Botan::CT::Choice Class Reference

#include <ct_utils.h>

Public Member Functions
constexpr bool	as_bool () const
constexpr	Choice (Choice &&other)=default
constexpr	Choice (const Choice &other)=default
constexpr Choice	operator! () const
constexpr Choice	operator!= (const Choice &other) const
constexpr Choice	operator&& (const Choice &other) const
constexpr Choice &	operator= (Choice &&other) noexcept=default
constexpr Choice &	operator= (const Choice &other) noexcept=default
constexpr Choice	operator|| (const Choice &other) const
constexpr uint32_t	value () const
	Return the masked value.

Static Public Member Functions
template<typename T > requires std::unsigned_integral<T> && (!std::same_as<bool, T>)
static constexpr Choice	from_int (T v)
static constexpr Choice	no ()
static constexpr Choice	yes ()

Detailed Description

A Choice is used for constant-time conditionals.

Internally it always is either |0| (all 0 bits) or |1| (all 1 bits) and measures are taken to block compilers from reasoning about the expected value of a Choice.

Definition at line 117 of file ct_utils.h.

Constructor & Destructor Documentation

Choice() [1/2]

Botan::CT::Choice::Choice ( const Choice & other )

constexprdefault

Referenced by from_int(), no(), operator!(), operator!=(), operator&&(), operator||(), and yes().

Choice() [2/2]

Botan::CT::Choice::Choice ( Choice && other )

constexprdefault

Member Function Documentation

as_bool()

bool Botan::CT::Choice::as_bool ( ) const

inlineconstexpr

Unsafe conversion to bool

This conversion itself is (probably) constant time, but once the choice is reduced to a simple bool, it's entirely possible for the compiler to perform range analysis on the values, since there are just the two. As a consequence even if the caller is not using this in an obviously branchy way (if(choice.as_bool()) ...) a smart compiler may introduce branches depending on the value.

Definition at line 160 of file ct_utils.h.

{ return m_value != 0; }

Referenced by Botan::ProjectiveCurvePoint< FieldElement, Params >::from_affine().

from_int()

template<typename T >
requires std::unsigned_integral<T> && (!std::same_as<bool, T>)

static constexpr Choice Botan::CT::Choice::from_int ( T v )

inlinestaticconstexpr

If v == 0 return an unset (false) Choice, otherwise a set Choice

Definition at line 124 of file ct_utils.h.

                                            {
         // Mask of T that is either |0| or |1|
         const T v_is_0 = ct_is_zero<T>(value_barrier<T>(v));
 
         // We want the mask to be set if v != 0 so we must check that
         // v_is_0 is itself zero.
         //
         // Also sizeof(T) may not equal sizeof(uint32_t) so we must
         // use ct_is_zero<uint32_t>. It's ok to either truncate or
         // zero extend v_is_0 to 32 bits since we know it is |0| or |1|
         // so even just the low bit is sufficient.
         return Choice(ct_is_zero<uint32_t>(static_cast<uint32_t>(v_is_0)));
      }

References Choice(), Botan::ct_is_zero(), T, and Botan::CT::value_barrier().

Referenced by Botan::CT::Mask< T >::as_choice(), and Botan::IntMod< Rep >::is_even().

no()

static constexpr Choice Botan::CT::Choice::no ( )

inlinestaticconstexpr

Definition at line 140 of file ct_utils.h.

{ return Choice(0); }

References Choice().

operator!()

Choice Botan::CT::Choice::operator! ( ) const

inlineconstexpr

Definition at line 142 of file ct_utils.h.

{ return Choice(~value()); }

References Choice(), and value().

operator!=()

Choice Botan::CT::Choice::operator!= ( const Choice & other ) const

inlineconstexpr

Definition at line 148 of file ct_utils.h.

{ return Choice(value() ^ other.value()); }

References Choice(), and value().

operator&&()

Choice Botan::CT::Choice::operator&& ( const Choice & other ) const

inlineconstexpr

Definition at line 144 of file ct_utils.h.

{ return Choice(value() & other.value()); }

References Choice(), and value().

operator=() [1/2]

Choice & Botan::CT::Choice::operator= ( Choice && other )

constexprdefaultnoexcept

operator=() [2/2]

Choice & Botan::CT::Choice::operator= ( const Choice & other )

constexprdefaultnoexcept

operator||()

Choice Botan::CT::Choice::operator|| ( const Choice & other ) const

inlineconstexpr

Definition at line 146 of file ct_utils.h.

{ return Choice(value() | other.value()); }

References Choice(), and value().

value()

uint32_t Botan::CT::Choice::value ( ) const

inlineconstexpr

Return the masked value.

Definition at line 163 of file ct_utils.h.

{ return value_barrier(m_value); }

References Botan::CT::value_barrier().

Referenced by Botan::CT::Mask< T >::from_choice(), operator!(), operator!=(), operator&&(), and operator||().

yes()

static constexpr Choice Botan::CT::Choice::yes ( )

inlinestaticconstexpr

Definition at line 138 of file ct_utils.h.

{ return Choice(static_cast<uint32_t>(-1)); }

References Choice().

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The documentation for this class was generated from the following file:

• src/lib/utils/ct_utils.h