RSA_check_key - validate private RSA keys

#include <openssl/rsa.h>

int RSA_check_key_ex(RSA *rsa, BN_GENCB *cb);

int RSA_check_key(RSA *rsa);

`RSA_check_key_ex()`

function validates RSA keys.
It checks that **p** and **q** are
in fact prime, and that **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 **d*e = 1 mod (p-1*q-1)**,
and that **dmp1**, **dmq1** and **iqmp** are set correctly or are **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 **cb** parameter is a callback that will be invoked in the same
manner as *BN_is_prime_ex*.

`RSA_check_key()`

is equivalent to `RSA_check_key_ex()`

with a NULL **cb**.

`RSA_check_key_ex()`

and `RSA_check_key()`

return 1 if **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.

Unlike most other RSA functions, this function does **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 **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.

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.

*BN_is_prime_ex*,
rsa,
ERR_get_error

`RSA_check_key()`

appeared in OpenSSL 0.9.4.
`RSA_check_key_ex()`

appeared after OpenSSL 1.0.2.