#include <stdio.h>
#include <string.h>
#include <ctype.h>
#define DIM 1000

char encode(char letter, int key);
char decode(char letter, int key);

int main()
{
    char phrase[DIM];
    char code_word[DIM];
    int keys[DIM];

    printf("Please enter your code phrase: ");
    gets(code_word);
    for (int i = 0; i < strlen(code_word); i++) {
        keys[i] = toupper(code_word[i]) - 'A' + 1;
    }
    const int MOD = strlen(code_word);

    printf("Please enter phrase you want to cipher: ");
    gets(phrase);

    char coded[DIM];
    for (int i = 0; i < strlen(phrase); i++) {
        coded[i] = encode(phrase[i], keys[i%MOD]);
    }

    char decoded[DIM];
    for (int i = 0; i < strlen(coded); i++) {
        decoded[i] = decode(coded[i], keys[i%MOD]);
    }

    printf("Coded phrase is: %s\n", coded);
    printf("Decoded phrase is: %s\n", decoded);
    return 0;
}

char encode(char letter, int key)
{
    if (!isalpha(letter)) return letter;
    char temp = letter;
    temp = toupper(temp);
    temp += key;
    if (temp > 'Z') temp = 'A' + temp - 'Z' - 1;
    return (isupper(letter) ? temp : tolower(temp));
}
char decode(char letter, int key)
{
    if (!isalpha(letter)) return letter;
    char temp = letter;
    temp = toupper(temp);
    temp -= key;
    if (temp < 'A') temp = 'Z' - ('A' - temp) + 1;
    return (isupper(letter) ? temp : tolower(temp));
}