Struct subtle::Choice

pub struct Choice(_);

The Choice struct represents a choice for use in conditional assignment.

It is a wrapper around a u8, which should have the value either 1 (true) or 0 (false).

With the nightly feature enabled, the conversion from u8 to Choice passes the value through an optimization barrier, as a best-effort attempt to prevent the compiler from inferring that the Choice value is a boolean. This strategy is based on Tim Maclean's work on rust-timing-shield, which attempts to provide a more comprehensive approach for preventing software side-channels in Rust code.

The Choice struct implements operators for AND, OR, XOR, and NOT, to allow combining Choice values. These operations do not short-circuit.

Methods

impl Choice

pub fn unwrap_u8(&self) -> u8

Unwrap the Choice wrapper to reveal the underlying u8.

Note

This function only exists as an escape hatch for the rare case where it's not possible to use one of the subtle-provided trait impls.

To convert a Choice to a bool, use the From implementation instead.

Trait Implementations

impl ConditionallySelectable for Choice

fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self

Select a or b according to choice.

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

Conditionally assign other to self, according to choice.

fn conditional_swap(a: &mut Self, b: &mut Self, choice: Choice)

Conditionally swap self and other if choice == 1; otherwise, reassign both unto themselves.

impl From<Choice> for bool

fn from(source: Choice) -> bool

Convert the Choice wrapper into a bool, depending on whether the underlying u8 was a 0 or a 1.

Note

This function exists to avoid having higher-level cryptographic protocol implementations duplicating this pattern.

The intended use case for this conversion is at the end of a higher-level primitive implementation: for example, in checking a keyed MAC, where the verification should happen in constant-time (and thus use a Choice) but it is safe to return a bool at the end of the verification.

impl From<u8> for Choice

fn from(input: u8) -> Choice

Performs the conversion.

impl Debug for Choice

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

Formats the value using the given formatter.

impl Not for Choice

type Output = Choice

The resulting type after applying the ! operator.

fn not(self) -> Choice

Performs the unary ! operation.

impl BitAnd<Choice> for Choice

type Output = Choice

The resulting type after applying the & operator.

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

Performs the & operation.

impl BitOr<Choice> for Choice

type Output = Choice

The resulting type after applying the | operator.

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

Performs the | operation.

impl BitXor<Choice> for Choice

type Output = Choice

The resulting type after applying the ^ operator.

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

Performs the ^ operation.

impl BitAndAssign<Choice> for Choice

fn bitand_assign(&mut self, rhs: Choice)

Performs the &= operation.

impl BitOrAssign<Choice> for Choice

fn bitor_assign(&mut self, rhs: Choice)

Performs the |= operation.

impl BitXorAssign<Choice> for Choice

fn bitxor_assign(&mut self, rhs: Choice)

Performs the ^= operation.

impl Copy for Choice

impl Clone for Choice

fn clone(&self) -> Choice

Returns a copy of the value.

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

Performs copy-assignment from source.