Store application-specific data on an EVP_PKEY at a CRYPTO ex_data index.
1 on success, or 0 on failure.
| Name | Description |
|---|---|
| key | Key that owns the ex_data table. |
| idx | Index obtained from EVP_PKEY_get_ex_new_index() (or a reserved index). |
| arg | Pointer to store; ownership remains with the caller unless free callbacks manage it. |