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.
Source§
Source§
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.