Something like Vigenere cipher but for bytes instead of letters

vigenere.c

/*
    Vigenere File Coder and Decoder
    Copyright (C) 2017 Farooq Karimi Zadeh <[email protected]>

    Permission is hereby granted, free of charge, to any person obtaining
    a copy of this software and associated documentation files
    (the "Software"), to deal in the Software without restriction,
    including without limitation the rights to use, copy, modify, merge,
    publish, distribute, sublicense, and/or sell copies of the Software,
    and to permit persons to whom the Software is furnished to do so,
    subject to the following conditions:

    The above copyright notice and this permission notice shall be included
    in all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
    DEALINGS IN THE SOFTWARE.
*/

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

unsigned char encrypt_chr(unsigned char input, unsigned char key){
    return input + key; // it does not matter if it performed an overflow!
}

unsigned char decrypt_chr(unsigned char input, unsigned char key){
    return input - key; // the same about this!
}

unsigned char crypt_chr(unsigned char input, unsigned char key, char mode){
    return mode == 'd'? decrypt_chr(input, key) : encrypt_chr(input, key);
}

void show_help(char *progname){
    fprintf(stderr, "\nVigenere Encoder and Decoder \nCopyright (C) 2017 "
           "Farooq Karimi Zadeh <[email protected]>\n"
           "This program comes with ABSOLUTELY NO WARRANTY; "
           "This is free software, and you\n are welcome to redistribute it"
           " under certain conditions. For more information\n see MIT"
           " License\n\n"
           "Usage: %s [-d|-e] [-i <input_file>] [-o <output_file>] [-k"
           " <key>]\n\tBy default it ecnodes the input file and <key> is"
           " \"key\". Maximum \n\tlength of <key> is 128 bytes\n\tdefault "
           "input and output files are -(which refers to"
           " STDIN and STDOUT \n\tfor input and output file.)\n"
           "Compiled at %s %s\n",
         progname, __DATE__, __TIME__);
    exit(0);
}

int main(int argc, char *argv[]){
    // these variables should be extracted from command line arguments
    char key[128 + 1] = "key";
    key[128] = '\0';
    char in_file_path[FILENAME_MAX + 1] = "-";
    char out_file_path[FILENAME_MAX + 1] = "-";
    char mode = 'e'; //d = decrypt, e = encrypt

    for (int i = 0; i < argc; i++){
        if (!strcmp(argv[i], "-h") || !strcmp(argv[i], "-?"))
            show_help(argv[0]);
        if (!strcmp(argv[i], "-d"))
            mode = 'd';
        if (!strcmp(argv[i], "-e"))
            mode = 'e';
        if (!strcmp(argv[i], "-i"))
            strcpy(in_file_path, argv[i + 1]);
        if (!strcmp(argv[i], "-o"))
            strcpy(out_file_path, argv[i + 1]);
        if (!strcmp(argv[i], "-k"))
            strcpy(key, argv[i + 1]);
    }
    FILE *in_file;
    FILE *out_file;

    if (!strcmp(in_file_path, "-"))
        in_file = stdin;
    else
        in_file = fopen(in_file_path, "r");
    if (!in_file){
        fprintf(stderr, "Could not open input file: %s\n", in_file_path);
        return -1;
    }
    if (!strcmp(out_file_path, "-"))
        out_file = stdout;
    else
        out_file = fopen(out_file_path, "w");
    if (!out_file){
        fprintf(stderr, "Could not open output file: %s\n", out_file_path);
    }

    char *kp;
    kp = key; // it starts with the first character of the key and when it
              //  reaches the last character of key(not the NULL character) it
              //  again goes to the first character.
    int key_len = strlen(key);
    fseek(in_file, 0, SEEK_END);
    unsigned size = ftell(in_file);
    fseek(in_file, 0, SEEK_SET);


    while (size--){
        fputc(crypt_chr(fgetc(in_file), *kp, mode), out_file);
        kp = (char) (kp - key) == key_len? key : ++kp; // a little complex, no?
        // ^ if the difference between kp and key is key_len, this means that
        // the kp pointer is on the last character, so key will be assigned to
        // kp and moves it to the first character, if it's not pointing to the
        // last character then just add one to it(points to the next character)
    }

    fclose(in_file);
    fclose(out_file);

    return 0;
}

Here's Wikipedia article about vigenere cipher.