MirBSD manpage: BN_num_bits(3), BN_num_bits_word(3), BN_num_bytes(3)

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 (some-
     thing like 512, 1024, 2048, ...).  This is because generat-
     ing 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.

MirBSD #10-current         2005-04-29                           1