How to implement SHA512/256 OpenSSL?

Question

The code below is the method by which a SHA152 hash is computed through using OpenSSL, however according to the changelog OpenSSL has supported SHA512/256 for some time - the issue is I cannot find information about how to use it?! If anyone knows how I'd be very grateful if you could let me know how.

SHA512_Init (&mdContext);
while ((bytes = fread (data, 1, 1024, inFile)) != 0)
    SHA512_Update (&mdContext, data, bytes);
SHA512_Final (c,&mdContext);
for(i = 0; i < SHA512_DIGEST_LENGTH; i++) printf("%02x", c[i]);
printf (" %s\n", filename);
return 0;

(this code is just an example, I know it will not run just like that)

EDIT: I've just changed SHA512_DIGEST_LENGTH to SHA256_DIGEST_LENGTH , is this the correct method? I'm concerned this will still compute the entire SHA512 hash (I'd like to see whether there is a performance increase) Not entirely sure how it works, so apologies in advance!)

Answer 1

Assume you are using a linux environment, you can reference the sha512-256 example (evptest.c) below:

#include <stdio.h>
#include <string.h>
#include <openssl/evp.h>
#define DIGEST_NAME "sha512-256"

int main(int argc, char *argv[]) {
  EVP_MD_CTX *mdctx;
  const EVP_MD *md;
  char mess1[] = "Test Message\n";
  char mess2[] = "Hello World\n";
  unsigned char md_value[EVP_MAX_MD_SIZE];
  int md_len, i;

  OpenSSL_add_all_digests();
  md = EVP_get_digestbyname(DIGEST_NAME);
  if(!md) {
    printf("Unknown message digest %s\n", DIGEST_NAME);
    exit(1);
  }
  mdctx = EVP_MD_CTX_create();
  EVP_DigestInit_ex(mdctx, md, NULL);
  EVP_DigestUpdate(mdctx, mess1, strlen(mess1));
  EVP_DigestUpdate(mdctx, mess2, strlen(mess2));
  EVP_DigestFinal_ex(mdctx, md_value, &md_len);
  EVP_MD_CTX_destroy(mdctx);

  printf("Digest %s is: ", DIGEST_NAME);
  for(i = 0; i < md_len; i++)
    printf("%02x", md_value[i]);
  printf("\n");

  /* Call this once before exit. */
  EVP_cleanup();
  exit(0);
}

In Linux with latest OpenSSL and build-essential, you can compile it:

gcc evptest.c -l crypto -o evptest

Then execute the evptest binary, you will see sha512-256 result

./evptest
Digest sha512-256 is: dd889041ed0cf4ff7a1cf77dd384444b2de9347b032b95e64fc1955d889da8fd