1 files changed, 30 insertions, 2 deletions

diff --git a/src/lib/libcrypto/doc/RSA_check_key.pod b/src/lib/libcrypto/doc/RSA_check_key.pod
index 79fed753ad..3d824a07f5 100644
--- a/src/lib/libcrypto/doc/RSA_check_key.pod
+++ b/src/lib/libcrypto/doc/RSA_check_key.pod
@@ -18,7 +18,9 @@ in fact prime, and that B<n = p*q>.
 It also checks that B<d*e = 1 mod (p-1*q-1)>,
 and that B<dmp1>, B<dmq1> and B<iqmp> are set correctly or are B<NULL>.
 
-The key's public components may not be B<NULL>.
+As such, this function can not be used with any arbitrary RSA key object,
+even if it is otherwise fit for regular RSA operation. See B<NOTES> for more
+information.
 
 =head1 RETURN VALUE
 
@@ -28,12 +30,38 @@ RSA_check_key() returns 1 if B<rsa> is a valid RSA key, and 0 otherwise.
 If the key is invalid or an error occurred, the reason code can be
 obtained using L<ERR_get_error(3)|ERR_get_error(3)>.
 
+=head1 NOTES
+
+This function does not work on RSA public keys that have only the modulus
+and public exponent elements populated. It performs integrity checks on all
+the RSA key material, so the RSA key structure must contain all the private
+key data too.
+
+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.
+
 =head1 SEE ALSO
 
 L<rsa(3)|rsa(3)>, L<err(3)|err(3)>
 
 =head1 HISTORY
 
-RSA_check() appeared in OpenSSL 0.9.4.
+RSA_check_key() appeared in OpenSSL 0.9.4.
 
 =cut