Verify file with OpenSSL

verification.c

#include <openssl/evp.h>
#include <openssl/err.h>
#include <openssl/x509.h>
#include <openssl/pem.h>

#include <stdio.h>
#include <stddef.h>
#include <inttypes.h>
#include <string.h>

void read_signature(char *buf, size_t buf_len, const char *filename)
{
  FILE *filp = fopen(filename, "rb");

  if (!filp)
  {
    printf("Cannot open file\n");

    return;
  }
  size_t offset = 0;

  while (buf_len > 0)
  {
    size_t read_bytes = fread(buf + offset, sizeof *buf, buf_len, filp);

    if (read_bytes == EOF)
    {
      printf("READ ERROR\n");
      fclose(filp);

      return;
    }

    buf_len -= read_bytes;
    offset += read_bytes;
  }

  fclose(filp);
}

int main()
{
  const char *fname = "./hello.jar";
  const char *sign = "./hello.jar.sha256";
  const char *certname = "./public.pem";
  char buf[512] = {0};
  BIO *certbio = NULL;
  X509 *cert = NULL;
  EVP_PKEY *pkey = NULL;

  OpenSSL_add_all_algorithms();
  ERR_load_BIO_strings();
  ERR_load_crypto_strings();
  read_signature(buf, 512, sign);
  printf("Read signature\n");
  certbio = BIO_new(BIO_s_file());

  BIO_read_filename(certbio, certname);
  cert = PEM_read_bio_X509(certbio, NULL, NULL, NULL);

  if (!cert)
  {
    printf("Cannot load certificate\n");

    return -1;
  }

  pkey = X509_get_pubkey(cert);
  int nid = X509_get_signature_nid(cert);

  if (!pkey)
  {
    printf("Cannot load public key\n");

    return -1;
  }

  EVP_MD_CTX *mdctx = EVP_MD_CTX_create();

  if (!mdctx)
  {
    printf("Cannot create MD CTX\n");

    return -1;
  }

  if (EVP_DigestVerifyInit(mdctx, NULL, EVP_get_digestbynid(nid), NULL, pkey) != 1)
  {
    printf("Cannot init EVP digest\n");

    return -1;
  }

  FILE *file_to_verif = fopen(fname, "rb");
  char fbuf[128];

  memset(fbuf, 0, 128);
  size_t read = 0;
  size_t offset = 0;

  while ((read = fread(fbuf, sizeof(char), 128, file_to_verif)) != 0)
  {
    offset += read;

    if (EVP_DigestUpdate(mdctx, fbuf, read) != 1)
    {
      printf("Cannot update EVP digest\n");

      return -1;
    }
  }

  printf("Read file: %" PRIu64 "\n", offset);
  printf("Verification status: ");
  if(EVP_DigestVerifyFinal(mdctx, (unsigned char *) buf, 512) == 1)
  {
    /* Success */
    printf("SUCCESS\n");
  }
  else
  {
    /* Failure */
    printf("FAILURE\n");
  }

  EVP_MD_CTX_destroy(mdctx);
  EVP_PKEY_free(pkey);
  X509_free(cert);
  BIO_free_all(certbio);

  return 0;
}