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

void Encrypt(char* msg) {
    int i, j, k, m1, m2, m;
    char enc[26];
    char alfabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    char key[26];

    printf("key: ");
    scanf("%s", key);

    k = 0;
    for (i = 0; i < strlen(msg); i++) {
        for (j = 0; j < strlen(alfabet); j++) {
            if (msg[i] == alfabet[j])
                m1 = j;
            if (key[k] == alfabet[j])
                m2 = j;
        }
        m = (m1 + m2) % strlen(alfabet);
        enc[i] = alfabet[m];
        if (k == strlen(key) - 1)
            k = 0;
        else
            k++;
    }
    enc[i] = '\0';
    printf("%s\n", enc);
}

void Decrypt(char* msg) {
    int i, j, k, m1, m2, m;
    char dec[26];
    char alfabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    char key[26];

    printf("key: ");
    scanf("%s", key);

    k = 0;
    for (i = 0; i < strlen(msg); i++) {
        for (j = 0; j < strlen(alfabet); j++) {
            if (msg[i] == alfabet[j])
                m1 = j;
            if (key[k] == alfabet[j])
                m2 = j;
        }
        m = (m1 - m2 + strlen(alfabet)) % strlen(alfabet);
        dec[i] = alfabet[m];
        if (k == strlen(key) - 1)
            k = 0;
        else
            k++;
    }
    dec[i] = '\0';
    printf("%s\n", dec);
}

int main(int argc, char* argv[]) {
    if (argc == 1)
        printf("%s: no opt & msg", argv[0]);
    else if (argc == 2)
        printf("%s: no opt or msg", argv[0]);
    else if (argc == 3) {
        if (strcmp(argv[1], "-Ecsr") == 0)
            Encrypt(argv[2]);
        else if (strcmp(argv[1], "-Dcsr") == 0)
            Decrypt(argv[2]);
        else
            printf("%s: no such option\n", argv[0]);
    }
    return 0;
}