Question

In cryptography, a Caesar Cipher is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caesar, who used it in his private correspondence. wiki/Caesar cipher A B C D E F A BCD E F In a Caesar Cipher, the encryption is performed against a key string, which is often just the 26 letters of the alphabet. An offset value is defined that consists of a positive or negative integer The plaintext (the normal text is encrypted, one letter at a time, by using that offset value in the following way: (a) the index position within the key string of the letter to be encrypted is determined, (b) the offset value is applied to that index in order to obtain a new index within the key string, (c) the character at the new index is written as the cipher text. For a simple example, let's assume the key string consists of the 26 letters of the alphabet plus a Space S1325-Introduction to Programming page: 2 Here we have the 26 letters of the English alphabet plus a space with their index positions listed above. To simplify, we'll stick with all caps for the moment.) We want to encrypt the phrase "I LOVE TO FLY" using an offset off +5. We get the following Plain text: I LOVE TO FLY Cipher text: NEQT JEYTEKQC Note that I' has an index of 8. When we add 5 we get an index of 13, which corresponds to the letter "N'. Similarly, the letter "Y' has an index of 24. When we add 5 we get 29, but since that index doesn't exist within the array, we "wrap around" and replace the Y' with a C To decrypt a message, we apply the offset value in reverse. If the offset were added to produce the cipher text, then it should be subtracted to reconstitute the plaintext.

Write the programming C please, not C++. The main function should be to find the offset value of the ciper text "wPL2KLK9PWWZ7K3ST24KZYKfPMKJ4SKLYOKRP4KFKP842LK0ZTY43 " and decrypt it.

Answer 1

#include<stdio.h>
#include<string.h>
int decrypt(char* text, int s)
{
    int i; char result[128];
    for(i=0; i<strlen(text); i++)
    {
         result[i] = text[i]-s;
    }
    printf("Decrypt: %s\n", result);
}
void encrypt(char text[], int s)
{
   char result[128];
   int i,length=0;
  
  
   for (i=0; i<strlen(text);i++)
   {
       // Encrypt Uppercase letters
       if (isupper(text[i]))
           result[i] = (text[i]+s-65)%26 +65;
            // Encrypt Lowercase letters
        else
           result[i]=(text[i]+s-97)%26 +97;
        }
    printf("Cipher: %s\n",result);
   decrypt(result, s);
}

int main()
{
   char text[128]="I LOVE FLY";
   int s =5;
   printf("Text: %s\n",text);
   encrypt(text, s);
   return 0;
}

sh-4.25 gcc -o maïn·c sh-4.2$ main Text: I LOVE FLY Cipher: NYQTAJYKQD Decrypt: ITLO ETFL? sh-4.2$

sh-4.2$ mairn Text: abcdef Cipher: fghijk Decrypt: abcdef sh-4.2$