Problem - Kevin Tafuro

There’s a similar function for assigning the value 1:
int BN_one(BIGNUM *bn);
You can also assign any nonnegative value that fits in an unsigned long using the
function BN_set_word( ):
int BN_set_word(BIGNUM *bn, unsigned long value);
The previous three functions return 1 on success.
If you need to assign a positive number that is too large to represent as an unsigned
long, you can represent it in binary as a sequence of bytes and have OpenSSLconvert
the binary buffer to a BIGNUM object. Note that the bytes must be in order from
most significant to least significant. That is, you can’t just point OpenSSLat memory
containing a 64-bit long long (__int64 on Windows) on a little-endian machine,
because the bytes will be backwards. Once your buffer is in the right format, you can
use the function BN_bin2bn( ), which has the following signature:
BIGNUM *BN_bin2bn(unsigned char *buf, int len, BIGNUM *c);
This function has the following arguments:
buf
Buffer containing the binary representation to be converted.
len
Length of the buffer in bits. It does not need to be a multiple of eight. Extra bits
in the buffer will be ignored.
c
BIGNUM object to be loaded with the value from the binary representation. This
may be specified as NULL, in which case a new BIGNUM object will be dynamically
allocated. The new BIGNUM object will be returned if one is allocated; otherwise,
the specified BIGNUM object will be returned.
None of the previously mentioned techniques allows us to represent a negative number.
The simplest technique is to get the corresponding positive integer, then use the
following macro that takes a pointer to a BIGNUM object and negates it (i.e., multiplies
by –1):
#define BN_negate(x) ((x)->neg = (!((x)->neg)) & 1)
Getting BIGNUM objects with random values
Before you can get BIGNUM objects with random values, you need to have seeded the
OpenSSLrandom number generator. (With newer versions of OpenSSL, the generator
will be seeded for you on most platforms; see Recipe 11.9).
One common thing to want to do is generate a random prime number. The API for
this is somewhat complex:
BIGNUM *BN_generate_prime(BIGNUM *ret, int num, int safe, BIGNUM *add, BIGNUM *rem,
void (*callback)(int, int, void *), void *cb_arg);
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