-RSA_check_key() can only check soft keys. If given hard keys, i.e. keys
-that were retreived from an ENGINE (with ENGINE_load_private_key()),
-RSA_check_key() will always return 0, as if the key was invalid. The
-reason is that the private components B<p>, B<q> and B<d> are normally
-not available to OpenSSL for hard keys, and the checks can therefore not
-be done.
+Unlike most other RSA functions, this function does B<not> work
+transparently with any underlying ENGINE implementation because it uses the
+key data in the RSA structure directly. An ENGINE implementation can
+override the way key data is stored and handled, and can even provide
+support for HSM keys - in which case the RSA structure may contain B<no>
+key data at all! If the ENGINE in question is only being used for
+acceleration or analysis purposes, then in all likelihood the RSA key data
+is complete and untouched, but this can't be assumed in the general case.
+
+=head1 BUGS
+
+A method of verifying the RSA key using opaque RSA API functions might need
+to be considered. Right now RSA_check_key() simply uses the RSA structure
+elements directly, bypassing the RSA_METHOD table altogether (and
+completely violating encapsulation and object-orientation in the process).
+The best fix will probably be to introduce a "check_key()" handler to the
+RSA_METHOD function table so that alternative implementations can also
+provide their own verifiers.