Add an implementation of mutual message exchange

[rust-lightning] / mutual-message-exchange / src / lib.rs

//#![cfg_attr(not(test), no_std)]
#![cfg_attr(not(test), deny(missing_docs))]
#![forbid(unsafe_code)]

#![deny(rustdoc::broken_intra_doc_links)]
#![deny(rustdoc::private_intra_doc_links)]

//! A simple mutual-authentication protocol which allows two parties to maintain a set of public
//! keys which they're willing to exchange messages with and exchange a message with an extra
//! half-round-trip.
//!
//! The protocol contains one party wishing to send a message to another. The message recipient is
//! the `initiator` in the protocol, and speaks first. Most of the CPU cost is born by the message
//! sender.
//!
//! Both parties first create a [`TrustedSet`] listing the public keys which they are willing to
//! exchange messages with.
//!
//! In order to exchange a message, the message recipient calls [`get_init_bytes`] and sends the
//! resulting message bytes to the message sender. That message sender then uses
//! [`respond_with_message`] to determine if both sides are mutually in each others' [`TrustedSet`]
//! and encrypt the message if so. Finally, the initiator uses [`decode_msg`]
//!
//! If the message sender is in the initiator's trusted set and the message sender has the public
//! key for the initiator, the message sender will learn who the initiator is upon receipt of the
//! init message (without any response). The initiator will only learn who the message sender is
//! (and the message sender will only respond) if both sides are mutually-trusting.
//!
//! In any other case, neither party learns anything about the other, apart from a rough estimate
//! of the trusted set size of the initiator.

extern crate alloc;

use bitcoin_hashes::cmp::fixed_time_eq;

#[allow(dead_code)]
mod chacha20;
#[allow(dead_code)]
mod poly1305;
#[allow(dead_code)]
mod chacha20poly1305rfc;

use alloc::vec;
use alloc::vec::Vec;

use secp256k1::{SecretKey, PublicKey};
use secp256k1::ecdh::SharedSecret;

use bitcoin_hashes::{Hash, HashEngine};
use bitcoin_hashes::sha256::Hash as Sha256;

use chacha20::ChaCha20;
use chacha20poly1305rfc::ChaCha20Poly1305RFC;

/// The maximum number of trusted counterparties which is allowed to be in a single [`TrustedSet`].
pub const MAX_TRUSTED_KEYS: usize = 1024;

/// A `TrustedSet` stores the set of peers which we are willing to talk to.
pub struct TrustedSet {
        trusted_ecdhs: Vec<[u8; 32]>,
        state_key: [u8; 32],
}

impl TrustedSet {
        /// Constructs a new [`TrustedSet`] given a list of trusted counterparties. The keys are not
        /// stored, only ECDH results are.
        ///
        /// `trusted_counterparties` must not exceed [`MAX_TRUSTED_KEYS`] entries or the construction
        /// will fail. In all other cases construction succeeds.
        pub fn new(our_key: &SecretKey, trusted_counterparties: &[PublicKey]) -> Result<Self, ()> {
                if trusted_counterparties.len() > MAX_TRUSTED_KEYS { return Err(()) }

                let mut trusted_ecdhs = Vec::with_capacity(trusted_counterparties.len());
                for counterparty in trusted_counterparties.iter() {
                        let mut ecdh_hash = Sha256::engine();
                        ecdh_hash.input(b"Mutual Message Exchange ECDH Result");
                        ecdh_hash.input(&SharedSecret::new(counterparty, &our_key).secret_bytes());
                        trusted_ecdhs.push(Sha256::from_engine(ecdh_hash).to_byte_array());
                }
                let mut state_key_hash = Sha256::engine();
                state_key_hash.input(b"Mutual Private Auth State Key Generation");
                state_key_hash.input(&Sha256::hash(&our_key[..]).to_byte_array());
                Ok(Self {
                        trusted_ecdhs,
                        state_key: Sha256::from_engine(state_key_hash).to_byte_array(),
                })
        }

        fn get_cover_trusted_count(&self) -> usize {
                // In order to avoid giving away exactly how many keys we trust, we include some fake
                // entries in our message. To avoid too much overhead we only round the trusted set up a
                // bit.
                debug_assert!(self.trusted_ecdhs.len() <= MAX_TRUSTED_KEYS);
                match self.trusted_ecdhs.len() {
                        0..=16 => 16,
                        17..=32 => 32,
                        33..=128 => 128,
                        129..=512 => 512,
                        _ => MAX_TRUSTED_KEYS,
                }
        }
}

/// The per-trusted-peer length in the intitial bytes.
///
/// Fixed by the protocol.
const PER_PEER_LEN: usize = 32 + 16;
/// The length of the repeated data we sent in the init bytes and expect to be repeated in the
/// response. This is floating in the protocol, but we fix it for ourselves.
const OUR_REPEATED_DATA_LEN: usize = 64 + 32 + 8 + 16;

/// In order to avoid a message recipient having any guess as to the size our trusted set is, we
/// shuffle the entries in our init deterministically, using the permutation calculated here.
fn get_idx_permutation(cover_trusted_set_len: usize, rng_seed: &[u8]) -> [u16; MAX_TRUSTED_KEYS] {
        debug_assert!(cover_trusted_set_len <= MAX_TRUSTED_KEYS);
        debug_assert!(MAX_TRUSTED_KEYS <= u16::MAX.into());
        debug_assert_eq!(rng_seed.len(), 32);

        let mut perm = [0; MAX_TRUSTED_KEYS];
        for i in 0..MAX_TRUSTED_KEYS {
                perm[i] = i as u16;
        }
        let mut rng = ChaCha20::new(rng_seed, b"MPA PERM RNG");
        for i in 0..cover_trusted_set_len {
                let mut pos;
                let max_pos = (cover_trusted_set_len - i) as u64;
                loop {
                        let mut rand = [0; 8];
                        rng.process_in_place(&mut rand);
                        pos = u64::from_le_bytes(rand);
                        if pos < u64::MAX / max_pos * max_pos{
                                pos %= max_pos;
                                break;
                        }
                }
                perm.swap(i, pos as usize);
        }
        perm
}

/// Gets the initial bytes this initiator should send to a (potential) message sender.
///
/// It requires 64 secure random bytes, a reference to a [`TrustedSet`], and a `salt` and `aad`.
///
/// The `salt` should uniquely describe this protocol the protocol being built using this mutual
/// authentication handshake. The `aad` should describe the particular message type being sent
/// (which the sender expects).
pub fn get_init_bytes(secure_random_nonce: [u8; 64], trusted_set: &TrustedSet, salt: [u8; 8], aad: &[u8]) -> Vec<u8> {
        let mut local_nonce = [0; 32];
        local_nonce.copy_from_slice(&secure_random_nonce[..32]);

        let mut chacha_salt = [0; 12];
        chacha_salt[4..].copy_from_slice(&salt);

        let mut rng = ChaCha20::new(&secure_random_nonce[32..], b"MPA INIT RNG");

        // Init message format is
        // 2 byte handshake count
        // PER_PEER_LEN * handshake count:
        //   32-byte encrypted initiator nonce
        //   16-byte poly1305 tag
        // 2 byte repeated data len
        // repeated data len bytes of data to be repeated
        // any further bytes uninterpreted (for extensibility)
        //
        // Our repeated data is a 40 byte IV (XOR'd into `state_key` and "NONCE KY" to form the ChaCha
        // key and nonce to encrypt remaining bytes), followed by 64 bytes containing the
        // ChaCha-encrypted `secure_random_nonce` and the 16 byte Poly1305 MAC tag for the same.

        let vec_cnt = trusted_set.get_cover_trusted_count();
        let repeated_data_offs = 2 + vec_cnt * PER_PEER_LEN;
        let mut res = Vec::with_capacity(repeated_data_offs + 2 + OUR_REPEATED_DATA_LEN);
        res.resize(2 + vec_cnt * PER_PEER_LEN + 2 + OUR_REPEATED_DATA_LEN, 0);
        res[..2].copy_from_slice(&(vec_cnt as u16).to_be_bytes());

        // First fill in the encrypted slots for our trusted peers and fill remaining slots with noise.
        let idx_permutation = get_idx_permutation(vec_cnt, &secure_random_nonce[32..]);
        for (pos, idx) in idx_permutation.iter().take(vec_cnt).enumerate() {
                let idx_slice = &mut res[2 + pos * PER_PEER_LEN..8 + (pos + 1) * PER_PEER_LEN];
                if *idx as usize >= trusted_set.trusted_ecdhs.len() {
                        rng.process_in_place(idx_slice);
                } else {
                        let ecdh = &trusted_set.trusted_ecdhs[*idx as usize];
                        let mut cryptoor = ChaCha20Poly1305RFC::new(ecdh, &chacha_salt, aad);
                        let (crypted, tag) = idx_slice.split_at_mut(32);
                        cryptoor.encrypt(&local_nonce, crypted, tag);
                }
        }

        // Pick a random nonce to XOR into the ChaCha key and nonce. Note that we reuse the `state_key`
        // here repeatedly, so need the IV here to make the ChaCha pad unique.
        let mut repeated_data_key_mask = [0; 32 + 8];
        rng.process_in_place(&mut repeated_data_key_mask);
        let mut repeated_data_key = [0; 32];
        for i in 0..32 {
                repeated_data_key[i] = trusted_set.state_key[i] ^ repeated_data_key_mask[i];
        }
        let mut repeated_data_nonce = [0; 12];
        for i in 0..8 {
                repeated_data_nonce[4 + i] = b"NONCE KY"[i] ^ repeated_data_key_mask[32 + i];
        }

        res[repeated_data_offs..repeated_data_offs + 2]
                .copy_from_slice(&(OUR_REPEATED_DATA_LEN as u16).to_be_bytes());
        res[repeated_data_offs + 2..repeated_data_offs + 2 + 32 + 8]
                .copy_from_slice(&repeated_data_key_mask);

        let (crypted_nonce, tag) = res[repeated_data_offs + 2 + 32 + 8..].split_at_mut(64);
        let mut state_store = ChaCha20Poly1305RFC::new(&repeated_data_key, &repeated_data_nonce, &[]);
        state_store.encrypt(&secure_random_nonce, crypted_nonce, tag);
        res
}

/// Decode the message our counterparty sent us. The `salt` and `aad` provided must match the one
/// set in [`get_init_bytes`] and the one used by the message sender in [`respond_with_message`].
///
/// Returns both the message sent to us by the counterparty (if any) and a shared key which can be
/// used to en/decrypt future messages with the message-sender.
pub fn decode_msg(trusted_set: &TrustedSet, salt: [u8; 8], aad: &[u8], wire_msg: &[u8]) -> Result<(Vec<u8>, [u8; 32]), ()> {
        // Message format is:
        // 2 byte selected challenge index
        // 32 + 16 byte encrypted + MAC'd nonce
        // 2 byte repeated data len
        // repeated data len bytes of repeated data
        // 2 byte message length (not counting mac)
        // message length of encrypted message data
        // 16 byte poly1305 message MAC
        //
        // Our repeated data is a 40 byte IV (XOR'd into `state_key` and "NONCE KY" to form the ChaCha
        // key and nonce to encrypt remaining bytes), followed by 64 bytes containing the
        // ChaCha-encrypted `secure_random_nonce` and the 16 byte Poly1305 MAC tag for the same.
        const REPEATED_DATA_OFFS: usize = 2 + 32 + 16;
        const CONST_OVERHEAD: usize = 2 + 32 + 16 + 2 + 2 + 16;

        let mut chacha_salt = [0; 12];
        chacha_salt[4..].copy_from_slice(&salt);
        for b in chacha_salt.iter_mut().skip(4) {
                *b ^= 0xff;
        }
        if wire_msg.len() < CONST_OVERHEAD { return Err(()); }

        // Read our state storage (i.e. the "secure_random_nonce" parameter from `get_init_bytes`.
        let mut secure_random_nonce = [0; 64];
        let repeated_part_len;
        let msg_offs: usize;
        {
                let mut repeated_part_len_bytes = [0; 2];
                repeated_part_len_bytes.copy_from_slice(&wire_msg[REPEATED_DATA_OFFS..REPEATED_DATA_OFFS + 2]);
                repeated_part_len = u16::from_be_bytes(repeated_part_len_bytes);
                if repeated_part_len as usize != OUR_REPEATED_DATA_LEN { return Err(()); }
                if wire_msg.len() < CONST_OVERHEAD + OUR_REPEATED_DATA_LEN { return Err(()); }
                msg_offs = REPEATED_DATA_OFFS + 2 + OUR_REPEATED_DATA_LEN;

                let mut repeated_data_key = [0; 32];
                for i in 0..32 {
                        repeated_data_key[i] = trusted_set.state_key[i] ^ wire_msg[REPEATED_DATA_OFFS + 2 + i];
                }
                let mut repeated_data_nonce = [0; 12];
                for i in 0..8 {
                        repeated_data_nonce[4 + i] = b"NONCE KY"[i] ^ wire_msg[REPEATED_DATA_OFFS + 2 + 32 + i];
                }

                let mut state_store = ChaCha20Poly1305RFC::new(&repeated_data_key, &repeated_data_nonce, &[]);
                let ciphertext = &wire_msg[REPEATED_DATA_OFFS + 2 + 32 + 8..REPEATED_DATA_OFFS + 2 + OUR_REPEATED_DATA_LEN - 16];
                let mac = &wire_msg[REPEATED_DATA_OFFS + 2 + OUR_REPEATED_DATA_LEN - 16..REPEATED_DATA_OFFS + 2 + OUR_REPEATED_DATA_LEN];
                // The message sender (presumably) knows if they modified the repeated data, so there's no
                // need to be constant-time wrt failures here (and thus we also return early).
                state_store.variable_time_decrypt(ciphertext, &mut secure_random_nonce, mac)?;
        }

        let mut local_nonce = [0; 32];
        local_nonce.copy_from_slice(&secure_random_nonce[..32]);

        let mut remote_nonce = [0; 32];

        // Decrypt and validate the remote nonce
        {
                let mut peer_idx_bytes = [0; 2];
                peer_idx_bytes.copy_from_slice(&wire_msg[..2]);
                let peer_idx = u16::from_be_bytes(peer_idx_bytes);
                let idx_permutation = get_idx_permutation(trusted_set.get_cover_trusted_count(), &secure_random_nonce[32..]);

                // The message sender has already learned if they're in our trusted peer list and if we're
                // in theirs. Same goes for any third-party observers who would detect the same by the fact
                // that some response was made. Thus, there's no need to worry about timing differences
                // giving that away here - if the idx is bogus we can simply return and we can do a
                // variable time decryption (and early return if it fails).
                let ecdh_idx = idx_permutation.get(peer_idx as usize).ok_or(())?;
                let ecdh = trusted_set.trusted_ecdhs.get(*ecdh_idx as usize).ok_or(())?;

                let mut remote_nonce_key = Sha256::engine();
                remote_nonce_key.input(ecdh);
                remote_nonce_key.input(&local_nonce);

                let mut cryptoor = ChaCha20Poly1305RFC::new(&Sha256::from_engine(remote_nonce_key)[..], &chacha_salt, aad);
                cryptoor.variable_time_decrypt(&wire_msg[2..2+32], &mut remote_nonce, &wire_msg[2+32..2+32+16])?;
        }

        for b in chacha_salt.iter_mut().skip(4) {
                *b ^= 0x0f;
        }

        let mut msg_key = local_nonce;
        for (out, remote) in msg_key.iter_mut().zip(remote_nonce.iter()) {
                *out ^= *remote;
        }

        let separated_msg_keys = ChaCha20::get_single_block(&msg_key, b"INLINE KEY STRCH");
        let mut oob_msg_key = [0; 32];
        oob_msg_key.copy_from_slice(&separated_msg_keys[32..]);

        let mut msg_len_bytes = [0; 2];
        msg_len_bytes.copy_from_slice(&wire_msg[msg_offs..msg_offs + 2]);
        let msg_len = u16::from_be_bytes(msg_len_bytes);

        let mut res = Vec::new();
        if msg_len != 0 {
                if msg_len < 16 { return Err(()); }
                if wire_msg.len() < msg_offs + msg_len as usize { return Err(()); }
                res = vec![0; msg_len as usize - 16];
                let ciphertext = &wire_msg[msg_offs + 2..msg_offs + 2 + msg_len as usize - 16];
                let mac = &wire_msg[msg_offs + 2 + msg_len as usize - 16..msg_offs + 2 + msg_len as usize];

                let mut msg_cryptoor = ChaCha20Poly1305RFC::new(&separated_msg_keys[..32], &chacha_salt, aad);
                if msg_cryptoor.variable_time_decrypt(ciphertext, &mut res, mac).is_err() {
                        return Err(());
                }
        }

        Ok((res, oob_msg_key))
}

/// Processes the initial message sent by the initiator and generates an encrypted response,
/// containing the given `msg`. Also returns a negotiated shared key which can be used to encrypt
/// further messages to the initiator.
///
/// Requires a random 64 bytes, a [`TrustedSet`] of peers, the `peer_init` message sent to us by
/// the initiator (via [`get_init_bytes`]), and a `salt` and `aad` which match those used by the
/// initiator.
///
/// The `salt` should uniquely describe this protocol the protocol being built using this mutual
/// authentication handshake. The `aad` should describe the particular message type being sent
/// (which the recipient expects).
pub fn respond_with_message(secure_random_nonce: [u8; 64], trusted_set: &TrustedSet, peer_init: &[u8], salt: &[u8; 8], aad: &[u8], msg: &[u8])
-> Result<(Vec<u8>, [u8; 32]), ()> {
        // Init message format is
        // 2 byte handshake count
        // PER_PEER_LEN * handshake count:
        //   32-byte encrypted initiator nonce
        //   16-byte poly1305 tag
        // 2 byte repeated data len
        // repeated data len bytes of data to be repeated
        // any further bytes uninterpreted (for extensibility)

        if peer_init.len() < 4 { return Err(()); }

        let mut handshake_count_bytes = [0; 2];
        handshake_count_bytes.copy_from_slice(&peer_init[..2]);
        let handshake_count = u16::from_be_bytes(handshake_count_bytes);
        if peer_init.len() < 4 + handshake_count as usize * PER_PEER_LEN { return Err(()); }

        let mut repeated_data_len_bytes = [0; 2];
        repeated_data_len_bytes.copy_from_slice(&peer_init[2 + handshake_count as usize * PER_PEER_LEN..2 + handshake_count as usize * PER_PEER_LEN + 2]);
        let repeated_data_len = u16::from_be_bytes(repeated_data_len_bytes);
        if peer_init.len() < 4 + handshake_count as usize * PER_PEER_LEN + repeated_data_len as usize { return Err(()); }

        let mut chacha_salt = [0; 12];
        chacha_salt[4..].copy_from_slice(salt);

        let mut local_nonce = [0; 32];
        local_nonce.copy_from_slice(&secure_random_nonce[..32]);
        let mut rng = ChaCha20::new(&secure_random_nonce[32..], b"MPA Key Salt");

        let mut default_peer_bytes = [0; 8];
        rng.process_in_place(&mut default_peer_bytes);
        let mut peer_match_idx =
                (u64::from_le_bytes(default_peer_bytes) as usize) %
                core::cmp::max(trusted_set.trusted_ecdhs.len(), 1);
        let mut remote_nonce = [0; 32];
        rng.process_in_place(&mut remote_nonce);
        let mut peer_ecdh = &remote_nonce;

        'match_search: for (idx, peer_enc) in peer_init[2..2 + handshake_count as usize * PER_PEER_LEN].chunks(PER_PEER_LEN).enumerate() {
                for ecdh in trusted_set.trusted_ecdhs.iter() {
                        let mut cryptoor = ChaCha20Poly1305RFC::new(ecdh, &chacha_salt, aad);

                        let mut peer_nonce = [0; 32];
                        // Because the sender (should have) randomized the order of their trusted-peers list,
                        // the time taken to find a matching ECDH entry shouldn't give away who they were to a
                        // third-party observer. Thus, variable-time decryption (and an early return) should be
                        // fine.
                        if cryptoor.variable_time_decrypt(&peer_enc[..32], &mut peer_nonce, &peer_enc[32..]).is_ok() {
                                peer_ecdh = ecdh;
                                remote_nonce = peer_nonce;
                                peer_match_idx = idx;
                                break 'match_search;
                        }
                }
        }

        for b in chacha_salt.iter_mut().skip(4) {
                *b ^= 0xff;
        }

        // Message format is:
        // 2 byte selected challenge index
        // 32 + 16 byte encrypted + MAC'd nonce
        // 2 byte repeated data len
        // repeated data len bytes of repeated data
        // 2 byte message length (not counting mac)
        // message length of encrypted message data
        // 16 byte poly1305 message MAC

        let mut res = Vec::with_capacity(2 + 32 + 16 + 2 + repeated_data_len as usize + 2 + msg.len() + 16);
        res.resize(2 + 32 + 16 + 2 + repeated_data_len as usize + 2 + msg.len() + 16, 0);
        res[0..2].copy_from_slice(&(peer_match_idx as u16).to_be_bytes());
        let mut res_write_pos = 2;

        let mut noise = [0; 32];
        rng.process_in_place(&mut noise);

        let mut local_nonce_key = Sha256::engine();
        local_nonce_key.input(peer_ecdh);
        local_nonce_key.input(&remote_nonce);
        {
                let mut cryptoor = ChaCha20Poly1305RFC::new(&Sha256::from_engine(local_nonce_key)[..], &chacha_salt, aad);
                let (crypted, tag) = res.split_at_mut(res_write_pos + 32);
                cryptoor.encrypt(&local_nonce, &mut crypted[res_write_pos..], &mut tag[..16]);
                res_write_pos += 32 + 16;
        }

        res[res_write_pos..res_write_pos + 2].copy_from_slice(&repeated_data_len_bytes);
        res_write_pos += 2;
        res[res_write_pos..res_write_pos + repeated_data_len as usize]
                .copy_from_slice(&peer_init[2 + handshake_count as usize * PER_PEER_LEN + 2..2 + handshake_count as usize * PER_PEER_LEN + 2 + repeated_data_len as usize]);
        res_write_pos += repeated_data_len as usize;

        for b in chacha_salt.iter_mut().skip(4) {
                *b ^= 0x0f;
        }

        let mut msg_key = local_nonce;
        for (out, remote) in msg_key.iter_mut().zip(remote_nonce.iter()) {
                *out ^= *remote;
        }

        let separated_msg_keys = ChaCha20::get_single_block(&msg_key, b"INLINE KEY STRCH");
        let mut oob_msg_key = [0; 32];
        oob_msg_key.copy_from_slice(&separated_msg_keys[32..]);

        let proto_msg_len = if msg.is_empty() { 0 } else { msg.len() as u16 + 16 };
        res[res_write_pos..res_write_pos + 2].copy_from_slice(&proto_msg_len.to_be_bytes());
        res_write_pos += 2;

        let mut msg_cryptoor = ChaCha20Poly1305RFC::new(&separated_msg_keys[0..32], &chacha_salt, aad);
        let (crypted, tag) = res.split_at_mut(res_write_pos + msg.len());
        debug_assert_eq!(tag.len(), 16);
        msg_cryptoor.encrypt(msg, &mut crypted[res_write_pos..], tag);
        res_write_pos += msg.len() + 16;
        debug_assert_eq!(res_write_pos, res.len());

        Ok((res, oob_msg_key))
}

#[cfg(test)]
mod tests {
        use super::*;

        use secp256k1::{SecretKey, PublicKey, Secp256k1};

        use std::hash::{BuildHasher, Hasher};

        fn rand_bytes() -> [u8; 32] {
                let random_number = std::collections::hash_map::RandomState::new().build_hasher().finish();
                [random_number as u8; 32]
        }
        fn rand_64_bytes() -> [u8; 64] {
                let mut res = [0; 64];
                res[..32].copy_from_slice(&rand_bytes());
                res[32..].copy_from_slice(&rand_bytes());
                res
        }

        #[test]
        fn simple_test() {
                let secp_ctx = Secp256k1::new();

                let initiator_key = SecretKey::from_slice(&rand_bytes()).unwrap();
                let initiator_pk = PublicKey::from_secret_key(&secp_ctx, &initiator_key);
                let receiver_key = SecretKey::from_slice(&rand_bytes()).unwrap();
                let receiver_pk = PublicKey::from_secret_key(&secp_ctx, &receiver_key);

                let initiator_state = TrustedSet::new(&initiator_key, &[receiver_pk]).unwrap();
                let init_msg = get_init_bytes(rand_64_bytes(), &initiator_state, *b"SALTSALT", b"42");

                let receiver_state = TrustedSet::new(&receiver_key, &[initiator_pk]).unwrap();
                let sent_msg = b"Hello Initiator!";
                let (receiver_wire, receiver_shared_key) =
                        respond_with_message(rand_64_bytes(), &receiver_state, &init_msg, b"SALTSALT", b"42", sent_msg).unwrap();

                let (initiator_msg, initiator_shared_key) =
                        decode_msg(&initiator_state, *b"SALTSALT", b"42", &receiver_wire).unwrap();
                assert_eq!(&initiator_msg[..], sent_msg);
                assert_eq!(receiver_shared_key, initiator_shared_key);
        }
}