## NAME

`BN_num_bits_word`

,
`BN_num_bits`

, `BN_num_bytes`

— get BIGNUM size

## SYNOPSIS

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

`int`

`BN_num_bits_word`

(`BN_ULONG
w`);

`int`

`BN_num_bits`

(`const BIGNUM
*a`);

`int`

`BN_num_bytes`

(`const BIGNUM
*a`);

## DESCRIPTION

`BN_num_bits_word`

()
returns the number of significant bits in `w`, that is,
the minimum number of digits needed to write `w` as a
binary number. Except for an argument of 0, this is

`w`)) + 1

`BN_ULONG` is a macro that expands to
`unsigned long` (= `uint64_t`) on
`_LP64`

platforms and `unsigned
int` (= `uint32_t`) elsewhere.

`BN_num_bits`

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

().

`BN_num_bytes`

()
is a macro that returns the number of significant bytes in
`a`, i.e. the minimum number of bytes needed to store
the value of `a`, that is,
`BN_num_bits`

(`a`) divided by
eight and rounded up to the next integer number.

## RETURN VALUES

`BN_num_bits_word`

() returns the number of
significant bits in `w` or 0 if `w`
is 0. The maximum return value that can occur is
`BN_BITS2`

, which is 64 on
`_LP64`

platforms and 32 elsewhere.

`BN_num_bits`

() returns the number of
significant bits and `BN_num_bytes`

() the number of
significant bytes in `a`, or 0 if the value of
`a` is 0.

## SEE ALSO

BN_new(3), BN_security_bits(3), DH_size(3), DSA_size(3), RSA_size(3)

## 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.

## CAVEATS

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 smaller. If you want to know the "key size" of such
a key, use functions like
RSA_size(3),
DH_size(3), and
DSA_size(3).