https://d19vezwu8eufl6.cloudfront.net/crypto/images%2Fpp3-fig.jpg

block_wise_hash.c

#include <sys/stat.h>

#include "otp.h"

#define WORD_LENGTH 32
#define BLOCK_SIZE  1024

// int fseek ( FILE * stream, long int offset, int origin );
// size_t fread ( void * ptr, size_t size, size_t count, FILE * stream );
// unsigned char *SHA256 ( const unsigned char *d, size_t n, unsigned char *md );

int
main(int argc, char *argv[])
{
    int      i;

    char     *filename,
             *result;
    FILE     *file;
    struct   stat st; 

    long int offset,
             iterations;

    uint8_t  *input,
             *hash;

    filename = "/Users/dce/Desktop/6 - 1 - Introduction (11 min).mp4";
    file     = fopen(filename, "rb");

    stat(filename, &st);

    iterations = st.st_size / BLOCK_SIZE;
    offset     = st.st_size % BLOCK_SIZE;

    input      = malloc(BLOCK_SIZE + WORD_LENGTH);
    hash       = malloc(WORD_LENGTH);

    // Compute the initial hash
    fseek(file, offset * -1, SEEK_END);
    fread(input, 1, offset, file);
    SHA256(input, offset, hash);

    for (i = 0; i < iterations; i++) {
        // Move back one block from end & read a block
        fseek(file,
              ((i + 1) * BLOCK_SIZE + offset) * -1,
              SEEK_END);

        fread(input, 1, BLOCK_SIZE, file);

        // Append previous result to block
        memcpy(input + BLOCK_SIZE, hash, WORD_LENGTH);

        // Hash block plus previous result
        SHA256(input, BLOCK_SIZE + WORD_LENGTH, hash);
    }

    result = bytes_to_hex(hash, WORD_LENGTH);

    printf("%s\n", result);

    free(input);
    free(hash);
    free(result);

    return 0;
}