[IS] Autokey Encryption

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

#define MAX 512

char offset(int c, int n)
{
    int isUpper = 0, off, sign = 1, rez ;
    if (c >= 65 && c <= 90) { isUpper = 1; c += 32; }
    if (n < 0) sign = -1;
    if (n*sign >= 65 && n*sign <= 90) n += sign * 32;

    rez = c;
    off = n % 97;
    rez += off;
    if (rez < 97) rez = 123 + off + (c - 97);
    if (rez > 122) rez = 96 + off - (122 - c);

    if (isUpper) rez -= 32;
    return rez;
}


char * encrypt(char *m, char *key)
{
    int i, j;
    char *cip;
    int mlen = strlen(m), klen = strlen(key);
    cip = (char*) malloc(mlen * sizeof(char));

    for (i = 0, j = 0; i < mlen && j < klen; i++)
    {
        if (isalpha(m[i]))
        {
            cip[i] = offset(m[i], (int) key[j]);
            j++;
        }
        else cip[i] = m[i];
    }

    for (j = 0; i < mlen; i++)
    {
        if (isalpha(m[i]))
        {
            while (!isalpha(m[j])) j++;
            cip[i] = offset(m[i], (int) m[j]);
            j++;
        }
        else cip[i] = m[i];
        if (j > mlen) j = 0;
    }

    cip[i] = '\0';

    return cip;
}

char *decrypt(char *cip, char *key)
{
    int i, j;
    char *m, *tkey;;
    int clen = strlen(cip), klen = strlen(key);
    m = (char*) malloc(clen * sizeof(char));
    tkey = (char*) malloc(klen * sizeof(char));

    for (i = 0, j = 0; i < clen && j < klen; i++)
    {
        if (isalpha(cip[i]))
        {
            m[i] = offset(cip[i], -1 * (int) key[j]);
            j++;
        }
        else m[i] = cip[i];
    }

    for (j = 0; i < clen; i++)
    {
        if (isalpha(cip[i]))
        {
            while (!isalpha(cip[j])) j++;
            m[i] = offset(cip[i], -1 * (int) m[j]);
            j++;
        }
        else m[i] = cip[i];
        if (j > clen) j = 0;
    }

    m[i] = '\0';
    return m;
}

void main()
{
    int i, op;
    char *s[MAX], *c[MAX], *key[MAX];
    FILE *f;
    f = fopen("input.txt", "r");
    fgets(s, MAX, f);
    fclose(f);

    printf("input key: "); gets(key);
    printf("select option: encrypt (1), decrypt (2): "); scanf("%d", &op);

    if (op == 1) strcpy(c, encrypt(s, key));
    else if (op == 2) strcpy(c, decrypt(s, key));
    f = fopen("output.txt", "w");
    fprintf(f, "%s", c);
    fclose(f);
}