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.