurz<\/p>\n

    Look at the example above, we have replaced, \u2018m\u2019 with \u2018p\u2019 which occur three places after, \u2018m\u2019. Similarly, \u2018e\u2019 is replaced with \u2018h\u2019 which occurs in three places after \u2018e\u2019.<\/p>\n

    Note:<\/strong> If we have to replace the letter \u2018z\u2019 then the next three alphabets counted after \u2018z\u2019 will be \u2018a\u2019 \u2018b\u2019 \u2018c\u2019. So, while counting further three alphabets if ‘z’ occurs it circularly follows ‘a’.<\/p>\n

    There are also some drawbacks of this simple substitution technique. If the hacker knows that the Caesar cipher is used then to perform brute force cryptanalysis, he has only to try 25 possible keys to decrypt the plain text.
    \nThe hacker is also aware of the encryption and decryption algorithm.
    \n    <\/a><\/p>\n

    Monoalphabetic Cipher<\/h3>\n

    Monoalphabetic cipher is a substitution cipher, where the cipher alphabet for each plain text alphabet is fixed, for the entire encryption.<\/p>\n

    In simple words, if the alphabet \u2018p\u2019 in the plain text is replaced by the cipher alphabet \u2018d\u2019. Then in the entire plain text wherever alphabet \u2018p\u2019 is used, it will be replaced by the alphabet \u2018d\u2019 to form the ciphertext.
    \n    <\/a><\/p>\n

    Playfair Cipher<\/h3>\n

    Playfair cipher is a substitution cipher which involves a 5X5 matrix. Let us discuss the technique of this Playfair cipher with the help of an example:<\/p>\n

    Plain Text:<\/strong> meet me tomorrow<\/p>\n

    Key:<\/strong> KEYWORD<\/p>\n

    Now, we have to convert this plain text to ciphertext using the given key. We will discuss the further process in steps.<\/p>\n

    Step 1:<\/strong> Create a 5X5 matrix and place the key in that matrix row-wise from left to right. Then put the remaining alphabet in the blank space.<\/p>\n

    \"Play<\/p>\n

     <\/p>\n

    Note:<\/strong> If a key has duplicate alphabets, then fill those alphabets only once in the matrix, and I & J should be kept together in the matrix even though they occur in the given key.<\/p>\n

    Step 2:<\/strong> Now, you have to break the plain text into a pair of alphabets.<\/p>\n

    Plain Text:<\/strong> meet me tomorrow<\/p>\n

    Pair:<\/strong> me<\/u> et<\/u> me<\/u> to<\/u> mo<\/u> rx<\/u> ro<\/u> wz<\/u><\/p>\n

    Note<\/strong><\/p>\n