Struct Choice 

pub struct Choice(/* private fields */);

Constant-time analogue of bool providing a “best effort” optimization barrier.

Attempts to hint to the compiler and its codegen backends that optimizations should not be applied which depend on a value.

This is used as a “belt-and-suspenders” defense in addition to mechanisms like constant-time predication intrinsics provided by the cmov crate, and is never expected to be the only line of defense.

Implementations

impl Choice

pub const FALSE: Self

Equivalent of false.

pub const TRUE: Self

Equivalent of true.

pub const fn new(value: u8) -> Self

Create a new Choice from the given u8 value, which should be either 0 or 1.

pub fn to_bool(self) -> bool

Convert Choice into a bool.

Security Warning

Using this function will introduce timing variability, since computing this at all currently requires a branch.

This is intended to be used as either the one and only branch at the end of a constant-time operation to e.g. differentiate between success and failure, or in contexts where constant-time doesn’t matter, e.g. variable-time code that operates on “maybe secret” types which aren’t secrets in a particular context.

If you are trying to use this in the context of a constant-time operation, be warned that the small amount of timing variability it introduces can potentially be exploited. Whenever possible, prefer fully constant-time approaches instead.

pub fn to_u8(self) -> u8

Convert Choice to a u8, attempting to apply a “best effort” optimization barrier.

pub const fn and(self, rhs: Choice) -> Choice

Apply an and conditional to the given Choices.

pub const fn or(self, rhs: Choice) -> Choice

Apply an or conditional to the given Choices.

pub const fn xor(self, rhs: Choice) -> Choice

Apply an xor conditional to the given Choices.

pub const fn not(self) -> Choice

Compute the boolean inverse of self.

Trait Implementations

impl BitAnd for Choice

type Output = Choice

The resulting type after applying the & operator.

fn bitand(self, rhs: Choice) -> Choice

Performs the & operation.

impl BitAndAssign for Choice

fn bitand_assign(&mut self, rhs: Choice)

Performs the &= operation.

impl BitOr for Choice

type Output = Choice

The resulting type after applying the | operator.

fn bitor(self, rhs: Choice) -> Choice

Performs the | operation.

impl BitOrAssign for Choice

fn bitor_assign(&mut self, rhs: Choice)

Performs the |= operation.

impl BitXor for Choice

type Output = Choice

The resulting type after applying the ^ operator.

fn bitxor(self, rhs: Choice) -> Choice

Performs the ^ operation.

impl BitXorAssign for Choice

fn bitxor_assign(&mut self, rhs: Choice)

Performs the ^= operation.

impl Clone for Choice

fn clone(&self) -> Choice

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl CtEq for Choice

fn ct_eq(&self, other: &Self) -> Self

Equality

fn ct_ne(&self, other: &Rhs) -> Choice

Inequality

impl CtSelect for Choice

fn ct_select(&self, other: &Self, choice: Choice) -> Self

Select between self and other based on choice, returning a copy of the value.

fn ct_assign(&mut self, other: &Self, choice: Choice)

Conditionally assign other to self if choice is Choice::TRUE.

fn ct_swap(&mut self, other: &mut Self, choice: Choice)

Conditionally swap self and other if choice is Choice::TRUE.

impl Debug for Choice

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl From<Choice> for Choice

Available on crate feature subtle only.

fn from(choice: Choice) -> Choice

Converts to this type from the input type.

impl From<Choice> for Choice

Available on crate feature subtle only.

fn from(choice: Choice) -> Choice

Converts to this type from the input type.

impl From<Choice> for bool

fn from(choice: Choice) -> bool

Converts to this type from the input type.

impl From<Choice> for u8

fn from(choice: Choice) -> u8

Converts to this type from the input type.

impl Not for Choice

type Output = Choice

The resulting type after applying the ! operator.

fn not(self) -> Choice

Performs the unary ! operation.

impl Copy for Choice