Struct subtle::Choice

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

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impl Choice

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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§

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impl BitAnd for Choice

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type Output = Choice

The resulting type after applying the & operator.
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fn bitand(self, rhs: Choice) -> Choice

Performs the & operation. Read more
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impl BitAndAssign for Choice

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fn bitand_assign(&mut self, rhs: Choice)

Performs the &= operation. Read more
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impl BitOr for Choice

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type Output = Choice

The resulting type after applying the | operator.
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fn bitor(self, rhs: Choice) -> Choice

Performs the | operation. Read more
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impl BitOrAssign for Choice

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fn bitor_assign(&mut self, rhs: Choice)

Performs the |= operation. Read more
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impl BitXor for Choice

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type Output = Choice

The resulting type after applying the ^ operator.
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fn bitxor(self, rhs: Choice) -> Choice

Performs the ^ operation. Read more
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impl BitXorAssign for Choice

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fn bitxor_assign(&mut self, rhs: Choice)

Performs the ^= operation. Read more
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impl Clone for Choice

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fn clone(&self) -> Choice

Returns a copy of the value. Read more
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fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source. Read more
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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. Read more
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fn conditional_assign(&mut self, other: &Self, choice: Choice)

Conditionally assign other to self, according to choice. Read more
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fn conditional_swap(a: &mut Self, b: &mut Self, choice: Choice)

Conditionally swap self and other if choice == 1; otherwise, reassign both unto themselves. Read more
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impl ConstantTimeEq for Choice

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

Determine if two items are equal. Read more
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impl Debug for Choice

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

Formats the value using the given formatter. Read more
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impl From<Choice> for bool

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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.

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impl From<u8> for Choice

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fn from(input: u8) -> Choice

Converts to this type from the input type.
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impl Not for Choice

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type Output = Choice

The resulting type after applying the ! operator.
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fn not(self) -> Choice

Performs the unary ! operation. Read more
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impl Copy for Choice

Auto Trait Implementations§

Blanket Implementations§

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impl<T> Any for T
where T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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impl<T> Borrow<T> for T
where T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for T
where T: ?Sized,

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fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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impl<T, U> Into<U> for T
where U: From<T>,

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fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

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impl<T, U> TryFrom<U> for T
where U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
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fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.