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126 .IX Title "BN_num_bytes 3"
127 .TH BN_num_bytes 3 "2010-11-16" "1.0.0b" "OpenSSL"
128 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
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133 BN_num_bits, BN_num_bytes, BN_num_bits_word \- get BIGNUM size
135 .IX Header "SYNOPSIS"
137 \& #include <openssl/bn.h>
139 \& int BN_num_bytes(const BIGNUM *a);
141 \& int BN_num_bits(const BIGNUM *a);
143 \& int BN_num_bits_word(BN_ULONG w);
146 .IX Header "DESCRIPTION"
147 \&\fIBN_num_bytes()\fR returns the size of a \fB\s-1BIGNUM\s0\fR in bytes.
149 \&\fIBN_num_bits_word()\fR returns the number of significant bits in a word.
150 If we take 0x00000432 as an example, it returns 11, not 16, not 32.
151 Basically, except for a zero, it returns \fIfloor(log2(w))+1\fR.
153 \&\fIBN_num_bits()\fR returns the number of significant bits in a \fB\s-1BIGNUM\s0\fR,
154 following the same principle as \fIBN_num_bits_word()\fR.
156 \&\fIBN_num_bytes()\fR is a macro.
158 .IX Header "RETURN VALUES"
162 Some have tried using \fIBN_num_bits()\fR on individual numbers in \s-1RSA\s0 keys,
163 \&\s-1DH\s0 keys and \s-1DSA\s0 keys, and found that they don't always come up with
164 the number of bits they expected (something like 512, 1024, 2048,
165 \&...). This is because generating a number with some specific number
166 of bits doesn't always set the highest bits, thereby making the number
167 of \fIsignificant\fR bits a little lower. If you want to know the \*(L"key
168 size\*(R" of such a key, either use functions like \fIRSA_size()\fR, \fIDH_size()\fR
169 and \fIDSA_size()\fR, or use \fIBN_num_bytes()\fR and multiply with 8 (although
170 there's no real guarantee that will match the \*(L"key size\*(R", just a lot
173 .IX Header "SEE ALSO"
174 \&\fIbn\fR\|(3), \fIDH_size\fR\|(3), \fIDSA_size\fR\|(3),
175 \&\fIRSA_size\fR\|(3)
178 \&\fIBN_num_bytes()\fR, \fIBN_num_bits()\fR and \fIBN_num_bits_word()\fR are available in
179 all versions of SSLeay and OpenSSL.