How to print the content of an OpenSSL RSA key

Question

I'd like to print the content of an RSA key created using OpenSSL and accessed by means of the pointer

RSA * rsa;

Now, I've tried to use the RSA_print and BIO_read functions as follows:

BIO * keybio ;
RSA_print(keybio, rsa, 0);
char buffer [1024];
std::string res = "";
while (BIO_read (keybio, buffer, 1023) > 0)
{
    std::cout << buffer;
}

but I get a segmentation fault at the very first execution of the BIO_read (sixth line). Can someone spot the error?

Answer 1

I think you are writing over unallocated memory. I don't think RSA_print() allocates memory for BIO, so you are passing an uninitialized pointer to it.

EDIT: As @jww pointed out in the comment, the BIO object should be instantiated with a set of dedicated options. I edited the answer accordingly. I also changed the buffer size passed to 1024.

Allocate space:

BIO * keybio = BIO_new(BIO_s_mem());
RSA_print(keybio, rsa, 0);
char buffer [1024];
std::string res = "";
while (BIO_read (keybio, buffer, 1024) > 0)
{
    std::cout << buffer;
}
BIO_free(keybio); //appropriate free "method"

BIO_s_mem() creates a memory BIO method function. And BIO_new() creates and initializes the object itself, using the supplied method.

Also please note that this code is c-style. There is a nice way of wrapping pure pointer into c++ smart pointers, shown here: How to properly print RSA* as string in C++?.