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1.\" $OpenBSD: BN_num_bytes.3,v 1.9 2022/11/22 18:55:04 schwarze Exp $
2.\" full merge up to: OpenSSL 9e183d22 Mar 11 08:56:44 2017 -0500
3.\"
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21.\" The original file was written by Ulf Moeller <ulf@openssl.org>
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69.Dd $Mdocdate: November 22 2022 $
70.Dt BN_NUM_BYTES 3
71.Os
72.Sh NAME
73.Nm BN_num_bits_word ,
74.Nm BN_num_bits ,
75.Nm BN_num_bytes
76.Nd get BIGNUM size
77.Sh SYNOPSIS
78.In openssl/bn.h
79.Ft int
80.Fo BN_num_bits_word
81.Fa "BN_ULONG w"
82.Fc
83.Ft int
84.Fo BN_num_bits
85.Fa "const BIGNUM *a"
86.Fc
87.Ft int
88.Fo BN_num_bytes
89.Fa "const BIGNUM *a"
90.Fc
91.Sh DESCRIPTION
92.Fn BN_num_bits_word
93returns the number of significant bits in
94.Fa w ,
95that is, the minimum number of digits needed to write
96.Fa w
97as a binary number.
98Except for an argument of 0, this is
99.Pp
100.D1 floor(log2( Ns Fa w ) ) No + 1 .
101.Pp
102.Vt BN_ULONG
103is a macro that expands to
104.Vt unsigned long Pq = Vt uint64_t
105on
106.Dv _LP64
107platforms and
108.Vt unsigned int Pq = Vt uint32_t
109elsewhere.
110.Pp
111.Fn BN_num_bits
112returns the number of significant bits in the value of the
113.Fa "BIGNUM *a" ,
114following the same principle as
115.Fn BN_num_bits_word .
116.Pp
117.Fn BN_num_bytes
118is a macro that returns the number of significant bytes in
119.Fa a ,
120i.e. the minimum number of bytes needed to store the value of
121.Fa a ,
122that is,
123.Fn BN_num_bits a
124divided by eight and rounded up to the next integer number.
125.Sh RETURN VALUES
126.Fn BN_num_bits_word
127returns the number of significant bits in
128.Fa w
129or 0 if
130.Fa w
131is 0.
132The maximum return value that can occur is
133.Dv BN_BITS2 ,
134which is 64 on
135.Dv _LP64
136platforms and 32 elsewhere.
137.Pp
138.Fn BN_num_bits
139returns the number of significant bits and
140.Fn BN_num_bytes
141the number of significant bytes in
142.Fa a ,
143or 0 if the value of
144.Fa a
145is 0.
146.Sh SEE ALSO
147.Xr BN_new 3 ,
148.Xr BN_security_bits 3 ,
149.Xr DH_size 3 ,
150.Xr DSA_size 3 ,
151.Xr RSA_size 3
152.Sh HISTORY
153.Fn BN_num_bytes
154and
155.Fn BN_num_bits
156first appeared in SSLeay 0.5.1.
157.Fn BN_num_bits_word
158first appeared in SSLeay 0.5.2.
159These functions have been available since
160.Ox 2.4 .
161.Sh CAVEATS
162Some have tried using
163.Fn BN_num_bits
164on individual numbers in RSA keys, DH keys and DSA keys, and found that
165they don't always come up with the number of bits they expected
166(something like 512, 1024, 2048, ...).
167This is because generating a number with some specific number of bits
168doesn't always set the highest bits, thereby making the number of
169.Em significant
170bits a little smaller.
171If you want to know the "key size" of such a key, use functions like
172.Xr RSA_size 3 ,
173.Xr DH_size 3 ,
174and
175.Xr DSA_size 3 .
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