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CentOS 7.0 - man page for bn_num_bytes (centos section 3)

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BN_num_bytes(3) 			     OpenSSL				  BN_num_bytes(3)

NAME
       BN_num_bits, BN_num_bytes, 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.  If we take
       0x00000432 as an example, it returns 11, not 16, not 32.  Basically, except for a zero, it
       returns floor(log2(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.

RETURN VALUES
       The size.

NOTES
       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(), DH_size() and DSA_size(), 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).

SEE ALSO
       bn(3), DH_size(3), DSA_size(3), RSA_size(3)

HISTORY
       BN_num_bytes(), BN_num_bits() and BN_num_bits_word() are available in all versions of
       SSLeay and OpenSSL.

1.0.1e					    2013-02-11				  BN_num_bytes(3)
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