-B<dmp1>, B<dmq1> and B<iqmp> may be B<NULL> in private keys, but the
-RSA operations are much faster when these values are available.
+B<p>, B<q>, B<dmp1>, B<dmq1> and B<iqmp> may be B<NULL> in private
+keys, but the RSA operations are much faster when these values are
+available.
+
+Note that RSA keys may use non-standard B<RSA_METHOD> implementations,
+either directly or by the use of B<ENGINE> modules. In some cases (eg. an
+ENGINE providing support for hardware-embedded keys), these BIGNUM values
+will not be used by the implementation or may be used for alternative data
+storage. For this reason, applications should generally avoid using RSA
+structure elements directly and instead use API functions to query or
+modify keys.