/*
 * Vigenere cipher
 */

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

#define N 128

int main(int argc, char *argv[])
{
    char buf[N+1], key[N+1], c;
    int i, shift, l, k, encrypt = 0;

    if (argc != 4)
    {
        fprintf(stderr, "Usage: %s (e|d) key text\n", argv[0]);
        exit(1);
    }

    if (!((strlen(argv[1]) == 1) && ((argv[1][0] == 'e') || (argv[1][0] == 'd'))))
    {
        fprintf(stderr, "Usage: %s (e|d) key text\n  e indicates encryption, d - decryption", argv[0]);
        exit(1);
    }
    if (argv[1][0] == 'e')
    {
        encrypt = 1;
    }

    if (strlen(argv[2]) > N)
    {
        fprintf(stderr, "%s: key is too long, only the first %d bytes will be used\n", argv[0], N);
    }
    strncpy(key, argv[2], N);
    l = strlen(key);
    for (i = 0; i < l; i++)
    {
        if (!islower(key[i]))
        {
            fprintf(stderr, "%s: only lower case letters  are allowed in key \n", argv[0]);
            exit(1);
        }
    }

    if (strlen(argv[3]) > N)
    {
        fprintf(stderr, "%s: text is too long, only the first %d bytes will be encrypted\n", argv[0], N);
    }
    strncpy(buf, argv[3], N);

    for (i = 0; i < strlen(buf); i++)
    {
        c = buf[i];
        k = i % l;
        shift = key[k] - 'a';
        if (!encrypt)
        {
            shift = 26 - shift;
        }

        if (islower(c))
        {
            c = 'a' + (c - 'a' + shift)%26;
        }
        else if (isupper(c))
        {
            c = 'A' + (c - 'A' + shift)%26;
        }
        printf("%c", c);
    }
    printf("\n");

    return 0;
}