[EMAIL PROTECTED] - Thu Jul 1 12:52:19 2004]: > I'd suggest to clarify the man page of 'BN_num_bits': > People (such as me) are tempted to use 'BN_num_bits' to get the key size > (class) of a private or public key, and expect that its size matches the > size (class) of its counterpart (public or private key). With "size > class" I mean 512, 1024, 2048 bit etc. > Now what I experience is that the size of the public key that was > generated from a given private key (e.g. with 'DH_generate_key') does > not always match the size of the private key, which is confusing...
I just committed the following change, is that satisfactory? Index: doc/crypto/BN_num_bytes.pod =================================================================== RCS file: /e/openssl/cvs/openssl/doc/crypto/BN_num_bytes.pod,v retrieving revision 1.3 retrieving revision 1.3.8.1 diff -u -r1.3 -r1.3.8.1 --- doc/crypto/BN_num_bytes.pod 24 Feb 2000 11:55:05 -0000 1.3 +++ doc/crypto/BN_num_bytes.pod 1 Jul 2004 12:33:44 -0000 1.3.8.1 @@ -16,8 +16,14 @@ =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 @@ 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 -- Richard Levitte [EMAIL PROTECTED] ______________________________________________________________________ OpenSSL Project http://www.openssl.org Development Mailing List [EMAIL PROTECTED] Automated List Manager [EMAIL PROTECTED]
