"../man7/provider.html">provider(7)</a>. Historically keys using the same algorithm could use different NIDs. For example an RSA key could use the NIDs corresponding to the NIDs <b>NID_rsaEncryption</b> (equivalent to <b>EVP_PKEY_RSA</b>) or <b>NID_rsa</b> (equivalent to <b>EVP_PKEY_RSA2</b>). The use of alternative non-standard NIDs is now rare so <b>EVP_PKEY_RSA2</b> et al are not often seen in practice. EVP_PKEY_get_id() returns -1 (<b>EVP_PKEY_KEYMGMT</b>) if the <i>pkey</i> is only implemented in a <a 