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131 .IX Title "BN_num_bytes 3"
132 .TH BN_num_bytes 3 "2007-03-15" "0.9.8e" "OpenSSL"
134 BN_num_bits, BN_num_bytes, BN_num_bits_word \- get BIGNUM size
136 .IX Header "SYNOPSIS"
138 \& #include <openssl/bn.h>
142 \& int BN_num_bytes(const BIGNUM *a);
146 \& int BN_num_bits(const BIGNUM *a);
150 \& int BN_num_bits_word(BN_ULONG w);
153 .IX Header "DESCRIPTION"
154 \&\fIBN_num_bytes()\fR returns the size of a \fB\s-1BIGNUM\s0\fR in bytes.
156 \&\fIBN_num_bits_word()\fR returns the number of significant bits in a word.
157 If we take 0x00000432 as an example, it returns 11, not 16, not 32.
158 Basically, except for a zero, it returns \fIfloor(log2(w))+1\fR.
160 \&\fIBN_num_bits()\fR returns the number of significant bits in a \fB\s-1BIGNUM\s0\fR,
161 following the same principle as \fIBN_num_bits_word()\fR.
163 \&\fIBN_num_bytes()\fR is a macro.
165 .IX Header "RETURN VALUES"
169 Some have tried using \fIBN_num_bits()\fR on individual numbers in \s-1RSA\s0 keys,
170 \&\s-1DH\s0 keys and \s-1DSA\s0 keys, and found that they don't always come up with
171 the number of bits they expected (something like 512, 1024, 2048,
172 \&...). This is because generating a number with some specific number
173 of bits doesn't always set the highest bits, thereby making the number
174 of \fIsignificant\fR bits a little lower. If you want to know the \*(L"key
175 size\*(R" of such a key, either use functions like \fIRSA_size()\fR, \fIDH_size()\fR
176 and \fIDSA_size()\fR, or use \fIBN_num_bytes()\fR and multiply with 8 (although
177 there's no real guarantee that will match the \*(L"key size\*(R", just a lot
180 .IX Header "SEE ALSO"
181 \&\fIbn\fR\|(3), \fIDH_size\fR\|(3), \fIDSA_size\fR\|(3),
182 \&\fIRSA_size\fR\|(3)
185 \&\fIBN_num_bytes()\fR, \fIBN_num_bits()\fR and \fIBN_num_bits_word()\fR are available in
186 all versions of SSLeay and OpenSSL.