How could I "manually" signify EOF when writing a file?

Question

I have this function :

int cipher_file(char *file_path, uint8_t *key, int key_size){

    FILE *file;
    size_t read_char_count, wrote_char_count;

    fpos_t *pos = malloc(sizeof(fpos_t));
    char *block = malloc(16*sizeof(uint8_t));

    if ( !(file = fopen(file_path, "rb+")) ) {
      return EXIT_FAILURE;
    }

    while(!feof(file)){

      while( ( read_char_count = fread(block, 1, 16*sizeof(uint8_t), file) ) > 0 ) {
        block = cipher_block(block, key, key_size);
        fseek(file, -read_char_count, SEEK_CUR);
        wrote_char_count = fwrite(block , 1, 16*sizeof(uint8_t), file);
      }

    }
    fclose(file);
    return EXIT_SUCCESS;
}

(I know ECB mode is not safe btw)

Which takes a file, break it down in blocks of 128 bits, cipher them using an AES and write them back to the file, effectively replacing plain text with cipher text. I also wrote a function decipher_file() to decipher the file.

The issue is that if the file size is not a multiple of 128 bits, at the end fread() only partially replace content of "block" (which is 16 bytes long) with the successfully read characters, leaving a bunch of garbage from the previous ciphered block.

When deciphering since decipher_file() has normally no way of knowing the size of the original file, it deciphers all the content, including the garbage characters, and write it back to the file.

I also tried re initializing "block" with zeros at each round but, without great surprise, they were added to the file too, which can be very problematic.

So my question is, is there a way (like a function) to signify where the file ends, or tell fwrite() to stop writing?

`while(!feof` is a mistake, you should check the return value of fread instead — M.M, Dec 09 '18 at 11:17
[this for `while (!feof`](https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) — Antti Haapala -- Слава Україні, Dec 09 '18 at 11:18
Use [PKCS#7 padding](https://en.wikipedia.org/wiki/Padding_(cryptography)#PKCS%235_and_PKCS%237) for example. And for understanding crypto better, consider hacking the challenges in Cryptopals; here's one for implementing [PKCS#7 padding](https://cryptopals.com/sets/2/challenges/9), and here [its validation](https://cryptopals.com/sets/2/challenges/15). — Antti Haapala -- Слава Україні, Dec 09 '18 at 11:23

score 1 · Accepted Answer · answered Dec 09 '18 at 11:14

1

You can't use a special character because the encrypted data might end up looking like it. It would not be an elegant solution anyway.

There are multiple solutions:

Prefix the file with the decrypted content length. That's very clean and easy to implement.
Use a cipher mode that retains length information. ECB does not. Use padding schemes or a scheme, that preserves the length such as counter mode.

answered Dec 09 '18 at 11:14

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I will look into that – Balocre Dec 09 '18 at 11:46

How could I "manually" signify EOF when writing a file?

1 Answers1