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BN_num_bits.3
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Always turn off hyphenation; it makes .\" way too many mistakes in technical documents. .if n .ad l .nh .SH "NAME" BN_num_bits, BN_num_bytes, BN_num_bits_word \- get BIGNUM size .SH "SYNOPSIS" .IX Header "SYNOPSIS" .Vb 1 \& #include <openssl/bn.h> \& \& int BN_num_bytes(const BIGNUM *a); \& \& int BN_num_bits(const BIGNUM *a); \& \& int BN_num_bits_word(BN_ULONG w); .Ve .SH "DESCRIPTION" .IX Header "DESCRIPTION" \&\fBBN_num_bytes()\fR returns the size of a \fB\s-1BIGNUM\s0\fR in bytes. .PP \&\fBBN_num_bits_word()\fR 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 \fIfloor(log2(w))+1\fR. .PP \&\fBBN_num_bits()\fR returns the number of significant bits in a \fB\s-1BIGNUM\s0\fR, following the same principle as \fBBN_num_bits_word()\fR. .PP \&\fBBN_num_bytes()\fR is a macro. .SH "RETURN VALUES" .IX Header "RETURN VALUES" The size. .SH "NOTES" .IX Header "NOTES" Some have tried using \fBBN_num_bits()\fR on individual numbers in \s-1RSA\s0 keys, \&\s-1DH\s0 keys and \s-1DSA\s0 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 \fIsignificant\fR bits a little lower. If you want to know the \*(L"key size\*(R" of such a key, either use functions like \fBRSA_size()\fR, \fBDH_size()\fR and \fBDSA_size()\fR, or use \fBBN_num_bytes()\fR and multiply with 8 (although there's no real guarantee that will match the \*(L"key size\*(R", just a lot more probability). .SH "SEE ALSO" .IX Header "SEE ALSO" \&\fBbn\fR\|(3), \fBDH_size\fR\|(3), \fBDSA_size\fR\|(3), \&\fBRSA_size\fR\|(3) .SH "HISTORY" .IX Header "HISTORY" \&\fBBN_num_bytes()\fR, \fBBN_num_bits()\fR and \fBBN_num_bits_word()\fR are available in all versions of SSLeay and OpenSSL.
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