diff options
Diffstat (limited to 'doc/crypto/BN_num_bytes.pod')
| -rw-r--r-- | doc/crypto/BN_num_bytes.pod | 26 |
1 files changed, 23 insertions, 3 deletions
diff --git a/doc/crypto/BN_num_bytes.pod b/doc/crypto/BN_num_bytes.pod index 61589fb9ac..a6a2e3f819 100644 --- a/doc/crypto/BN_num_bytes.pod +++ b/doc/crypto/BN_num_bytes.pod @@ -16,8 +16,14 @@ BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size =head1 DESCRIPTION -These functions return the size of a B<BIGNUM> in bytes or bits, -and the size of an unsigned integer in bits. +BN_num_bytes() returns the size of a B<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 I<floor(log2(w))+1>. + +BN_num_bits() returns the number of significant bits in a B<BIGNUM>, +following the same principle as BN_num_bits_word(). BN_num_bytes() is a macro. @@ -25,9 +31,23 @@ BN_num_bytes() is a macro. The size. +=head1 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 I<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). + =head1 SEE ALSO -L<bn(3)|bn(3)> +L<bn(3)|bn(3)>, L<DH_size(3)|DH_size(3)>, L<DSA_size(3)|DSA_size(3)>, +L<RSA_size(3)|RSA_size(3)> =head1 HISTORY |