HMAC-SHA256

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <openssl/evp.h>
#include <openssl/hmac.h>

int main()
{
    char msg[] = "A simple string";
    char key[] = "key";

    HMAC_CTX hmac;
    HMAC_Init_ex(&hmac, key, strlen(key), EVP_sha256(), NULL);
    HMAC_Update(&hmac, (const unsigned char*)msg, strlen(msg));

    unsigned int md_len = HMAC_size(&hmac);
    unsigned char md[md_len];
    HMAC_Final(&hmac, md, &md_len);

    // dump resulting digest
    for (size_t i=0; i<md_len; ++i)
        printf("%02X", md[i]);
    printf("\n");

    return 0;

}