Trouble with OpenSSL's BN_bn2bin function

Question

I'm trying to use the BN_* functions in OpenSSL. Specifically, I have the following code:

#import <openssl/bn.h>
BIGNUM * num = BN_new();
BN_set_word(num, 42);
char * buffer = malloc((BN_num_bytes(num)+1) * sizeof(char));
buffer[BN_num_bytes(num)] = '\0';
int len = BN_bn2bin(num, buffer);
printf("42 in binary is %s\n", buffer);

However, when I do this, I don't get a string of ones and zeros. Instead it prints "42 in binary is *". As far as I can tell, and from the very limited number of examples available on the web that I've compared this to, I've implemented this correctly.

Any ideas why it isn't working?

Answer 1

BN_bn2bin doesn't create a printable string - instead, it creates a representation that is truly binary (i.e. a sequence of bits). More specifically, it createas a big-endian representation of the number. Since 42 fits into one byte, you get one byte 0x2a, which is "*" in ASCII.

If you want a 0/1 representation, you need to iterate over all bytes, and do the printing yourself (e.g. with shifting or a lookup table).

Answer 2

Here's some code that actually turns the BN_bn2bin output into a printable string just like the output of BN_bn2dec and BN_bn2hex. It's in a NodeJS library but is written in C++ for speed. Took me all day and probably isn't optimal (because I haven't written any C++ code since first year of uni). But it passes a bunch of unit tests so I know it works!

https://github.com/malcolmocean/node-bignum

if (BN_is_zero(&bignum->bignum_)) {
  to = (char*) OPENSSL_malloc(2*sizeof(char));
  to[0] = '0';
  to[1] = '\0';
} else {
  unsigned char *binary = (unsigned char*) OPENSSL_malloc(BN_num_bytes(&bignum->bignum_)*sizeof(unsigned char));
  int len = BN_bn2bin(&bignum->bignum_, binary);
  to = (char*) OPENSSL_malloc((len*8+2)*sizeof(char));
  int offset = 0;
  if (BN_is_negative(&bignum->bignum_)) {
    to[0] = '-';
    offset--;
  }
  unsigned char x = binary[0];
  while (!(x & 128) && x) {
    x = x << 1;
    offset++;
  }
  for (int i = 0; i < len; i++) {
    unsigned char bits = binary[i];

    int j=7;
    while(bits) {
      if (bits & 1) {
        to[8*i+j-offset] = '1';
      } else {
        to[8*i+j-offset] = '0';
      }
      bits = bits >> 1;
      j--;
    }
    if (i > 0) {
      while (j >= 0) {
        to[8*i+j-offset] = '0';
        j--;
      }
    }
  }
  to[8*len-offset] = '\0';
  OPENSSL_free(binary);
}