.\" $OpenBSD: BN_num_bytes.3,v 1.9 2022/11/22 18:55:04 schwarze Exp $ .\" full merge up to: OpenSSL 9e183d22 Mar 11 08:56:44 2017 -0500 .\" .\" This file is a derived work. .\" The changes are covered by the following Copyright and license: .\" .\" Copyright (c) 2022 Ingo Schwarze .\" .\" Permission to use, copy, modify, and distribute this software for any .\" purpose with or without fee is hereby granted, provided that the above .\" copyright notice and this permission notice appear in all copies. .\" .\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES .\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS. 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IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: November 22 2022 $ .Dt BN_NUM_BYTES 3 .Os .Sh NAME .Nm BN_num_bits_word , .Nm BN_num_bits , .Nm BN_num_bytes .Nd get BIGNUM size .Sh SYNOPSIS .In openssl/bn.h .Ft int .Fo BN_num_bits_word .Fa "BN_ULONG w" .Fc .Ft int .Fo BN_num_bits .Fa "const BIGNUM *a" .Fc .Ft int .Fo BN_num_bytes .Fa "const BIGNUM *a" .Fc .Sh DESCRIPTION .Fn BN_num_bits_word returns the number of significant bits in .Fa w , that is, the minimum number of digits needed to write .Fa w as a binary number. Except for an argument of 0, this is .Pp .D1 floor(log2( Ns Fa w ) ) No + 1 . .Pp .Vt BN_ULONG is a macro that expands to .Vt unsigned long Pq = Vt uint64_t on .Dv _LP64 platforms and .Vt unsigned int Pq = Vt uint32_t elsewhere. .Pp .Fn BN_num_bits returns the number of significant bits in the value of the .Fa "BIGNUM *a" , following the same principle as .Fn BN_num_bits_word . .Pp .Fn BN_num_bytes is a macro that returns the number of significant bytes in .Fa a , i.e. the minimum number of bytes needed to store the value of .Fa a , that is, .Fn BN_num_bits a divided by eight and rounded up to the next integer number. .Sh RETURN VALUES .Fn BN_num_bits_word returns the number of significant bits in .Fa w or 0 if .Fa w is 0. The maximum return value that can occur is .Dv BN_BITS2 , which is 64 on .Dv _LP64 platforms and 32 elsewhere. .Pp .Fn BN_num_bits returns the number of significant bits and .Fn BN_num_bytes the number of significant bytes in .Fa a , or 0 if the value of .Fa a is 0. .Sh SEE ALSO .Xr BN_new 3 , .Xr BN_security_bits 3 , .Xr DH_size 3 , .Xr DSA_size 3 , .Xr RSA_size 3 .Sh HISTORY .Fn BN_num_bytes and .Fn BN_num_bits first appeared in SSLeay 0.5.1. .Fn BN_num_bits_word first appeared in SSLeay 0.5.2. These functions have been available since .Ox 2.4 . .Sh CAVEATS Some have tried using .Fn BN_num_bits on individual numbers in RSA keys, DH keys and DSA 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 .Em significant bits a little smaller. If you want to know the "key size" of such a key, use functions like .Xr RSA_size 3 , .Xr DH_size 3 , and .Xr DSA_size 3 .