大文件上的 HMAC SHA256 计算（使用系统调用）

Question

I have the below code to generate a SHA256 HMAC hash:

HMAC(EVP_sha256(), key, key_len, (const unsigned char *)data, strlen(data), digest, &digest_len);

I would however like to generate HMAC SHA256 hash of a file. Please note the file is a big file (~80 MB) I will need to do this using system calls rather than directly using a shell command.

Regards

Answer 1

If you have a large file then you can use HMAC_Update() and HMAC_Final() . Here is a quick and dirty example:

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <ssl/hmac.h>


int main(int argc, char *argv[])
{
    FILE *f;
    uint8_t buf[1024];
    HMAC_CTX *ctx;
    size_t ret;
    uint8_t key[8] = { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07 };
    uint8_t *md;
    int md_len;
    int i;

    if (argc < 2) {
        printf("Usage: %s [FILE]\n", argv[0]);
        return 1;
    }

    f = fopen(argv[1], "r");

    if (!f) {
        printf("Could not open file\n");
        return 1;
    }

    ctx = HMAC_CTX_new();
    HMAC_Init_ex(ctx, key, 8, EVP_sha256(), NULL);

    md = (uint8_t *) malloc(HMAC_size(ctx));

    while (!feof(f)) {
        ret = fread(buf, 1, sizeof(buf), f);
        HMAC_Update(ctx, &buf[0], ret);
    }

    HMAC_Final(ctx, md, &md_len);

    for (i = 0; i < md_len; i ++)
        printf("%02X", md[i]);

    printf("\n");
    HMAC_CTX_free(ctx);
    fclose(f);
    return 0;
}

The output:

$ ./hmac hmac
C477E7AA9EC9F0DD8C98F85B94A18BC954E0648BB0C7D302F5EDE3679A93A6E8
$