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.\" $NetBSD: BN_set_bit.3,v 1.5 2026/04/08 17:06:41 christos Exp $
.\"
.\" -*- mode: troff; coding: utf-8 -*-
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.\" titles (.TH), headers (.SH), subsections (.SS), items (.Ip), and index
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.\" ========================================================================
.\"
.IX Title "BN_set_bit 3"
.TH BN_set_bit 3 2026-04-07 3.5.6 OpenSSL
.\" For nroff, turn off justification. Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH NAME
BN_set_bit, BN_clear_bit, BN_is_bit_set, BN_mask_bits, BN_lshift,
BN_lshift1, BN_rshift, BN_rshift1 \- bit operations on BIGNUMs
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/bn.h>
\&
\& int BN_set_bit(BIGNUM *a, int n);
\& int BN_clear_bit(BIGNUM *a, int n);
\&
\& int BN_is_bit_set(const BIGNUM *a, int n);
\&
\& int BN_mask_bits(BIGNUM *a, int n);
\&
\& int BN_lshift(BIGNUM *r, const BIGNUM *a, int n);
\& int BN_lshift1(BIGNUM *r, BIGNUM *a);
\&
\& int BN_rshift(BIGNUM *r, BIGNUM *a, int n);
\& int BN_rshift1(BIGNUM *r, BIGNUM *a);
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\fBBN_set_bit()\fR sets bit \fBn\fR in \fBa\fR to 1 (\f(CW\*(C`a|=(1<<n)\*(C'\fR). The
number is expanded if necessary.
.PP
\&\fBBN_clear_bit()\fR sets bit \fBn\fR in \fBa\fR to 0 (\f(CW\*(C`a&=~(1<<n)\*(C'\fR). An
error occurs if \fBa\fR is shorter than \fBn\fR bits.
.PP
\&\fBBN_is_bit_set()\fR tests if bit \fBn\fR in \fBa\fR is set.
.PP
\&\fBBN_mask_bits()\fR truncates \fBa\fR to an \fBn\fR bit number
(\f(CW\*(C`a&=~((~0)<<n)\*(C'\fR). An error occurs if \fBn\fR is negative. An error is
also returned if the internal representation of \fBa\fR is already shorter than
\&\fBn\fR bits. The internal representation depends on the platform\*(Aqs word size, and
this error can be safely ignored. Use \fBBN_num_bits\fR\|(3) to determine the exact
number of bits if needed.
.PP
\&\fBBN_lshift()\fR shifts \fBa\fR left by \fBn\fR bits and places the result in
\&\fBr\fR (\f(CW\*(C`r=a*2^n\*(C'\fR). Note that \fBn\fR must be nonnegative. \fBBN_lshift1()\fR shifts
\&\fBa\fR left by one and places the result in \fBr\fR (\f(CW\*(C`r=2*a\*(C'\fR).
.PP
\&\fBBN_rshift()\fR shifts \fBa\fR right by \fBn\fR bits and places the result in
\&\fBr\fR (\f(CW\*(C`r=a/2^n\*(C'\fR). Note that \fBn\fR must be nonnegative. \fBBN_rshift1()\fR shifts
\&\fBa\fR right by one and places the result in \fBr\fR (\f(CW\*(C`r=a/2\*(C'\fR).
.PP
For the shift functions, \fBr\fR and \fBa\fR may be the same variable.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBBN_is_bit_set()\fR returns 1 if the bit is set, 0 otherwise.
.PP
All other functions return 1 for success, 0 on error. The error codes
can be obtained by \fBERR_get_error\fR\|(3).
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBBN_num_bytes\fR\|(3), \fBBN_add\fR\|(3)
.SH COPYRIGHT
.IX Header "COPYRIGHT"
Copyright 2000\-2024 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the Apache License 2.0 (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file LICENSE in the source distribution or at
<https://www.openssl.org/source/license.html>.
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