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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.



50                                               2013-03-05                                  BN_num_bytes(3)