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//! Types related to binding messages to specific circuits
#[cfg(feature = "hs-service")]
use tor_hscrypto::ops::HsMacKey;
use zeroize::Zeroizing;
/// Number of bytes of circuit binding material negotiated per circuit hop.
pub(crate) const CIRC_BINDING_LEN: usize = 20;
/// Cryptographic information used to bind a message to a specific circuit.
///
/// This information is used in some of our protocols (currently only the onion
/// services protocol) to prove that a given message was referring to a specific
/// hop on a specific circuit, and was not replayed from another circuit.
///
/// In `tor-spec` and `rend-spec`, this value is called `KH`.
#[derive(Clone)]
pub struct CircuitBinding(
// We use a Box here to avoid moves that would bypass the zeroize-on-drop
// semantics.
//
// (This is not super-critical, since the impact of leaking one of these
// keys is slight, but it's best not to leak them at all.)
Box<Zeroizing<[u8; CIRC_BINDING_LEN]>>,
);
impl From<[u8; CIRC_BINDING_LEN]> for CircuitBinding {
fn from(value: [u8; CIRC_BINDING_LEN]) -> Self {
Self(Box::new(Zeroizing::new(value)))
}
}
impl CircuitBinding {
/// Return a view of this key suitable for computing the MAC function used
/// to authenticate onion services' ESTABLISH_INTRODUCE messages.
///
/// Note that this is not a general-purpose MAC; please avoid adding new
/// users of it. See notes on [`hs_mac`](tor_hscrypto::ops::hs_mac) for
/// more information.
#[cfg(feature = "hs-service")]
pub fn hs_mac(&self) -> HsMacKey<'_> {
HsMacKey::from(self.dangerously_into_bytes())
}
/// Return a view of this key as a byte-slice.
///
/// This is potentially dangerous, since we don't want to expose this
/// information: We only want to use it as a MAC key.
#[cfg(feature = "hs-service")]
fn dangerously_into_bytes(&self) -> &[u8] {
&(**self.0)[..]
}
}
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