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

#define STR_LEN 50

void decrypt(char* cipher, int n);
void strRev(char *str);

int main(void)
{
    char str[STR_LEN] = { 0 };
    int decKey = 0;

    printf("Enter cipher to decrypt: ");
    fgets(str, STR_LEN, stdin);
    *(str + strlen(str) - 1) = 0;

    printf("Enter decryption key: ");
    scanf("%d", &decKey);

    decrypt(str, decKey);
    printf("Decrypted text: %s", str);

    system("PAUSE");
}

void decrypt(char* cipher, int n)
{
    int i = 0, j = 0;

    strRev(cipher);

    for (; *cipher; cipher += 1)
    {
        *cipher = (((*cipher - 'a') + n) % 26) + 'a';
    }
}

void strRev(char *str)
{
    char tmp;
    int i = 0, j = 0;
    for (i = strlen(str) - 1, j = 0; i > j; i--, j++)
    {
        tmp = *(str + i);
        *(str + i) = *(str + j);
        *(str + j) = tmp;
    }
}