sign

static VALUE
ossl_pkey_sign(VALUE self, VALUE digest, VALUE data)
{
    EVP_PKEY *pkey;
    EVP_MD_CTX ctx;
    unsigned int buf_len;
    VALUE str;

    if (rb_funcall(self, id_private_q, 0, NULL) != Qtrue) {
        ossl_raise(rb_eArgError, "Private key is needed.");
    }
    GetPKey(self, pkey);
    EVP_SignInit(&ctx, GetDigestPtr(digest));
    StringValue(data);
    EVP_SignUpdate(&ctx, RSTRING_PTR(data), RSTRING_LEN(data));
    str = rb_str_new(0, EVP_PKEY_size(pkey)+16);
    if (!EVP_SignFinal(&ctx, (unsigned char *)RSTRING_PTR(str), &buf_len, pkey))
        ossl_raise(ePKeyError, NULL);
    assert((long)buf_len <= RSTRING_LEN(str));
    rb_str_set_len(str, buf_len);

    return str;
}