BIGNUM strange behavior in a calculation loop - ios

Question

I'm trying to implement a basic routine to perform some calculation on BIGNUM(s) and I've found a strange behavior. The functions are as follows
unsigned char *char_array_as_hex(unsigned char *chr_a, int len)
{
unsigned char *chr_s = (unsigned char *)malloc(len * 2);
char buffer[5];
for (int i = 0; i < len; i++)
{
sprintf(buffer, "%02X", chr_a[i]);
chr_s[(2 * i) + 0] = buffer[0];
chr_s[(2 * i) + 1] = buffer[1];
}
return chr_s;
}
and
char *big_number_as_decimal_from_hex_array(unsigned char *chr_a, int len, BN_CTX *bn_ctx)
{
unsigned char *hex_s = char_array_as_hex(chr_a, len);
BIGNUM *big_number = BN_CTX_get(bn_ctx);
BN_hex2bn(&big_number, (char *)hex_s);
char *big_number_as_decimal = BN_bn2dec(big_number);
free(hex_s);
BN_free(big_number);
return big_number_as_decimal;
}
and
void test_compute_prime256v1()
{
BN_CTX *bn_ctx = BN_CTX_new();
BN_CTX_start(bn_ctx);
unsigned char seed_a[20] = {
0xC4,0x9D,0x36,0x08,0x86,0xE7,0x04,0x93,0x6A,0x66, /* seed */
0x78,0xE1,0x13,0x9D,0x26,0xB7,0x81,0x9F,0x7E,0x90
};
printf("s = %s\n", big_number_as_decimal_from_hex_array(seed_a, 20, bn_ctx));
unsigned char p_a[32] = {
0xFF,0xFF,0xFF,0xFF,0x00,0x00,0x00,0x01,0x00,0x00, /* p */
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
0xFF,0xFF
};
printf("p = %s\n", big_number_as_decimal_from_hex_array(p_a, 32, bn_ctx));
BN_CTX_end(bn_ctx);
BN_CTX_free(bn_ctx);
}
then I call "test_compute_prime256v1" in an Objective-C method. If I call it once or multiple times with a reasonable delay between each call it produces correct result however, when I call that function in a loop it produces different incorrect values
- (IBAction)btnOK_Clicked:(id)sender
{
for (int i = 1; i < 10; i++)
{
printf("i = %d\n", i);
test_compute_prime256v1();
}
}
and a sample output was
i = 1
s = 1122468115042657169822351801880191947498376363664
p = 115792089210356248762697446949407573530086143415290314195533631308867097853951
i = 2
s = 1122468115042657169822351801880191947498376363664
p = 966134380529368896499052403318808180610643774633026536153469502543482958881555881553276...
i = 3
s = 1122468115042657169822351801880191947498376363664
p = 115792089210356248762697446949407573530086143415290314195533631308867097853951
Note: some numbers are trimmed to fit in. I have followed the suggestion in here.
Am I missing something? Is there any mistake somewhere?
Anyone can help?
Thanks
EDITED:
I made some modification to code but the issue still exists. I changed big_number_as_decimal_from_hex_array as follows
char *big_number_as_decimal_from_hex_array_ex(unsigned char *chr_a, int len)
{
BN_CTX *bn_ctx = BN_CTX_new();
BN_CTX_start(bn_ctx);
unsigned char *hex_s = char_array_as_hex(chr_a, len);
BIGNUM *big_number = BN_CTX_get(bn_ctx);
BN_hex2bn(&big_number, (char *)hex_s);
char *big_number_as_decimal = BN_bn2dec(big_number);
free(hex_s);
BN_free(big_number);
BN_CTX_end(bn_ctx);
BN_CTX_free(bn_ctx);
return big_number_as_decimal;
}
and also
char *big_number_as_decimal_from_hex_array_ex_2(unsigned char *chr_a, int len)
{
BN_CTX *bn_ctx = BN_CTX_new();
unsigned char *hex_s = char_array_as_hex(chr_a, len);
BIGNUM *big_number = BN_CTX_get(bn_ctx);
BN_hex2bn(&big_number, (char *)hex_s);
char *big_number_as_decimal = BN_bn2dec(big_number);
free(hex_s);
BN_free(big_number);
BN_CTX_free(bn_ctx);
return big_number_as_decimal;
}
I modified the test_compute_prime256v1 as
void test_compute_prime256v1_ex()
{
unsigned char seed_a[20] = {...};
printf("s = %s\n", big_number_as_decimal_from_hex_array_ex(seed_a, 20));
unsigned char p_a[32] = {...};
printf("p = %s\n", big_number_as_decimal_from_hex_array_ex(p_a, 32));
// or
unsigned char seed_a[20] = {...};
printf("s = %s\n", big_number_as_decimal_from_hex_array_ex_2(seed_a, 20));
unsigned char p_a[32] = {...};
printf("p = %s\n", big_number_as_decimal_from_hex_array_ex_2(p_a, 32));
}
but the code produces the same incorrect result in a looped calculation

Answer 1

BN_hex2bn(&big_number, (char *)hex_s); expects a C string as second argument, ie a '\0' terminated one since it has no other way to know the size of your string.