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1.\" $OpenBSD: BN_num_bytes.3,v 1.7 2018/03/27 17:35:50 schwarze Exp $
2.\" OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
3.\"
4.\" This file was written by Ulf Moeller <ulf@openssl.org>
5.\" and Richard Levitte <levitte@openssl.org>.
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52.Dd $Mdocdate: March 27 2018 $
53.Dt BN_NUM_BYTES 3
54.Os
55.Sh NAME
56.Nm BN_num_bytes ,
57.Nm BN_num_bits ,
58.Nm BN_num_bits_word
59.Nd get BIGNUM size
60.Sh SYNOPSIS
61.In openssl/bn.h
62.Ft int
63.Fo BN_num_bytes
64.Fa "const BIGNUM *a"
65.Fc
66.Ft int
67.Fo BN_num_bits
68.Fa "const BIGNUM *a"
69.Fc
70.Ft int
71.Fo BN_num_bits_word
72.Fa "BN_ULONG w"
73.Fc
74.Sh DESCRIPTION
75.Fn BN_num_bytes
76returns the size of a
77.Vt BIGNUM
78in bytes.
79.Pp
80.Fn BN_num_bits_word
81returns the number of significant bits in a word.
82As an example, 0x00000432 returns 11, not 16 or 32.
83Basically, except for a zero, it returns
84.Pp
85.D1 floor(log2( Ns Fa w ) ) No + 1 .
86.Pp
87.Fn BN_num_bits
88returns the number of significant bits in a
89.Sy BIGNUM ,
90following the same principle as
91.Fn BN_num_bits_word .
92.Pp
93.Fn BN_num_bytes
94is a macro.
95.Pp
96Some have tried using
97.Fn BN_num_bits
98on individual numbers in RSA keys, DH keys and DSA keys, and found that
99they don't always come up with the number of bits they expected
100(something like 512, 1024, 2048, ...). This is because generating a
101number with some specific number of bits doesn't always set the highest
102bits, thereby making the number of
103.Em significant
104bits a little lower.
105If you want to know the "key size" of such a key, either use functions
106like
107.Xr RSA_size 3 ,
108.Xr DH_size 3 ,
109and
110.Xr DSA_size 3 ,
111or use
112.Fn BN_num_bytes
113and multiply with 8 (although there's no real guarantee that will match
114the "key size", just a lot more probability).
115.Sh RETURN VALUES
116The size.
117.Sh SEE ALSO
118.Xr BN_new 3 ,
119.Xr DH_size 3 ,
120.Xr DSA_size 3 ,
121.Xr RSA_size 3
122.Sh HISTORY
123.Fn BN_num_bytes
124and
125.Fn BN_num_bits
126first appeared in SSLeay 0.5.1.
127.Fn BN_num_bits_word
128first appeared in SSLeay 0.5.2.
129These functions have been available since
130.Ox 2.4 .
generated by cgit v1.2.3-55-g6feb (git 2.39.1) at 2026-02-14 03:13:42 +0000