Caesar.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void encrypt(char* text, size_t text_len, char key){
    int i = 0;

    for (i = 0; i < text_len; i++){
            if (('a' <= text[i]) && (text[i] <= 'z')){
                text[i] = 'a' + (1 + (text[i] + key) % 'a') % 26;
            }
            else if(('A' <= text[i]) && (text[i] <= 'Z')){
                text[i] = 'A' + (1 + (text[i] + key + 'A' - 'a') % 'A') % 26;
            }
    }
}

void decrypt1(char* text, size_t text_len, char key){
    int i = 0;
    key = 'z' - key + 'a' + 1;

    for (i = 0; i < text_len; i++){
            if (('a' <= text[i]) && (text[i] <= 'z')){
                text[i] = 'a' + (text[i] + key - 1) % 'a' % 26;
            }
            else if(('A' <= text[i]) && (text[i] <= 'Z')){
                text[i] = 'A' + (text[i] + key + 'A' - 'a' - 1) % 'A' % 26;
            }
    }
}

void decrypt2(char* text, size_t text_len, char key){
    int i = 0;

    for (i = 0; i < text_len; i++){
            if (('a' <= text[i]) && (text[i] <= 'z')){
                text[i] = 'a' + ('z' + text[i] - key) % 'a' % 26;
            }
            else if(('A' <= text[i]) && (text[i] <= 'Z')){
                text[i] = 'A' + ('Z' + text[i] - key - 'A' + 'a') % 'A' % 26;
            }
    }
}

int main(){
    char text[256];
    size_t text_len;
    char k = 'c';

    scanf("%s", &text);
    text_len = strlen(text);
    encrypt(text, text_len, k);
    printf("Plain  text: %s\n", text);

    decrypt1(text, text_len, 'c');
    printf("decrypt1\nCipher text: %s\n", text);

    encrypt(text, text_len, k);

    decrypt2(text, text_len, k);
    printf("decrypt2\nCipher text: %s\n", text);

    return 0;
}