Struct subtle::Choice

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pub struct Choice(_);

[−] Expand description

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).

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.

Implementations

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 BitAnd<Choice> for Choice[+]

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type Output = Choice

The resulting type after applying the & operator.

fn bitand(self, rhs: Choice) -> Choice[−]

Performs the & operation.

impl BitAndAssign<Choice> for Choice[+]

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fn bitand_assign(&mut self, rhs: Choice)[−]

Performs the &= operation.

impl BitOr<Choice> for Choice[+]

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type Output = Choice

The resulting type after applying the | operator.

fn bitor(self, rhs: Choice) -> Choice[−]

Performs the | operation.

impl BitOrAssign<Choice> for Choice[+]

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fn bitor_assign(&mut self, rhs: Choice)[−]

Performs the |= operation.

impl BitXor<Choice> for Choice[+]

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type Output = Choice

The resulting type after applying the ^ operator.

fn bitxor(self, rhs: Choice) -> Choice[−]

Performs the ^ operation.

impl BitXorAssign<Choice> for Choice[+]

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fn bitxor_assign(&mut self, rhs: Choice)[−]

Performs the ^= operation.

impl Clone for Choice[+]

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fn clone(&self) -> Choice[−]

Returns a copy of the value.

pub fn clone_from(&mut self, source: &Self)[−]

Performs copy-assignment from source.

impl ConditionallySelectable for Choice[+]

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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 ConstantTimeEq for Choice[+]

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fn ct_eq(&self, rhs: &Choice) -> Choice[−]

Determine if two items are equal.

impl Copy for Choice

impl Debug for Choice[+]

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

Formats the value using the given formatter.

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[+]

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fn from(input: u8) -> Choice[−]

Performs the conversion.

impl Not for Choice[+]

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type Output = Choice

The resulting type after applying the ! operator.

fn not(self) -> Choice[−]

Performs the unary ! operation.