digest

static VALUE
ossl_hmac_s_digest(VALUE klass, VALUE digest, VALUE key, VALUE data)
{
    unsigned char *buf;
    unsigned int buf_len;

    StringValue(key);
    StringValue(data);
    buf = HMAC(GetDigestPtr(digest), RSTRING_PTR(key), RSTRING_LEN(key),
               (unsigned char *)RSTRING_PTR(data), RSTRING_LEN(data), NULL, &buf_len);

    return rb_str_new((const char *)buf, buf_len);
}