Convert a big number given as a string to an OpenSSL BIGNUM

I am trying to convert a string `p_str` representing a big integer to a `BIGNUM` `p` using the OpenSSL library.

#include <stdio.h>
#include <openssl/bn.h>

int main ()
{
/* I shortened the integer */
unsigned char *p_str = "82019154470699086128524248488673846867876336512717";

BIGNUM *p = BN_bin2bn(p_str, sizeof(p_str), NULL);

BN_print_fp(stdout, p);
puts("");

BN_free(p);
return 0;
}

Compiled it with:

gcc -Wall -Wextra -g -o convert convert.c -lcrypto

But, when I execute it, I get the following result:

3832303139313534

> unsigned char *p_str = "82019154470699086128524248488673846867876336512717";
>
> BIGNUM *p = BN_bin2bn(p_str, sizeof(p_str), NULL);

Use `int BN_dec2bn(BIGNUM **a, const char *str)` instead.

You would use `BN_bin2bn` when you have an array of `bytes` (and not a NULL terminated ASCII string).

The man pages are located at [`BN_bin2bn(3)`](

[To see links please register here]

).

The correct code would look like this:

#include <stdio.h>
#include <openssl/bn.h>

int main ()
{
static const
char p_str[] = "82019154470699086128524248488673846867876336512717";

BIGNUM *p = BN_new();
BN_dec2bn(&p, p_str);

char * number_str = BN_bn2hex(p);
printf("%s\n", number_str);

OPENSSL_free(number_str);
BN_free(p);

return 0;
}