PEM_read_RSAPrivateKey: Getting RSA key public modulus and exponent

Question

I'm using PEM_read_RSAPrivateKey function in this way:

void test(void)
{
    RSA * privateKey = NULL;
    FILE * fp;

    if(NULL != (fp= fopen("./my_file.key", "r")) )
    {
          privateKey=PEM_read_RSAPrivateKey(fp,NULL,NULL,NULL);
          if(privateKey==NULL)
          {
              printf("\n\tCould NOT read RSA private key file");
          }
          else
          {
              printf("\n\tRSA structure filled");
          }

         // This is working OK and privateKey is NOT NULL

    }
}

Then, I try to retrieve modulus and public exponent to fill them into a personal structure:

struct
{
    unsigned char   modulus[256];
    unsigned char   pub_exp[8];
} s;

But all accesses I tried (I tried a lot) to privateKey->n will result in a segmentation fault.

for example:

unsigned char modulus [2048];
unsigned char exp[2048];
BN_bn2bin(privateKey->n, modulus);  // Segmentation fault results from this call

So my question is: how to copy modulus or public exponent from RSA structure to my structure "s" fields?

May someone help about this? Many thanks, Regards,

Sylvain

Answer 1

how to copy modulus or public exponent from RSA structure

int req = BN_num_bytes(rsa->n);
assert(rc > 0);

unsigned char* buff = malloc(req);
assert(buff != NULL);

int rc = BN_bn2bin(rsa->n, buff);
assert(req == rc);

Be wary of trying to copy the byte buffers into fixed sized arrays. Someone might come along and get you to copy a 4096-bit modulus into your 2048-bit array.

Answer 2

From OpenSSL 1.1.0 many structures were made opaque and you can no longer do things like look directly at the fields in a struct. You should use the provided accessor functions instead. Here you can find more infos: https://wiki.openssl.org/index.php/OpenSSL_1.1.0_Changes

---

Acces rsa->n in OpenSSL 1.1.0:

RSA *pRsa;
BIGNUM *n;
FILE *pFile = fopen("private.key","r");

pRsa = PEM_read_RSAPrivateKey(pFile, NULL, NULL, password);
RSA_get0_key(pRSA, &n, NULL,NULL);

BN_print_fp(stdout, n);