BN_NUM_BYTES(3)            Library Functions Manual            BN_NUM_BYTES(3)

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

           floor(log2(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).

Linux 6.8.2-arch2-1            November 22, 2022           Linux 6.8.2-arch2-1