1.\" $OpenBSD: BN_num_bytes.3,v 1.8 2022/07/13 21:51:35 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>. 6.\" Copyright (c) 2000, 2004 The OpenSSL Project. All rights reserved. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 15.\" 2. Redistributions in binary form must reproduce the above copyright 16.\" notice, this list of conditions and the following disclaimer in 17.\" the documentation and/or other materials provided with the 18.\" distribution. 19.\" 20.\" 3. All advertising materials mentioning features or use of this 21.\" software must display the following acknowledgment: 22.\" "This product includes software developed by the OpenSSL Project 23.\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)" 24.\" 25.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to 26.\" endorse or promote products derived from this software without 27.\" prior written permission. For written permission, please contact 28.\" openssl-core@openssl.org. 29.\" 30.\" 5. Products derived from this software may not be called "OpenSSL" 31.\" nor may "OpenSSL" appear in their names without prior written 32.\" permission of the OpenSSL Project. 33.\" 34.\" 6. Redistributions of any form whatsoever must retain the following 35.\" acknowledgment: 36.\" "This product includes software developed by the OpenSSL Project 37.\" for use in the OpenSSL Toolkit (http://www.openssl.org/)" 38.\" 39.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY 40.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 41.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 42.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR 43.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 44.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 45.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; 46.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 47.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 48.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 49.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED 50.\" OF THE POSSIBILITY OF SUCH DAMAGE. 51.\" 52.Dd $Mdocdate: July 13 2022 $ 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 BN_security_bits 3 , 120.Xr DH_size 3 , 121.Xr DSA_size 3 , 122.Xr RSA_size 3 123.Sh HISTORY 124.Fn BN_num_bytes 125and 126.Fn BN_num_bits 127first appeared in SSLeay 0.5.1. 128.Fn BN_num_bits_word 129first appeared in SSLeay 0.5.2. 130These functions have been available since 131.Ox 2.4 . 132