Question

Write a java code that will Implement Simple Substitution cipher with permutations and Implement Letter Frequency Analysis. As well as Apply Letter Frequency Analysis to chosen outputs of your Simple Substitution encryption.

Answer 1

The code to write simple substitution,

class substitution

{ public static StringBuffer encrypt(String text, int s)

    { StringBuffer result= new StringBuffer();

        for (int i=0; i<text.length(); i++)

        { if (Character.isUpperCase(text.charAt(i))) // since we are using the ASCII we need all of the word to be same case for the conversation so we'll do small and upper one by one

            {

char ch = (char)(((int)text.charAt(i) +s - 65) % 26 + 65);

                result.append(ch); // this is actually converting the acsii and then adding the position and them getting the character corresponding to that and replacing that in the result at the end.

            }else {

char ch = (char)(((int)text.charAt(i) + s - 97) % 26 + 97);

                result.append(ch); } }

        return result;

    }

Now we just have to call the function with the input and the number of places we want to shift this so if we need a simple substitution then the number of places will be same for each character, where as if you say as per the question you need a particular permuting then you can use the value of N from a array just make that edit, but simple substitution cipher does have only 1 value for n and not a variable so

String text = "put the plain text here";

int s = 4;

System.out.println("Cipher: " + encrypt(text, s));

Now the time to make a frequency for all the characters,

int n = str.length();

int[] freq = new int[26];

// we make table for frequency

for (int i = 0; i < n; i++)

freq[str.charAt(i) - 'a']++;

//Above we took the character and then subtracted the 'a' so that the number left is the index of frequency count of that character and we increased it by ++

Now it's upto the analysis posting who they want to take care of the crypto Analysis part, which means that we can just look at the frequency of characters and say the most frequently used is e and then make out the words like "the" or "is" as the most frequently coming three and two character words and like that but that all depends on the strategy we follow

for (int i = 0; i < n; i++) {

  if (freq[str.charAt(i) - 'a'] != 0) {

  System.out.print(str.charAt(i));

  System.out.print(freq[str.charAt(i) - 'a'] + " ");}

This will print the character and there frequency to visualise