185 lines
5.6 KiB
Groff
185 lines
5.6 KiB
Groff
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.\" ========================================================================
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.\"
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.IX Title "BN_num_bytes 3"
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.TH BN_num_bytes 3 "2018-03-27" "1.0.2o" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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BN_num_bits, BN_num_bytes, BN_num_bits_word \- get BIGNUM size
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/bn.h>
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\&
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\& int BN_num_bytes(const BIGNUM *a);
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\&
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\& int BN_num_bits(const BIGNUM *a);
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\&
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\& int BN_num_bits_word(BN_ULONG w);
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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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\&\fIBN_num_bytes()\fR returns the size of a \fB\s-1BIGNUM\s0\fR in bytes.
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.PP
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\&\fIBN_num_bits_word()\fR returns the number of significant bits in a word.
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If we take 0x00000432 as an example, it returns 11, not 16, not 32.
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Basically, except for a zero, it returns \fIfloor(log2(w))+1\fR.
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.PP
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\&\fIBN_num_bits()\fR returns the number of significant bits in a \fB\s-1BIGNUM\s0\fR,
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following the same principle as \fIBN_num_bits_word()\fR.
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.PP
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\&\fIBN_num_bytes()\fR is a macro.
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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The size.
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.SH "NOTES"
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.IX Header "NOTES"
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Some have tried using \fIBN_num_bits()\fR on individual numbers in \s-1RSA\s0 keys,
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\&\s-1DH\s0 keys and \s-1DSA\s0 keys, and found that they don't always come up with
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the number of bits they expected (something like 512, 1024, 2048,
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\&...). This is because generating a number with some specific number
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of bits doesn't always set the highest bits, thereby making the number
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of \fIsignificant\fR bits a little lower. If you want to know the \*(L"key
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size\*(R" of such a key, either use functions like \fIRSA_size()\fR, \fIDH_size()\fR
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and \fIDSA_size()\fR, or use \fIBN_num_bytes()\fR and multiply with 8 (although
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there's no real guarantee that will match the \*(L"key size\*(R", just a lot
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more probability).
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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\&\fIbn\fR\|(3), \fIDH_size\fR\|(3), \fIDSA_size\fR\|(3),
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\&\fIRSA_size\fR\|(3)
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.SH "HISTORY"
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.IX Header "HISTORY"
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\&\fIBN_num_bytes()\fR, \fIBN_num_bits()\fR and \fIBN_num_bits_word()\fR are available in
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all versions of SSLeay and OpenSSL.
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