## NAME

`BN_num_bytes`

,
`BN_num_bits`

,
`BN_num_bits_word`

—
get BIGNUM size

## SYNOPSIS

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

`int`

`BN_num_bytes`

(`const BIGNUM
*a`);

`int`

`BN_num_bits`

(`const BIGNUM
*a`);

`int`

`BN_num_bits_word`

(`BN_ULONG
w`);

## DESCRIPTION

`BN_num_bytes`

()
returns the size of a `BIGNUM` in bytes.

`BN_num_bits_word`

()
returns the number of significant bits in a word. As an example, 0x00000432
returns 11, not 16 or 32. Basically, except for a zero, it returns

`w`)) + 1

`BN_num_bits`

()
returns the number of significant bits in a
**BIGNUM**,
following the same principle as
`BN_num_bits_word`

().

`BN_num_bytes`

()
is a macro.

Some have tried using
`BN_num_bits`

()
on individual numbers in RSA keys, DH keys and DSA keys, and found that they
don't always come up with the number of bits they expected (something like
512, 1024, 2048, ...). This is because generating a number with some
specific number of bits doesn't always set the highest bits, thereby making
the number of
*significant*
bits a little lower. If you want to know the "key size" of such a
key, either use functions like
RSA_size(3),
DH_size(3), and
DSA_size(3), or use `BN_num_bytes`

() and
multiply with 8 (although there's no real guarantee that will match the
"key size", just a lot more probability).

## RETURN VALUES

The size.

## SEE ALSO

## HISTORY

`BN_num_bytes`

() and
`BN_num_bits`

() first appeared in SSLeay 0.5.1.
`BN_num_bits_word`

() first appeared in SSLeay 0.5.2.
These functions have been available since OpenBSD
2.4.