From: Richard Levitte
Date: Thu, 1 Jul 2004 12:33:39 +0000 (+0000)
Subject: Explain a little better what BN_num_bits() and BN_num_bits_word() do.
X-Git-Tag: BEN_FIPS_TEST_6~14^2~181
X-Git-Url: https://git.openssl.org/?p=openssl.git;a=commitdiff_plain;h=70696f4525d0af89c527a1f84e9a4bc8bb3bdd88
Explain a little better what BN_num_bits() and BN_num_bits_word() do.
Add a note as to how these functions do not always return the key size, and
how one can deal with that.
PR: 907
---
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 in bytes or bits,
-and the size of an unsigned integer in bits.
+BN_num_bytes() returns the size of a B 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.
+
+BN_num_bits() returns the number of significant bits in a B,
+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 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
+L, L, L,
+L
=head1 HISTORY