tor-proto: Code to construct crypto layers for virtual hops.

This is fairly straightforward, thanks to our existing design work
on this code.

crates/tor-proto/src/circuit.rs

@@ -410,6 +410,8 @@ impl ClientCirc {
         role: handshake::HandshakeRole,
         seed: impl handshake::KeyGenerator,
     ) -> Result<()> {
+        let (outbound, inbound) = protocol.construct_layers(role, seed)?;
+
         todo!() // TODO hs implement
     }
 

crates/tor-proto/src/circuit/handshake.rs

@@ -11,6 +11,11 @@
 // that can wait IMO until we have a second circuit creation mechanism for use
 // with ntor.
 
+use crate::crypto::cell::{
+    ClientLayer, CryptInit, InboundClientLayer, OutboundClientLayer, Tor1Hsv3RelayCrypto,
+};
+use crate::Result;
+
 pub use crate::crypto::handshake::hs_ntor;
 pub use crate::crypto::handshake::KeyGenerator;
 
@@ -27,7 +32,7 @@ pub enum RelayProtocol {
 }
 
 /// What role we are playing in a handshake.
-#[derive(Copy, Clone, Debug)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
 #[non_exhaustive]
 pub enum HandshakeRole {
     /// We are the party initiating the handshake.
@@ -35,3 +40,30 @@ pub enum HandshakeRole {
     /// We are the party responding to the handshake.
     Responder,
 }
+
+impl RelayProtocol {
+    /// Construct the cell-crypto layers that are needed for a given set of
+    /// circuit hop parameters.
+    pub(crate) fn construct_layers(
+        self,
+        role: HandshakeRole,
+        keygen: impl KeyGenerator,
+    ) -> Result<(
+        Box<dyn OutboundClientLayer + Send>,
+        Box<dyn InboundClientLayer + Send>,
+    )> {
+        match self {
+            RelayProtocol::HsV3 => {
+                let seed_needed = Tor1Hsv3RelayCrypto::seed_len();
+                let seed = keygen.expand(seed_needed)?;
+                let layer = Tor1Hsv3RelayCrypto::initialize(&seed)?;
+                let (fwd, back) = layer.split();
+                let (fwd, back) = match role {
+                    HandshakeRole::Initiator => (fwd, back),
+                    HandshakeRole::Responder => (back, fwd),
+                };
+                Ok((Box::new(fwd), Box::new(back)))
+            }
+        }
+    }
+}

crates/tor-proto/src/crypto/cell.rs

@@ -211,6 +211,13 @@ impl InboundClientCrypt {
 pub(crate) type Tor1RelayCrypto =
     tor1::CryptStatePair<tor_llcrypto::cipher::aes::Aes128Ctr, tor_llcrypto::d::Sha1>;
 
+/// Standard Tor relay crypto, as instantiated for the HSv3 protocol.
+///
+/// (The use of SHA3 is ridiculously overkill.)
+#[cfg(feature = "hs-common")]
+pub(crate) type Tor1Hsv3RelayCrypto =
+    tor1::CryptStatePair<tor_llcrypto::cipher::aes::Aes256Ctr, tor_llcrypto::d::Sha3_256>;
+
 /// Incomplete untested implementation of Tor's current cell crypto.
 pub(crate) mod tor1 {
     use super::*;