feat(federation): implement v2 inbound federation handlers

Replace stub federation handlers with full implementations that accept
relay and proxy requests from peer servers. Adds federation_client and
local_domain fields to ServerState for outbound relay and federated
address resolution. All six handlers (relay_enqueue, relay_batch_enqueue,
proxy_fetch_key_package, proxy_fetch_hybrid_key, proxy_resolve_user,
federation_health) now validate federation auth, interact with local
storage, and wake waiters on message delivery.
This commit is contained in:
2026-03-04 21:06:31 +01:00
parent 12717979ba
commit 1d59a052ad
4 changed files with 906 additions and 51 deletions

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@@ -0,0 +1,689 @@
//! Hybrid Noise_XX + ML-KEM-768 handshake for post-quantum transport security.
//!
//! Implements a three-message Noise_XX pattern with an embedded ML-KEM-768
//! encapsulation to produce a hybrid shared secret that is secure against
//! both classical and quantum adversaries.
//!
//! # Handshake pattern
//!
//! ```text
//! XX(s, rs):
//! -> e (initiator ephemeral)
//! <- e, ee, s, es, mlkem_ct (responder ephemeral + static + ML-KEM ciphertext)
//! -> s, se (initiator static)
//! ```
//!
//! After message 2, the ML-KEM shared secret is mixed into the chaining key
//! via HKDF. The final transport keys incorporate both the X25519 DH chain
//! and the ML-KEM shared secret.
//!
//! # Wire format
//!
//! Each handshake message is a simple length-prefixed blob:
//! ```text
//! [msg_len: u32 BE][handshake message bytes]
//! ```
//!
//! # Feature gate
//!
//! This module is always compiled but the `pq-noise` feature enables it
//! in the RPC layer for server/client negotiation.
use chacha20poly1305::{
aead::{Aead, KeyInit, Payload},
ChaCha20Poly1305, Key, Nonce,
};
use hkdf::Hkdf;
use ml_kem::{
array::Array,
kem::{Decapsulate, Encapsulate},
EncodedSizeUser, KemCore, MlKem768, MlKem768Params,
};
use ml_kem::kem::{DecapsulationKey, EncapsulationKey};
use rand::rngs::OsRng;
use sha2::Sha256;
use x25519_dalek::{PublicKey as X25519Public, StaticSecret};
use zeroize::Zeroizing;
use crate::error::CoreError;
/// Domain separation label for the hybrid Noise handshake.
const PROTOCOL_NAME: &[u8] = b"quicproquo-pq-noise-v1";
/// ML-KEM-768 encapsulation key length.
const MLKEM_EK_LEN: usize = 1184;
/// ML-KEM-768 ciphertext length.
const MLKEM_CT_LEN: usize = 1088;
/// AEAD tag length (ChaCha20-Poly1305).
const TAG_LEN: usize = 16;
// ── Keypair ──────────────────────────────────────────────────────────────────
/// A static keypair for the hybrid Noise handshake.
///
/// Contains both an X25519 static key and an ML-KEM-768 key pair.
pub struct NoiseKeypair {
x25519_sk: StaticSecret,
x25519_pk: X25519Public,
mlkem_dk: DecapsulationKey<MlKem768Params>,
mlkem_ek: EncapsulationKey<MlKem768Params>,
}
impl NoiseKeypair {
/// Generate a fresh keypair from OS CSPRNG.
pub fn generate() -> Self {
let x25519_sk = StaticSecret::random_from_rng(OsRng);
let x25519_pk = X25519Public::from(&x25519_sk);
let (mlkem_dk, mlkem_ek) = MlKem768::generate(&mut OsRng);
Self {
x25519_sk,
x25519_pk,
mlkem_dk,
mlkem_ek,
}
}
/// Return the X25519 public key bytes.
pub fn x25519_public(&self) -> [u8; 32] {
self.x25519_pk.to_bytes()
}
/// Return the ML-KEM-768 encapsulation key bytes.
pub fn mlkem_public(&self) -> Vec<u8> {
self.mlkem_ek.as_bytes().to_vec()
}
}
// ── Chaining key state ───────────────────────────────────────────────────────
/// Internal handshake state tracking the Noise chaining key and handshake hash.
struct HandshakeState {
/// Chaining key — evolved by each MixKey operation.
ck: Zeroizing<[u8; 32]>,
/// Handshake hash — commits to all handshake transcript data.
h: [u8; 32],
/// Current encryption key (derived from ck after MixKey).
k: Option<Zeroizing<[u8; 32]>>,
/// Nonce counter for in-handshake encryption.
n: u64,
}
impl HandshakeState {
fn new() -> Self {
// Initialize h = SHA-256(protocol_name), ck = h.
use sha2::{Digest, Sha256};
let h: [u8; 32] = Sha256::digest(PROTOCOL_NAME).into();
Self {
ck: Zeroizing::new(h),
h,
k: None,
n: 0,
}
}
/// MixHash: h = SHA-256(h || data)
fn mix_hash(&mut self, data: &[u8]) {
use sha2::{Digest, Sha256};
let mut hasher = Sha256::new();
hasher.update(self.h);
hasher.update(data);
self.h = hasher.finalize().into();
}
/// MixKey: (ck, k) = HKDF(ck, input_key_material)
fn mix_key(&mut self, ikm: &[u8]) {
let hk = Hkdf::<Sha256>::new(Some(&*self.ck), ikm);
let mut ck = Zeroizing::new([0u8; 32]);
let mut k = Zeroizing::new([0u8; 32]);
hk.expand(b"ck", &mut *ck)
.expect("32 bytes is valid HKDF output");
hk.expand(b"k", &mut *k)
.expect("32 bytes is valid HKDF output");
self.ck = ck;
self.k = Some(k);
self.n = 0;
}
/// Encrypt plaintext with the current key and nonce, using h as AAD.
fn encrypt_and_hash(&mut self, plaintext: &[u8]) -> Result<Vec<u8>, CoreError> {
let key = self
.k
.as_ref()
.ok_or_else(|| CoreError::Mls("pq_noise: no encryption key set".into()))?;
let cipher = ChaCha20Poly1305::new(Key::from_slice(&**key));
let nonce = nonce_from_counter(self.n);
let ct = cipher
.encrypt(
Nonce::from_slice(&nonce),
Payload {
msg: plaintext,
aad: &self.h,
},
)
.map_err(|_| CoreError::Mls("pq_noise: encrypt failed".into()))?;
self.mix_hash(&ct);
self.n += 1;
Ok(ct)
}
/// Decrypt ciphertext with the current key and nonce, using h as AAD.
fn decrypt_and_hash(&mut self, ciphertext: &[u8]) -> Result<Vec<u8>, CoreError> {
let key = self
.k
.as_ref()
.ok_or_else(|| CoreError::Mls("pq_noise: no decryption key set".into()))?;
let cipher = ChaCha20Poly1305::new(Key::from_slice(&**key));
let nonce = nonce_from_counter(self.n);
let ct_for_hash = ciphertext.to_vec();
let pt = cipher
.decrypt(
Nonce::from_slice(&nonce),
Payload {
msg: ciphertext,
aad: &self.h,
},
)
.map_err(|_| CoreError::Mls("pq_noise: decrypt failed".into()))?;
self.mix_hash(&ct_for_hash);
self.n += 1;
Ok(pt)
}
/// Split the handshake state into two transport keys (initiator->responder, responder->initiator).
fn split(&self) -> (TransportKey, TransportKey) {
let hk = Hkdf::<Sha256>::new(Some(&*self.ck), &[]);
let mut k1 = Zeroizing::new([0u8; 32]);
let mut k2 = Zeroizing::new([0u8; 32]);
hk.expand(b"initiator", &mut *k1)
.expect("32 bytes is valid HKDF output");
hk.expand(b"responder", &mut *k2)
.expect("32 bytes is valid HKDF output");
(
TransportKey { key: k1, nonce: 0 },
TransportKey { key: k2, nonce: 0 },
)
}
}
fn nonce_from_counter(n: u64) -> [u8; 12] {
let mut nonce = [0u8; 12];
nonce[4..].copy_from_slice(&n.to_le_bytes());
nonce
}
// ── Transport ────────────────────────────────────────────────────────────────
/// A transport encryption key with a nonce counter.
pub struct TransportKey {
key: Zeroizing<[u8; 32]>,
nonce: u64,
}
impl TransportKey {
/// Encrypt a message for transport.
pub fn encrypt(&mut self, plaintext: &[u8]) -> Result<Vec<u8>, CoreError> {
let cipher = ChaCha20Poly1305::new(Key::from_slice(&*self.key));
let nonce = nonce_from_counter(self.nonce);
let ct = cipher
.encrypt(Nonce::from_slice(&nonce), plaintext)
.map_err(|_| CoreError::Mls("pq_noise transport: encrypt failed".into()))?;
self.nonce += 1;
Ok(ct)
}
/// Decrypt a transport message.
pub fn decrypt(&mut self, ciphertext: &[u8]) -> Result<Vec<u8>, CoreError> {
let cipher = ChaCha20Poly1305::new(Key::from_slice(&*self.key));
let nonce = nonce_from_counter(self.nonce);
let pt = cipher
.decrypt(Nonce::from_slice(&nonce), ciphertext)
.map_err(|_| CoreError::Mls("pq_noise transport: decrypt failed".into()))?;
self.nonce += 1;
Ok(pt)
}
}
// ── Initiator ────────────────────────────────────────────────────────────────
/// Initiator side of the hybrid Noise_XX handshake.
pub struct Initiator {
state: HandshakeState,
/// Ephemeral secret stored as StaticSecret so DH doesn't consume it.
/// Generated from OsRng; we use StaticSecret purely for the non-consuming
/// `diffie_hellman(&self, ...)` API — the key is still ephemeral.
e_sk: StaticSecret,
e_pk: X25519Public,
s: NoiseKeypair,
/// Stored after reading message 2 so we can compute se = DH(s, re) in msg3.
re_pk: Option<X25519Public>,
}
impl Initiator {
/// Create a new initiator with the given static keypair.
pub fn new(static_keypair: NoiseKeypair) -> Self {
let e_sk = StaticSecret::random_from_rng(OsRng);
let e_pk = X25519Public::from(&e_sk);
Self {
state: HandshakeState::new(),
e_sk,
e_pk,
s: static_keypair,
re_pk: None,
}
}
/// Write message 1: `-> e`
///
/// Returns the initiator's ephemeral X25519 public key (32 bytes).
pub fn write_message_1(&mut self) -> Vec<u8> {
let e_pk_bytes = self.e_pk.to_bytes();
self.state.mix_hash(&e_pk_bytes);
e_pk_bytes.to_vec()
}
/// Read message 2 from responder: `<- e, ee, s, es, mlkem_ct`
///
/// Expects: `re_pk(32) || encrypted_rs_pk(32+TAG) || mlkem_ct(1088)`
///
/// Returns the responder's static X25519 public key.
pub fn read_message_2(&mut self, msg: &[u8]) -> Result<[u8; 32], CoreError> {
let expected_len = 32 + 32 + TAG_LEN + MLKEM_CT_LEN;
if msg.len() != expected_len {
return Err(CoreError::Mls(format!(
"pq_noise msg2: expected {expected_len} bytes, got {}",
msg.len()
)));
}
let mut cursor = 0;
// re = responder ephemeral public key
let mut re_pk_bytes = [0u8; 32];
re_pk_bytes.copy_from_slice(&msg[cursor..cursor + 32]);
cursor += 32;
let re_pk = X25519Public::from(re_pk_bytes);
self.state.mix_hash(&re_pk_bytes);
self.re_pk = Some(re_pk);
// ee = DH(e, re)
let ee_ss = self.e_sk.diffie_hellman(&re_pk);
self.state.mix_key(ee_ss.as_bytes());
// Decrypt responder's static key: s = Dec(encrypted_rs_pk)
let encrypted_rs = &msg[cursor..cursor + 32 + TAG_LEN];
cursor += 32 + TAG_LEN;
let rs_pk_bytes = self.state.decrypt_and_hash(encrypted_rs)?;
let mut rs_pk_arr = [0u8; 32];
if rs_pk_bytes.len() != 32 {
return Err(CoreError::Mls("pq_noise: decrypted rs not 32 bytes".into()));
}
rs_pk_arr.copy_from_slice(&rs_pk_bytes);
let rs_pk = X25519Public::from(rs_pk_arr);
// es = DH(e, rs)
let es_ss = self.e_sk.diffie_hellman(&rs_pk);
self.state.mix_key(es_ss.as_bytes());
// ML-KEM: decapsulate the ciphertext from the responder
let mlkem_ct = &msg[cursor..cursor + MLKEM_CT_LEN];
let mlkem_ct_arr = Array::try_from(mlkem_ct)
.map_err(|_| CoreError::Mls("pq_noise: invalid ML-KEM ciphertext".into()))?;
let mlkem_ss: ml_kem::SharedKey<MlKem768> = self
.s
.mlkem_dk
.decapsulate(&mlkem_ct_arr)
.map_err(|_| CoreError::Mls("pq_noise: ML-KEM decapsulation failed".into()))?;
self.state.mix_key(&mlkem_ss);
Ok(rs_pk_arr)
}
/// Write message 3: `-> s, se`
///
/// Returns the encrypted initiator static key.
pub fn write_message_3(&mut self) -> Result<Vec<u8>, CoreError> {
let re_pk = self
.re_pk
.ok_or_else(|| CoreError::Mls("pq_noise: must read msg2 before writing msg3".into()))?;
// Encrypt our static key
let s_pk_bytes = self.s.x25519_pk.to_bytes();
let encrypted_s = self.state.encrypt_and_hash(&s_pk_bytes)?;
// se = DH(s, re)
let se_ss = self.s.x25519_sk.diffie_hellman(&re_pk);
self.state.mix_key(se_ss.as_bytes());
Ok(encrypted_s)
}
/// Finalize the handshake and return transport keys.
///
/// Returns `(send_key, recv_key)` — initiator sends with send_key.
pub fn finalize(self) -> (TransportKey, TransportKey) {
self.state.split()
}
}
// ── Responder ────────────────────────────────────────────────────────────────
/// Responder side of the hybrid Noise_XX handshake.
pub struct Responder {
state: HandshakeState,
/// Ephemeral secret stored as StaticSecret so DH doesn't consume it.
e_sk: StaticSecret,
e_pk: X25519Public,
s: NoiseKeypair,
}
impl Responder {
/// Create a new responder with the given static keypair.
pub fn new(static_keypair: NoiseKeypair) -> Self {
let e_sk = StaticSecret::random_from_rng(OsRng);
let e_pk = X25519Public::from(&e_sk);
Self {
state: HandshakeState::new(),
e_sk,
e_pk,
s: static_keypair,
}
}
/// Read message 1 from initiator: `-> e`
///
/// Expects the initiator's ephemeral X25519 public key (32 bytes).
pub fn read_message_1(&mut self, msg: &[u8]) -> Result<(), CoreError> {
if msg.len() != 32 {
return Err(CoreError::Mls(format!(
"pq_noise msg1: expected 32 bytes, got {}",
msg.len()
)));
}
self.state.mix_hash(msg);
Ok(())
}
/// Write message 2: `<- e, ee, s, es, mlkem_ct`
///
/// `initiator_ek` is the initiator's ML-KEM encapsulation key.
///
/// Returns the message bytes.
pub fn write_message_2(
&mut self,
initiator_e_pk: &[u8; 32],
initiator_mlkem_ek: &[u8],
) -> Result<Vec<u8>, CoreError> {
let ie_pk = X25519Public::from(*initiator_e_pk);
// Our ephemeral key
let e_pk_bytes = self.e_pk.to_bytes();
self.state.mix_hash(&e_pk_bytes);
// ee = DH(e, ie)
let ee_ss = self.e_sk.diffie_hellman(&ie_pk);
self.state.mix_key(ee_ss.as_bytes());
// Encrypt our static key
let s_pk_bytes = self.s.x25519_pk.to_bytes();
let encrypted_s = self.state.encrypt_and_hash(&s_pk_bytes)?;
// es = DH(s, ie)
let es_ss = self.s.x25519_sk.diffie_hellman(&ie_pk);
self.state.mix_key(es_ss.as_bytes());
// ML-KEM: encapsulate to the initiator's encapsulation key
if initiator_mlkem_ek.len() != MLKEM_EK_LEN {
return Err(CoreError::Mls(format!(
"pq_noise: expected ML-KEM EK {} bytes, got {}",
MLKEM_EK_LEN,
initiator_mlkem_ek.len()
)));
}
let ek_arr = Array::try_from(initiator_mlkem_ek)
.map_err(|_| CoreError::Mls("pq_noise: invalid ML-KEM encapsulation key".into()))?;
let ek = EncapsulationKey::<MlKem768Params>::from_bytes(&ek_arr);
let (mlkem_ct, mlkem_ss): (ml_kem::Ciphertext<MlKem768>, ml_kem::SharedKey<MlKem768>) = ek
.encapsulate(&mut OsRng)
.map_err(|_| CoreError::Mls("pq_noise: ML-KEM encapsulation failed".into()))?;
self.state.mix_key(&mlkem_ss);
// Assemble: e_pk || encrypted_s || mlkem_ct
let mut out = Vec::with_capacity(32 + encrypted_s.len() + MLKEM_CT_LEN);
out.extend_from_slice(&e_pk_bytes);
out.extend_from_slice(&encrypted_s);
out.extend_from_slice(&mlkem_ct);
Ok(out)
}
/// Read message 3 from initiator: `-> s, se`
///
/// Returns the initiator's static X25519 public key.
pub fn read_message_3(&mut self, msg: &[u8]) -> Result<[u8; 32], CoreError> {
if msg.len() != 32 + TAG_LEN {
return Err(CoreError::Mls(format!(
"pq_noise msg3: expected {} bytes, got {}",
32 + TAG_LEN,
msg.len()
)));
}
// Decrypt initiator's static key
let is_pk_bytes = self.state.decrypt_and_hash(msg)?;
let mut is_pk_arr = [0u8; 32];
if is_pk_bytes.len() != 32 {
return Err(CoreError::Mls(
"pq_noise: decrypted initiator static not 32 bytes".into(),
));
}
is_pk_arr.copy_from_slice(&is_pk_bytes);
let is_pk = X25519Public::from(is_pk_arr);
// se = DH(e, is) — responder computes using ephemeral key
let se_ss = self.e_sk.diffie_hellman(&is_pk);
self.state.mix_key(se_ss.as_bytes());
Ok(is_pk_arr)
}
/// Finalize the handshake and return transport keys.
///
/// Returns `(recv_key, send_key)` — responder receives with recv_key.
pub fn finalize(self) -> (TransportKey, TransportKey) {
let (i2r, r2i) = self.state.split();
(i2r, r2i)
}
}
// ── Tests ────────────────────────────────────────────────────────────────────
#[cfg(test)]
#[allow(clippy::unwrap_used)]
mod tests {
use super::*;
#[test]
fn full_handshake_round_trip() {
let initiator_kp = NoiseKeypair::generate();
let responder_kp = NoiseKeypair::generate();
// Initiator's ML-KEM public key is sent out-of-band (or in a pre-message).
let initiator_mlkem_ek = initiator_kp.mlkem_public();
let mut initiator = Initiator::new(initiator_kp);
let mut responder = Responder::new(responder_kp);
// Message 1: initiator -> responder
let msg1 = initiator.write_message_1();
assert_eq!(msg1.len(), 32);
responder.read_message_1(&msg1).unwrap();
// Message 2: responder -> initiator
let ie_pk: [u8; 32] = msg1.as_slice().try_into().unwrap();
let msg2 = responder
.write_message_2(&ie_pk, &initiator_mlkem_ek)
.unwrap();
let _responder_static = initiator.read_message_2(&msg2).unwrap();
// Message 3: initiator -> responder
let msg3 = initiator.write_message_3().unwrap();
let _initiator_static = responder.read_message_3(&msg3).unwrap();
// Derive transport keys
let (mut i_send, mut i_recv) = initiator.finalize();
let (mut r_recv, mut r_send) = responder.finalize();
// Test transport: initiator -> responder
let plaintext = b"hello post-quantum world!";
let ct = i_send.encrypt(plaintext).unwrap();
let pt = r_recv.decrypt(&ct).unwrap();
assert_eq!(pt, plaintext);
// Test transport: responder -> initiator
let plaintext2 = b"reply from responder";
let ct2 = r_send.encrypt(plaintext2).unwrap();
let pt2 = i_recv.decrypt(&ct2).unwrap();
assert_eq!(pt2, plaintext2);
}
#[test]
fn tampered_msg2_fails() {
let initiator_kp = NoiseKeypair::generate();
let responder_kp = NoiseKeypair::generate();
let initiator_mlkem_ek = initiator_kp.mlkem_public();
let mut initiator = Initiator::new(initiator_kp);
let mut responder = Responder::new(responder_kp);
let msg1 = initiator.write_message_1();
responder.read_message_1(&msg1).unwrap();
let ie_pk: [u8; 32] = msg1.as_slice().try_into().unwrap();
let mut msg2 = responder
.write_message_2(&ie_pk, &initiator_mlkem_ek)
.unwrap();
// Tamper with the encrypted static key region
msg2[40] ^= 0xFF;
let result = initiator.read_message_2(&msg2);
assert!(result.is_err());
}
#[test]
fn wrong_mlkem_key_fails() {
let initiator_kp = NoiseKeypair::generate();
let responder_kp = NoiseKeypair::generate();
// Use a different keypair's ML-KEM key — decapsulation will use
// implicit rejection, producing a pseudorandom (wrong) shared secret.
let wrong_kp = NoiseKeypair::generate();
let wrong_mlkem_ek = wrong_kp.mlkem_public();
let mut initiator = Initiator::new(initiator_kp);
let mut responder = Responder::new(responder_kp);
let msg1 = initiator.write_message_1();
responder.read_message_1(&msg1).unwrap();
let ie_pk: [u8; 32] = msg1.as_slice().try_into().unwrap();
let msg2 = responder
.write_message_2(&ie_pk, &wrong_mlkem_ek)
.unwrap();
// ML-KEM implicit rejection: decap succeeds but returns wrong ss.
// The ML-KEM mix_key happens after the AEAD decrypt of the static key,
// so read_message_2 itself may succeed. But the chaining keys diverge,
// causing msg3 AEAD decrypt to fail on the responder side.
let read2 = initiator.read_message_2(&msg2);
if read2.is_err() {
// If msg2 processing itself failed, the test passes.
return;
}
// msg2 succeeded — chaining keys now diverge due to wrong ML-KEM ss.
// msg3 from initiator will use the wrong key, so responder can't decrypt.
let msg3 = initiator.write_message_3().unwrap();
let result = responder.read_message_3(&msg3);
assert!(result.is_err(), "msg3 should fail due to ML-KEM shared secret mismatch");
}
#[test]
fn multiple_transport_messages() {
let initiator_kp = NoiseKeypair::generate();
let responder_kp = NoiseKeypair::generate();
let initiator_mlkem_ek = initiator_kp.mlkem_public();
let mut initiator = Initiator::new(initiator_kp);
let mut responder = Responder::new(responder_kp);
let msg1 = initiator.write_message_1();
responder.read_message_1(&msg1).unwrap();
let ie_pk: [u8; 32] = msg1.as_slice().try_into().unwrap();
let msg2 = responder
.write_message_2(&ie_pk, &initiator_mlkem_ek)
.unwrap();
initiator.read_message_2(&msg2).unwrap();
let msg3 = initiator.write_message_3().unwrap();
responder.read_message_3(&msg3).unwrap();
let (mut i_send, mut i_recv) = initiator.finalize();
let (mut r_recv, mut r_send) = responder.finalize();
// Send multiple messages in each direction
for i in 0..10u32 {
let msg = format!("initiator message {i}");
let ct = i_send.encrypt(msg.as_bytes()).unwrap();
let pt = r_recv.decrypt(&ct).unwrap();
assert_eq!(pt, msg.as_bytes());
let reply = format!("responder reply {i}");
let ct2 = r_send.encrypt(reply.as_bytes()).unwrap();
let pt2 = i_recv.decrypt(&ct2).unwrap();
assert_eq!(pt2, reply.as_bytes());
}
}
#[test]
fn nonce_reuse_detected() {
let initiator_kp = NoiseKeypair::generate();
let responder_kp = NoiseKeypair::generate();
let initiator_mlkem_ek = initiator_kp.mlkem_public();
let mut initiator = Initiator::new(initiator_kp);
let mut responder = Responder::new(responder_kp);
let msg1 = initiator.write_message_1();
responder.read_message_1(&msg1).unwrap();
let ie_pk: [u8; 32] = msg1.as_slice().try_into().unwrap();
let msg2 = responder
.write_message_2(&ie_pk, &initiator_mlkem_ek)
.unwrap();
initiator.read_message_2(&msg2).unwrap();
let msg3 = initiator.write_message_3().unwrap();
responder.read_message_3(&msg3).unwrap();
let (mut i_send, _) = initiator.finalize();
let (mut r_recv, _) = responder.finalize();
// Encrypt two messages
let ct1 = i_send.encrypt(b"msg1").unwrap();
let _ct2 = i_send.encrypt(b"msg2").unwrap();
// Decrypt in order works
r_recv.decrypt(&ct1).unwrap();
// Replaying ct1 (wrong nonce) should fail
let result = r_recv.decrypt(&ct1);
assert!(result.is_err());
// But ct2 at the right nonce works
// (we already consumed nonce 1 trying ct1, so ct2 at nonce 2 fails too)
// This tests that the nonce counter prevents replay.
}
}