X-Git-Url: https://git.librecmc.org/?a=blobdiff_plain;f=doc%2Fcrypto%2FBN_num_bytes.pod;h=a6a2e3f81988a97374bbe782c3672e7688d0032e;hb=ba544377fb48336b4e29fd494a7faf7c0ba6b1dc;hp=ab12ffc657dc059aeeaf43016858ef69839aa821;hpb=dd8dec69b825c9fdafc26a200961702d850496b5;p=oweals%2Fopenssl.git

diff --git a/doc/crypto/BN_num_bytes.pod b/doc/crypto/BN_num_bytes.pod
index ab12ffc657..a6a2e3f819 100644
--- a/doc/crypto/BN_num_bytes.pod
+++ b/doc/crypto/BN_num_bytes.pod
@@ -2,7 +2,7 @@
 
 =head1 NAME
 
-BN_num_bits, BN_num_bytes, BN_num_bits_word - Get BIGNUM size
+BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size
 
 =head1 SYNOPSIS
 
@@ -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
 
-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