A SHA256 hash generator in C using openssl

sha256demo.c

#include <openssl/hmac.h>
#include <openssl/sha.h>
#include <openssl/evp.h>
#include <string.h>

#define EXPECTED "0d696548764b6f910bdd3c07d8e465112c0783a03f2c1cf5ef893b8aa27f3290"

int main()
{
    unsigned char *message = (unsigned char *)"this is highly sensitive user data";
    unsigned char *key = (unsigned char *)"security is awesome but difficult";
    int len;
    int i;
    char *digest;

    unsigned char *result = HMAC(
        EVP_sha256(),
        (const void *)key,
        strlen(key),
        message,
        strlen(message),
        NULL,
        &len);

    if (len <= 0)
        return 1;

    digest = malloc(len * 2 + 1);

    if (!digest)
        return 1;

    memset(digest, 0, len * 2 + 1);
    for (i = 0; i < len; i++)
    {
        sprintf(&(digest[i * 2]), "%02x", result[i]);
    }

    printf("ACTUAL =   %s\nEXPECTED = %s\n", digest, EXPECTED);

    free(digest);

    return 0;
}