struct HMAC_context *
Curl_HMAC_init(const struct HMAC_params *hashparams,
               const unsigned char *key,
               unsigned int keylen)
{
  size_t i;
  struct HMAC_context *ctxt;
  unsigned char *hkey;
  unsigned char b;

  /* Create HMAC context. */
  i = sizeof(*ctxt) + 2 * hashparams->ctxtsize + hashparams->resultlen;
  ctxt = malloc(i);

  if(!ctxt)
    return ctxt;

  ctxt->hash = hashparams;
  ctxt->hashctxt1 = (void *) (ctxt + 1);
  ctxt->hashctxt2 = (void *) ((char *) ctxt->hashctxt1 + hashparams->ctxtsize);

  /* If the key is too long, replace it by its hash digest. */
  if(keylen > hashparams->maxkeylen) {
    if(hashparams->hinit(ctxt->hashctxt1))
      return NULL;
    hashparams->hupdate(ctxt->hashctxt1, key, keylen);
    hkey = (unsigned char *) ctxt->hashctxt2 + hashparams->ctxtsize;
    hashparams->hfinal(hkey, ctxt->hashctxt1);
    key = hkey;
    keylen = hashparams->resultlen;
  }

  /* Prime the two hash contexts with the modified key. */
  if(hashparams->hinit(ctxt->hashctxt1) ||
     hashparams->hinit(ctxt->hashctxt2))
    return NULL;