=head1 NAME
-RSA_check_key - Validate RSA keys
+RSA_check_key - validate private RSA keys
=head1 SYNOPSIS
#include <openssl/rsa.h>
+ int RSA_check_key_ex(RSA *rsa, BN_GENCB *cb);
+
int RSA_check_key(RSA *rsa);
=head1 DESCRIPTION
-This function validates RSA keys. It checks that B<p> and B<q> are
+RSA_check_key_ex() function validates RSA keys.
+It checks that B<p> and B<q> are
in fact prime, and that B<n = p*q>.
+It does not work on RSA public keys that have only the modulus
+and public exponent elements populated.
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>.
+It performs integrity checks on all
+the RSA key material, so the RSA key structure must contain all the private
+key data too.
+Therefore, it cannot be used with any arbitrary RSA key object,
+even if it is otherwise fit for regular RSA operation.
+
+The B<cb> parameter is a callback that will be invoked in the same
+manner as L<BN_is_prime_ex(3)|BN_is_prime_ex(3)>.
-The key's public components may not be B<NULL>.
+RSA_check_key() is equivalent to RSA_check_key_ex() with a NULL B<cb>.
=head1 RETURN VALUE
-RSA_check_key() returns 1 if B<rsa> is a valid RSA key, and 0 otherwise.
--1 is returned if an error occurs while checking the key.
+RSA_check_key_ex() and RSA_check_key()
+return 1 if B<rsa> is a valid RSA key, and 0 otherwise.
+They return -1 if an error occurs while checking the key.
If the key is invalid or an error occurred, the reason code can be
-obtained using ERR_get_error(3).
+obtained using L<ERR_get_error(3)|ERR_get_error(3)>.
+
+=head1 NOTES
+
+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
-rsa(3), err(3)
+L<BN_is_prime_ex(3)|BN_is_prime_ex(3)>,
+L<rsa(3)|rsa(3)>,
+L<ERR_get_error(3)|ERR_get_error(3)>
=head1 HISTORY
-RSA_check() appeared in OpenSSL 0.9.4.
+RSA_check_key() appeared in OpenSSL 0.9.4.
+RSA_check_key_ex() appeared after OpenSSL 1.0.2.
=cut