Caesar Cipher in C - Sanfoundry

This is a C Program to implement Caesar cipher. This is the simplest of all, where every character of the message is replaced by its next 3rd character.

Here is source code of the C Program to Implement Caesar Cypher. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

```
#include <stdio.h>
```
```
#include <ctype.h>
```
```
 
```
```
#define MAXSIZE 1024
```
```
 
```
```
void encrypt(char*);
```
```
void decrypt(char*);
```
```
 
```
```
int menu();
```
```
 
```
```
int main(void) {
```
```
 
```
```
    char c, choice[2], s[MAXSIZE];
```
```
 
```
```
    while (1) {
```
```
        menu();
```
```
 
```
```
        gets(choice);
```
```
 
```

        if ((choice[0] == 'e') || (choice[0] == 'E')) {

            puts("Input text to encrypt->");

```
            gets(s);
```
```
            encrypt(s);
```

        } else if ((choice[0] == 'd') || (choice[0] == 'D')) {

            puts("Input text to decrypt->");

```
            gets(s);
```
```
            decrypt(s);
```
```
        } else
```
```
            break;
```
```
    }
```
```
 
```
```
    return 0;
```
```
}
```
```
 
```
```
void encrypt(char*str) {
```
```
    int n = 0;
```
```
    char *p = str, q[MAXSIZE];
```
```
 
```
```
    while (*p) {
```
```
        if (islower(*p)) {
```

            if ((*p >= 'a') && (*p < 'x'))

                q[n] = toupper(*p + (char) 3);

```
            else if (*p == 'x')
```
```
                q[n] = 'A';
```
```
            else if (*p == 'y')
```
```
                q[n] = 'B';
```
```
            else
```
```
                q[n] = 'C';
```
```
        } else {
```
```
            q[n] = *p;
```
```
        }
```
```
        n++;
```
```
        p++;
```
```
    }
```
```
    q[n++] = '\0';
```
```
    puts(q);
```
```
}
```
```
 
```
```
void decrypt(char*str) {
```
```
    int n = 0;
```
```
    char *p = str, q[MAXSIZE];
```
```
 
```
```
    while (*p) {
```
```
        if (isupper(*p)) {
```

            if ((*p >= 'D') && (*p <= 'Z'))

                q[n] = tolower(*p - (char) 3);

```
            else if (*p == 'A')
```
```
                q[n] = 'x';
```
```
            else if (*p == 'B')
```
```
                q[n] = 'y';
```
```
            else
```
```
                q[n] = 'z';
```
```
        } else {
```
```
            q[n] = *p;
```
```
        }
```
```
        n++;
```
```
        p++;
```
```
    }
```
```
    q[n++] = '\0';
```
```
    puts(q);
```
```
}
```
```
 
```
```
int menu() {
```
```
    puts("To encrypt, input e or E\n");
```
```
    puts("To decrypt, input d or D\n");
```

    puts("To exit, input any other letter\n");

```
    puts("Your choice:->\n");
```
```
    return 0;
```
```
}
```

Output:

$ gcc CaesarCipher.c
$ ./a.out
 
To encrypt, input e or E
To decrypt, input d or D
To exit, input any other letter
Your choice:-> e
Input text to encrypt-> DharmendraHingu
 
DKDUPHQGUDHLQJX
 
To encrypt, input e or E
To decrypt, input d or D
To exit, input any other letter
Your choice:-> x

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