vigenere.c (incomplete)

/* Implements the vigenere cipher: https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher
 */

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

#define get_offset(ch) \
    (toupper((ch)) - 'A')

#define fix_offset(off) \
    (((off) + ((off) < 0 ? 26 : 0)) % 26)

#define encipher(text, key) \
    (cipher((text), (key), 0))

#define decipher(text, key) \
    (cipher((text), (key), 1))

typedef ssize_t offset;

void
cipher(char *text, char *key, int mode)
{
    size_t keylen = strlen(key);
    size_t textlen = strlen(text);
    int outch;
    offset toff;
    offset koff;

    for(int i = 0, j = 0; i < textlen; i++, j++){
        toff = get_offset(text[i]);
        koff = get_offset(key[j % keylen]);
        outch = 'A' + fix_offset(toff + (mode ? -koff : +koff));

        if(isalpha(text[i]))
            putchar(outch);

        else {
            putchar(text[i]);
            j--;
        }
    }

    putchar('\n');
}

// TODO: add a UI
// probably: vigenere key text [-i (read from stdin) -e (encipher [default]) -d (decipher) -- (start ignoring '-')]
// Functions/objects probably to implement: usage, parse_arg, parse_opt, find_arg, dispatch table {flag, descr, callback}
// Probably just copy it from the other cipher I made

int
main(int argc, char **argv)
{
    char *plaintext = "What is a man without knowing the rich aroma of the future; the hot, complex balance of the present; and the bittersweet aftertaste of the past?";
    char *key = "LIBITINA";

    encipher(plaintext, key);
    //sanity testing to make sure the program didn't suddenly break
    encipher("attackatdawn", "lemon");
    decipher("LXFOPVEFRNHR", "lemon");
    encipher("Attack At Dawn!", "lemon");
    decipher("LXFOPV EF RNHR!", "lemon");
    encipher("Attack At 420 Dawn!", "lemon");
    decipher("LXFOPV EF 420 RNHR!", "lemon");
    return 0;
}