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

Question

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

Answer 1

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).

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;
}