Merge bitcoin/bitcoin#28196: BIP324 connection support

db9888feec48c6220a2fcf92865503bbbdab02a4 net: detect wrong-network V1 talking to V2Transport (Pieter Wuille)
91e1ef8684997fb4b3e8b64ef3935a936445066b test: add unit tests for V2Transport (Pieter Wuille)
297c8889975a18258d6cc39b1ec1e94fed6630fb net: make V2Transport preallocate receive buffer space (Pieter Wuille)
3ffa5fb49ee4a6d9502aa957093bd94058630282 net: make V2Transport send uniformly random number garbage bytes (Pieter Wuille)
0be752d9f8ca27320bc3e82498c7640fabd7e8de net: add short message encoding/decoding support to V2Transport (Pieter Wuille)
8da8642062fa2c7aa2f49995b832c3d0897e37ed net: make V2Transport auto-detect incoming V1 and fall back to it (Pieter Wuille)
13a7f01557272db652b3f333af3f06af6897253f net: add V2Transport class with subset of BIP324 functionality (Pieter Wuille)
dc2d7eb810ef95b06620f334c198687579916435 crypto: Spanify EllSwiftPubKey constructor (Pieter Wuille)
5f4b2c6d79e81ee0445752ad558fcc17831f4b2f net: remove unused Transport::SetReceiveVersion (Pieter Wuille)
c3fad1f29df093e8fd03d70eb43f25ee9d531bf7 net: add have_next_message argument to Transport::GetBytesToSend() (Pieter Wuille)

Pull request description:

  This is part of #27634.

  This implements the BIP324 v2 transport (which implements all of what the BIP calls transport layer *and* application layer), though in a non-exposed way. It is tested through an extensive fuzz test, which verifies that v2 transports can talk to v2 transports, and v1 transports can talk to v2 transports, and a unit test that exercises a number of unusual scenarios. The transport is functionally complete, including:
  * Autodetection of incoming V1 connections.
  * Garbage, both sending and receiving.
  * Short message type IDs, both sending and receiving.
  * Ignore packets (receiving only, but tested in a unit test).
  * Session IDs are visible in `getpeerinfo` output (for manual comparison).

  Things that are not included, left for future PRs, are:
  * Actually using the v2 transport for connections.
  * Support for the `NODE_P2P_V2` service flag.
  * Retrying downgrade to V1 when attempted outbound V2 connections immediately fail.
  * P2P functional and unit tests

ACKs for top commit:
  naumenkogs:
    ACK db9888feec48c6220a2fcf92865503bbbdab02a4
  theStack:
    re-ACK db9888feec48c6220a2fcf92865503bbbdab02a4
  mzumsande:
    Code Review ACK db9888feec48c6220a2fcf92865503bbbdab02a4

Tree-SHA512: 8906ac1e733a99e1f31c9111055611f706d80bbfc2edf6a07fa6e47b21bb65baacd1ff17993cbbf588063b2f5ad30b3af674a50c7bc8e8ebf4671483a21bbfeb

9 files changed, 1558 insertions, 59 deletions

diff --git a/src/net.cpp b/src/net.cpp
index 4addca0982..3955005dfa 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -867,20 +867,22 @@ bool V1Transport::SetMessageToSend(CSerializedNetMsg& msg) noexcept
     return true;
 }
 
-Transport::BytesToSend V1Transport::GetBytesToSend() const noexcept
+Transport::BytesToSend V1Transport::GetBytesToSend(bool have_next_message) const noexcept
 {
     AssertLockNotHeld(m_send_mutex);
     LOCK(m_send_mutex);
     if (m_sending_header) {
         return {Span{m_header_to_send}.subspan(m_bytes_sent),
-                // We have more to send after the header if the message has payload.
-                !m_message_to_send.data.empty(),
+                // We have more to send after the header if the message has payload, or if there
+                // is a next message after that.
+                have_next_message || !m_message_to_send.data.empty(),
                 m_message_to_send.m_type
                };
     } else {
         return {Span{m_message_to_send.data}.subspan(m_bytes_sent),
-                // We never have more to send after this message's payload.
-                false,
+                // We only have more to send after this message's payload if there is another
+                // message.
+                have_next_message,
                 m_message_to_send.m_type
                };
     }
@@ -911,16 +913,676 @@ size_t V1Transport::GetSendMemoryUsage() const noexcept
     return m_message_to_send.GetMemoryUsage();
 }
 
+namespace {
+
+/** List of short messages as defined in BIP324, in order.
+ *
+ * Only message types that are actually implemented in this codebase need to be listed, as other
+ * messages get ignored anyway - whether we know how to decode them or not.
+ */
+const std::array<std::string, 33> V2_MESSAGE_IDS = {
+    "", // 12 bytes follow encoding the message type like in V1
+    NetMsgType::ADDR,
+    NetMsgType::BLOCK,
+    NetMsgType::BLOCKTXN,
+    NetMsgType::CMPCTBLOCK,
+    NetMsgType::FEEFILTER,
+    NetMsgType::FILTERADD,
+    NetMsgType::FILTERCLEAR,
+    NetMsgType::FILTERLOAD,
+    NetMsgType::GETBLOCKS,
+    NetMsgType::GETBLOCKTXN,
+    NetMsgType::GETDATA,
+    NetMsgType::GETHEADERS,
+    NetMsgType::HEADERS,
+    NetMsgType::INV,
+    NetMsgType::MEMPOOL,
+    NetMsgType::MERKLEBLOCK,
+    NetMsgType::NOTFOUND,
+    NetMsgType::PING,
+    NetMsgType::PONG,
+    NetMsgType::SENDCMPCT,
+    NetMsgType::TX,
+    NetMsgType::GETCFILTERS,
+    NetMsgType::CFILTER,
+    NetMsgType::GETCFHEADERS,
+    NetMsgType::CFHEADERS,
+    NetMsgType::GETCFCHECKPT,
+    NetMsgType::CFCHECKPT,
+    NetMsgType::ADDRV2,
+    // Unimplemented message types that are assigned in BIP324:
+    "",
+    "",
+    "",
+    ""
+};
+
+class V2MessageMap
+{
+    std::unordered_map<std::string, uint8_t> m_map;
+
+public:
+    V2MessageMap() noexcept
+    {
+        for (size_t i = 1; i < std::size(V2_MESSAGE_IDS); ++i) {
+            m_map.emplace(V2_MESSAGE_IDS[i], i);
+        }
+    }
+
+    std::optional<uint8_t> operator()(const std::string& message_name) const noexcept
+    {
+        auto it = m_map.find(message_name);
+        if (it == m_map.end()) return std::nullopt;
+        return it->second;
+    }
+};
+
+const V2MessageMap V2_MESSAGE_MAP;
+
+} // namespace
+
+V2Transport::V2Transport(NodeId nodeid, bool initiating, int type_in, int version_in) noexcept :
+    m_cipher{}, m_initiating{initiating}, m_nodeid{nodeid},
+    m_v1_fallback{nodeid, type_in, version_in}, m_recv_type{type_in}, m_recv_version{version_in},
+    m_recv_state{initiating ? RecvState::KEY : RecvState::KEY_MAYBE_V1},
+    m_send_state{initiating ? SendState::AWAITING_KEY : SendState::MAYBE_V1}
+{
+    // Construct garbage (including its length) using a FastRandomContext.
+    FastRandomContext rng;
+    size_t garbage_len = rng.randrange(MAX_GARBAGE_LEN + 1);
+    // Initialize the send buffer with ellswift pubkey + garbage.
+    m_send_buffer.resize(EllSwiftPubKey::size() + garbage_len);
+    std::copy(std::begin(m_cipher.GetOurPubKey()), std::end(m_cipher.GetOurPubKey()), MakeWritableByteSpan(m_send_buffer).begin());
+    rng.fillrand(MakeWritableByteSpan(m_send_buffer).subspan(EllSwiftPubKey::size()));
+}
+
+V2Transport::V2Transport(NodeId nodeid, bool initiating, int type_in, int version_in, const CKey& key, Span<const std::byte> ent32, Span<const uint8_t> garbage) noexcept :
+    m_cipher{key, ent32}, m_initiating{initiating}, m_nodeid{nodeid},
+    m_v1_fallback{nodeid, type_in, version_in}, m_recv_type{type_in}, m_recv_version{version_in},
+    m_recv_state{initiating ? RecvState::KEY : RecvState::KEY_MAYBE_V1},
+    m_send_state{initiating ? SendState::AWAITING_KEY : SendState::MAYBE_V1}
+{
+    assert(garbage.size() <= MAX_GARBAGE_LEN);
+    // Initialize the send buffer with ellswift pubkey + provided garbage.
+    m_send_buffer.resize(EllSwiftPubKey::size() + garbage.size());
+    std::copy(std::begin(m_cipher.GetOurPubKey()), std::end(m_cipher.GetOurPubKey()), MakeWritableByteSpan(m_send_buffer).begin());
+    std::copy(garbage.begin(), garbage.end(), m_send_buffer.begin() + EllSwiftPubKey::size());
+}
+
+void V2Transport::SetReceiveState(RecvState recv_state) noexcept
+{
+    AssertLockHeld(m_recv_mutex);
+    // Enforce allowed state transitions.
+    switch (m_recv_state) {
+    case RecvState::KEY_MAYBE_V1:
+        Assume(recv_state == RecvState::KEY || recv_state == RecvState::V1);
+        break;
+    case RecvState::KEY:
+        Assume(recv_state == RecvState::GARB_GARBTERM);
+        break;
+    case RecvState::GARB_GARBTERM:
+        Assume(recv_state == RecvState::GARBAUTH);
+        break;
+    case RecvState::GARBAUTH:
+        Assume(recv_state == RecvState::VERSION);
+        break;
+    case RecvState::VERSION:
+        Assume(recv_state == RecvState::APP);
+        break;
+    case RecvState::APP:
+        Assume(recv_state == RecvState::APP_READY);
+        break;
+    case RecvState::APP_READY:
+        Assume(recv_state == RecvState::APP);
+        break;
+    case RecvState::V1:
+        Assume(false); // V1 state cannot be left
+        break;
+    }
+    // Change state.
+    m_recv_state = recv_state;
+}
+
+void V2Transport::SetSendState(SendState send_state) noexcept
+{
+    AssertLockHeld(m_send_mutex);
+    // Enforce allowed state transitions.
+    switch (m_send_state) {
+    case SendState::MAYBE_V1:
+        Assume(send_state == SendState::V1 || send_state == SendState::AWAITING_KEY);
+        break;
+    case SendState::AWAITING_KEY:
+        Assume(send_state == SendState::READY);
+        break;
+    case SendState::READY:
+    case SendState::V1:
+        Assume(false); // Final states
+        break;
+    }
+    // Change state.
+    m_send_state = send_state;
+}
+
+bool V2Transport::ReceivedMessageComplete() const noexcept
+{
+    AssertLockNotHeld(m_recv_mutex);
+    LOCK(m_recv_mutex);
+    if (m_recv_state == RecvState::V1) return m_v1_fallback.ReceivedMessageComplete();
+
+    return m_recv_state == RecvState::APP_READY;
+}
+
+void V2Transport::ProcessReceivedMaybeV1Bytes() noexcept
+{
+    AssertLockHeld(m_recv_mutex);
+    AssertLockNotHeld(m_send_mutex);
+    Assume(m_recv_state == RecvState::KEY_MAYBE_V1);
+    // We still have to determine if this is a v1 or v2 connection. The bytes being received could
+    // be the beginning of either a v1 packet (network magic + "version\x00"), or of a v2 public
+    // key. BIP324 specifies that a mismatch with this 12-byte string should trigger sending of the
+    // key.
+    std::array<uint8_t, V1_PREFIX_LEN> v1_prefix = {0, 0, 0, 0, 'v', 'e', 'r', 's', 'i', 'o', 'n', 0};
+    std::copy(std::begin(Params().MessageStart()), std::end(Params().MessageStart()), v1_prefix.begin());
+    Assume(m_recv_buffer.size() <= v1_prefix.size());
+    if (!std::equal(m_recv_buffer.begin(), m_recv_buffer.end(), v1_prefix.begin())) {
+        // Mismatch with v1 prefix, so we can assume a v2 connection.
+        SetReceiveState(RecvState::KEY); // Convert to KEY state, leaving received bytes around.
+        // Transition the sender to AWAITING_KEY state (if not already).
+        LOCK(m_send_mutex);
+        SetSendState(SendState::AWAITING_KEY);
+    } else if (m_recv_buffer.size() == v1_prefix.size()) {
+        // Full match with the v1 prefix, so fall back to v1 behavior.
+        LOCK(m_send_mutex);
+        Span<const uint8_t> feedback{m_recv_buffer};
+        // Feed already received bytes to v1 transport. It should always accept these, because it's
+        // less than the size of a v1 header, and these are the first bytes fed to m_v1_fallback.
+        bool ret = m_v1_fallback.ReceivedBytes(feedback);
+        Assume(feedback.empty());
+        Assume(ret);
+        SetReceiveState(RecvState::V1);
+        SetSendState(SendState::V1);
+        // Reset v2 transport buffers to save memory.
+        m_recv_buffer = {};
+        m_send_buffer = {};
+    } else {
+        // We have not received enough to distinguish v1 from v2 yet. Wait until more bytes come.
+    }
+}
+
+bool V2Transport::ProcessReceivedKeyBytes() noexcept
+{
+    AssertLockHeld(m_recv_mutex);
+    AssertLockNotHeld(m_send_mutex);
+    Assume(m_recv_state == RecvState::KEY);
+    Assume(m_recv_buffer.size() <= EllSwiftPubKey::size());
+
+    // As a special exception, if bytes 4-16 of the key on a responder connection match the
+    // corresponding bytes of a V1 version message, but bytes 0-4 don't match the network magic
+    // (if they did, we'd have switched to V1 state already), assume this is a peer from
+    // another network, and disconnect them. They will almost certainly disconnect us too when
+    // they receive our uniformly random key and garbage, but detecting this case specially
+    // means we can log it.
+    static constexpr std::array<uint8_t, 12> MATCH = {'v', 'e', 'r', 's', 'i', 'o', 'n', 0, 0, 0, 0, 0};
+    static constexpr size_t OFFSET = sizeof(CMessageHeader::MessageStartChars);
+    if (!m_initiating && m_recv_buffer.size() >= OFFSET + MATCH.size()) {
+        if (std::equal(MATCH.begin(), MATCH.end(), m_recv_buffer.begin() + OFFSET)) {
+            LogPrint(BCLog::NET, "V2 transport error: V1 peer with wrong MessageStart %s\n",
+                     HexStr(Span(m_recv_buffer).first(OFFSET)));
+            return false;
+        }
+    }
+
+    if (m_recv_buffer.size() == EllSwiftPubKey::size()) {
+        // Other side's key has been fully received, and can now be Diffie-Hellman combined with
+        // our key to initialize the encryption ciphers.
+
+        // Initialize the ciphers.
+        EllSwiftPubKey ellswift(MakeByteSpan(m_recv_buffer));
+        LOCK(m_send_mutex);
+        m_cipher.Initialize(ellswift, m_initiating);
+
+        // Switch receiver state to GARB_GARBTERM.
+        SetReceiveState(RecvState::GARB_GARBTERM);
+        m_recv_buffer.clear();
+
+        // Switch sender state to READY.
+        SetSendState(SendState::READY);
+
+        // Append the garbage terminator to the send buffer.
+        size_t garbage_len = m_send_buffer.size() - EllSwiftPubKey::size();
+        m_send_buffer.resize(m_send_buffer.size() + BIP324Cipher::GARBAGE_TERMINATOR_LEN);
+        std::copy(m_cipher.GetSendGarbageTerminator().begin(),
+                  m_cipher.GetSendGarbageTerminator().end(),
+                  MakeWritableByteSpan(m_send_buffer).last(BIP324Cipher::GARBAGE_TERMINATOR_LEN).begin());
+
+        // Construct garbage authentication packet in the send buffer (using the garbage data which
+        // is still there).
+        m_send_buffer.resize(m_send_buffer.size() + BIP324Cipher::EXPANSION);
+        m_cipher.Encrypt(
+            /*contents=*/{},
+            /*aad=*/MakeByteSpan(m_send_buffer).subspan(EllSwiftPubKey::size(), garbage_len),
+            /*ignore=*/false,
+            /*output=*/MakeWritableByteSpan(m_send_buffer).last(BIP324Cipher::EXPANSION));
+
+        // Construct version packet in the send buffer.
+        m_send_buffer.resize(m_send_buffer.size() + BIP324Cipher::EXPANSION + VERSION_CONTENTS.size());
+        m_cipher.Encrypt(
+            /*contents=*/VERSION_CONTENTS,
+            /*aad=*/{},
+            /*ignore=*/false,
+            /*output=*/MakeWritableByteSpan(m_send_buffer).last(BIP324Cipher::EXPANSION + VERSION_CONTENTS.size()));
+    } else {
+        // We still have to receive more key bytes.
+    }
+    return true;
+}
+
+bool V2Transport::ProcessReceivedGarbageBytes() noexcept
+{
+    AssertLockHeld(m_recv_mutex);
+    Assume(m_recv_state == RecvState::GARB_GARBTERM);
+    Assume(m_recv_buffer.size() <= MAX_GARBAGE_LEN + BIP324Cipher::GARBAGE_TERMINATOR_LEN);
+    if (m_recv_buffer.size() >= BIP324Cipher::GARBAGE_TERMINATOR_LEN) {
+        if (MakeByteSpan(m_recv_buffer).last(BIP324Cipher::GARBAGE_TERMINATOR_LEN) == m_cipher.GetReceiveGarbageTerminator()) {
+            // Garbage terminator received. Switch to receiving garbage authentication packet.
+            m_recv_garbage = std::move(m_recv_buffer);
+            m_recv_garbage.resize(m_recv_garbage.size() - BIP324Cipher::GARBAGE_TERMINATOR_LEN);
+            m_recv_buffer.clear();
+            SetReceiveState(RecvState::GARBAUTH);
+        } else if (m_recv_buffer.size() == MAX_GARBAGE_LEN + BIP324Cipher::GARBAGE_TERMINATOR_LEN) {
+            // We've reached the maximum length for garbage + garbage terminator, and the
+            // terminator still does not match. Abort.
+            LogPrint(BCLog::NET, "V2 transport error: missing garbage terminator, peer=%d\n", m_nodeid);
+            return false;
+        } else {
+            // We still need to receive more garbage and/or garbage terminator bytes.
+        }
+    } else {
+        // We have less than GARBAGE_TERMINATOR_LEN (16) bytes, so we certainly need to receive
+        // more first.
+    }
+    return true;
+}
+
+bool V2Transport::ProcessReceivedPacketBytes() noexcept
+{
+    AssertLockHeld(m_recv_mutex);
+    Assume(m_recv_state == RecvState::GARBAUTH || m_recv_state == RecvState::VERSION ||
+           m_recv_state == RecvState::APP);
+
+    // The maximum permitted contents length for a packet, consisting of:
+    // - 0x00 byte: indicating long message type encoding
+    // - 12 bytes of message type
+    // - payload
+    static constexpr size_t MAX_CONTENTS_LEN =
+        1 + CMessageHeader::COMMAND_SIZE +
+        std::min<size_t>(MAX_SIZE, MAX_PROTOCOL_MESSAGE_LENGTH);
+
+    if (m_recv_buffer.size() == BIP324Cipher::LENGTH_LEN) {
+        // Length descriptor received.
+        m_recv_len = m_cipher.DecryptLength(MakeByteSpan(m_recv_buffer));
+        if (m_recv_len > MAX_CONTENTS_LEN) {
+            LogPrint(BCLog::NET, "V2 transport error: packet too large (%u bytes), peer=%d\n", m_recv_len, m_nodeid);
+            return false;
+        }
+    } else if (m_recv_buffer.size() > BIP324Cipher::LENGTH_LEN && m_recv_buffer.size() == m_recv_len + BIP324Cipher::EXPANSION) {
+        // Ciphertext received, decrypt it into m_recv_decode_buffer.
+        // Note that it is impossible to reach this branch without hitting the branch above first,
+        // as GetMaxBytesToProcess only allows up to LENGTH_LEN into the buffer before that point.
+        m_recv_decode_buffer.resize(m_recv_len);
+        bool ignore{false};
+        Span<const std::byte> aad;
+        if (m_recv_state == RecvState::GARBAUTH) aad = MakeByteSpan(m_recv_garbage);
+        bool ret = m_cipher.Decrypt(
+            /*input=*/MakeByteSpan(m_recv_buffer).subspan(BIP324Cipher::LENGTH_LEN),
+            /*aad=*/aad,
+            /*ignore=*/ignore,
+            /*contents=*/MakeWritableByteSpan(m_recv_decode_buffer));
+        if (!ret) {
+            LogPrint(BCLog::NET, "V2 transport error: packet decryption failure (%u bytes), peer=%d\n", m_recv_len, m_nodeid);
+            return false;
+        }
+        // Feed the last 4 bytes of the Poly1305 authentication tag (and its timing) into our RNG.
+        RandAddEvent(ReadLE32(m_recv_buffer.data() + m_recv_buffer.size() - 4));
+
+        // At this point we have a valid packet decrypted into m_recv_decode_buffer. Depending on
+        // the current state, decide what to do with it.
+        switch (m_recv_state) {
+        case RecvState::GARBAUTH:
+            // Ignore flag does not matter for garbage authentication. Any valid packet functions
+            // as authentication. Receive and process the version packet next.
+            SetReceiveState(RecvState::VERSION);
+            m_recv_garbage = {};
+            break;
+        case RecvState::VERSION:
+            if (!ignore) {
+                // Version message received; transition to application phase. The contents is
+                // ignored, but can be used for future extensions.
+                SetReceiveState(RecvState::APP);
+            }
+            break;
+        case RecvState::APP:
+            if (!ignore) {
+                // Application message decrypted correctly. It can be extracted using GetMessage().
+                SetReceiveState(RecvState::APP_READY);
+            }
+            break;
+        default:
+            // Any other state is invalid (this function should not have been called).
+            Assume(false);
+        }
+        // Wipe the receive buffer where the next packet will be received into.
+        m_recv_buffer = {};
+        // In all but APP_READY state, we can wipe the decoded contents.
+        if (m_recv_state != RecvState::APP_READY) m_recv_decode_buffer = {};
+    } else {
+        // We either have less than 3 bytes, so we don't know the packet's length yet, or more
+        // than 3 bytes but less than the packet's full ciphertext. Wait until those arrive.
+    }
+    return true;
+}
+
+size_t V2Transport::GetMaxBytesToProcess() noexcept
+{
+    AssertLockHeld(m_recv_mutex);
+    switch (m_recv_state) {
+    case RecvState::KEY_MAYBE_V1:
+        // During the KEY_MAYBE_V1 state we do not allow more than the length of v1 prefix into the
+        // receive buffer.
+        Assume(m_recv_buffer.size() <= V1_PREFIX_LEN);
+        // As long as we're not sure if this is a v1 or v2 connection, don't receive more than what
+        // is strictly necessary to distinguish the two (12 bytes). If we permitted more than
+        // the v1 header size (24 bytes), we may not be able to feed the already-received bytes
+        // back into the m_v1_fallback V1 transport.
+        return V1_PREFIX_LEN - m_recv_buffer.size();
+    case RecvState::KEY:
+        // During the KEY state, we only allow the 64-byte key into the receive buffer.
+        Assume(m_recv_buffer.size() <= EllSwiftPubKey::size());
+        // As long as we have not received the other side's public key, don't receive more than
+        // that (64 bytes), as garbage follows, and locating the garbage terminator requires the
+        // key exchange first.
+        return EllSwiftPubKey::size() - m_recv_buffer.size();
+    case RecvState::GARB_GARBTERM:
+        // Process garbage bytes one by one (because terminator may appear anywhere).
+        return 1;
+    case RecvState::GARBAUTH:
+    case RecvState::VERSION:
+    case RecvState::APP:
+        // These three states all involve decoding a packet. Process the length descriptor first,
+        // so that we know where the current packet ends (and we don't process bytes from the next
+        // packet or decoy yet). Then, process the ciphertext bytes of the current packet.
+        if (m_recv_buffer.size() < BIP324Cipher::LENGTH_LEN) {
+            return BIP324Cipher::LENGTH_LEN - m_recv_buffer.size();
+        } else {
+            // Note that BIP324Cipher::EXPANSION is the total difference between contents size
+            // and encoded packet size, which includes the 3 bytes due to the packet length.
+            // When transitioning from receiving the packet length to receiving its ciphertext,
+            // the encrypted packet length is left in the receive buffer.
+            return BIP324Cipher::EXPANSION + m_recv_len - m_recv_buffer.size();
+        }
+    case RecvState::APP_READY:
+        // No bytes can be processed until GetMessage() is called.
+        return 0;
+    case RecvState::V1:
+        // Not allowed (must be dealt with by the caller).
+        Assume(false);
+        return 0;
+    }
+    Assume(false); // unreachable
+    return 0;
+}
+
+bool V2Transport::ReceivedBytes(Span<const uint8_t>& msg_bytes) noexcept
+{
+    AssertLockNotHeld(m_recv_mutex);
+    /** How many bytes to allocate in the receive buffer at most above what is received so far. */
+    static constexpr size_t MAX_RESERVE_AHEAD = 256 * 1024;
+
+    LOCK(m_recv_mutex);
+    if (m_recv_state == RecvState::V1) return m_v1_fallback.ReceivedBytes(msg_bytes);
+
+    // Process the provided bytes in msg_bytes in a loop. In each iteration a nonzero number of
+    // bytes (decided by GetMaxBytesToProcess) are taken from the beginning om msg_bytes, and
+    // appended to m_recv_buffer. Then, depending on the receiver state, one of the
+    // ProcessReceived*Bytes functions is called to process the bytes in that buffer.
+    while (!msg_bytes.empty()) {
+        // Decide how many bytes to copy from msg_bytes to m_recv_buffer.
+        size_t max_read = GetMaxBytesToProcess();
+
+        // Reserve space in the buffer if there is not enough.
+        if (m_recv_buffer.size() + std::min(msg_bytes.size(), max_read) > m_recv_buffer.capacity()) {
+            switch (m_recv_state) {
+            case RecvState::KEY_MAYBE_V1:
+            case RecvState::KEY:
+            case RecvState::GARB_GARBTERM:
+                // During the initial states (key/garbage), allocate once to fit the maximum (4111
+                // bytes).
+                m_recv_buffer.reserve(MAX_GARBAGE_LEN + BIP324Cipher::GARBAGE_TERMINATOR_LEN);
+                break;
+            case RecvState::GARBAUTH:
+            case RecvState::VERSION:
+            case RecvState::APP: {
+                // During states where a packet is being received, as much as is expected but never
+                // more than MAX_RESERVE_AHEAD bytes in addition to what is received so far.
+                // This means attackers that want to cause us to waste allocated memory are limited
+                // to MAX_RESERVE_AHEAD above the largest allowed message contents size, and to
+                // MAX_RESERVE_AHEAD more than they've actually sent us.
+                size_t alloc_add = std::min(max_read, msg_bytes.size() + MAX_RESERVE_AHEAD);
+                m_recv_buffer.reserve(m_recv_buffer.size() + alloc_add);
+                break;
+            }
+            case RecvState::APP_READY:
+                // The buffer is empty in this state.
+                Assume(m_recv_buffer.empty());
+                break;
+            case RecvState::V1:
+                // Should have bailed out above.
+                Assume(false);
+                break;
+            }
+        }
+
+        // Can't read more than provided input.
+        max_read = std::min(msg_bytes.size(), max_read);
+        // Copy data to buffer.
+        m_recv_buffer.insert(m_recv_buffer.end(), UCharCast(msg_bytes.data()), UCharCast(msg_bytes.data() + max_read));
+        msg_bytes = msg_bytes.subspan(max_read);
+
+        // Process data in the buffer.
+        switch (m_recv_state) {
+        case RecvState::KEY_MAYBE_V1:
+            ProcessReceivedMaybeV1Bytes();
+            if (m_recv_state == RecvState::V1) return true;
+            break;
+
+        case RecvState::KEY:
+            if (!ProcessReceivedKeyBytes()) return false;
+            break;
+
+        case RecvState::GARB_GARBTERM:
+            if (!ProcessReceivedGarbageBytes()) return false;
+            break;
+
+        case RecvState::GARBAUTH:
+        case RecvState::VERSION:
+        case RecvState::APP:
+            if (!ProcessReceivedPacketBytes()) return false;
+            break;
+
+        case RecvState::APP_READY:
+            return true;
+
+        case RecvState::V1:
+            // We should have bailed out before.
+            Assume(false);
+            break;
+        }
+        // Make sure we have made progress before continuing.
+        Assume(max_read > 0);
+    }
+
+    return true;
+}
+
+std::optional<std::string> V2Transport::GetMessageType(Span<const uint8_t>& contents) noexcept
+{
+    if (contents.size() == 0) return std::nullopt; // Empty contents
+    uint8_t first_byte = contents[0];
+    contents = contents.subspan(1); // Strip first byte.
+
+    if (first_byte != 0) {
+        // Short (1 byte) encoding.
+        if (first_byte < std::size(V2_MESSAGE_IDS)) {
+            // Valid short message id.
+            return V2_MESSAGE_IDS[first_byte];
+        } else {
+            // Unknown short message id.
+            return std::nullopt;
+        }
+    }
+
+    if (contents.size() < CMessageHeader::COMMAND_SIZE) {
+        return std::nullopt; // Long encoding needs 12 message type bytes.
+    }
+
+    size_t msg_type_len{0};
+    while (msg_type_len < CMessageHeader::COMMAND_SIZE && contents[msg_type_len] != 0) {
+        // Verify that message type bytes before the first 0x00 are in range.
+        if (contents[msg_type_len] < ' ' || contents[msg_type_len] > 0x7F) {
+            return {};
+        }
+        ++msg_type_len;
+    }
+    std::string ret{reinterpret_cast<const char*>(contents.data()), msg_type_len};
+    while (msg_type_len < CMessageHeader::COMMAND_SIZE) {
+        // Verify that message type bytes after the first 0x00 are also 0x00.
+        if (contents[msg_type_len] != 0) return {};
+        ++msg_type_len;
+    }
+    // Strip message type bytes of contents.
+    contents = contents.subspan(CMessageHeader::COMMAND_SIZE);
+    return {std::move(ret)};
+}
+
+CNetMessage V2Transport::GetReceivedMessage(std::chrono::microseconds time, bool& reject_message) noexcept
+{
+    AssertLockNotHeld(m_recv_mutex);
+    LOCK(m_recv_mutex);
+    if (m_recv_state == RecvState::V1) return m_v1_fallback.GetReceivedMessage(time, reject_message);
+
+    Assume(m_recv_state == RecvState::APP_READY);
+    Span<const uint8_t> contents{m_recv_decode_buffer};
+    auto msg_type = GetMessageType(contents);
+    CDataStream ret(m_recv_type, m_recv_version);
+    CNetMessage msg{std::move(ret)};
+    // Note that BIP324Cipher::EXPANSION also includes the length descriptor size.
+    msg.m_raw_message_size = m_recv_decode_buffer.size() + BIP324Cipher::EXPANSION;
+    if (msg_type) {
+        reject_message = false;
+        msg.m_type = std::move(*msg_type);
+        msg.m_time = time;
+        msg.m_message_size = contents.size();
+        msg.m_recv.resize(contents.size());
+        std::copy(contents.begin(), contents.end(), UCharCast(msg.m_recv.data()));
+    } else {
+        LogPrint(BCLog::NET, "V2 transport error: invalid message type (%u bytes contents), peer=%d\n", m_recv_decode_buffer.size(), m_nodeid);
+        reject_message = true;
+    }
+    m_recv_decode_buffer = {};
+    SetReceiveState(RecvState::APP);
+
+    return msg;
+}
+
+bool V2Transport::SetMessageToSend(CSerializedNetMsg& msg) noexcept
+{
+    AssertLockNotHeld(m_send_mutex);
+    LOCK(m_send_mutex);
+    if (m_send_state == SendState::V1) return m_v1_fallback.SetMessageToSend(msg);
+    // We only allow adding a new message to be sent when in the READY state (so the packet cipher
+    // is available) and the send buffer is empty. This limits the number of messages in the send
+    // buffer to just one, and leaves the responsibility for queueing them up to the caller.
+    if (!(m_send_state == SendState::READY && m_send_buffer.empty())) return false;
+    // Construct contents (encoding message type + payload).
+    std::vector<uint8_t> contents;
+    auto short_message_id = V2_MESSAGE_MAP(msg.m_type);
+    if (short_message_id) {
+        contents.resize(1 + msg.data.size());
+        contents[0] = *short_message_id;
+        std::copy(msg.data.begin(), msg.data.end(), contents.begin() + 1);
+    } else {
+        // Initialize with zeroes, and then write the message type string starting at offset 1.
+        // This means contents[0] and the unused positions in contents[1..13] remain 0x00.
+        contents.resize(1 + CMessageHeader::COMMAND_SIZE + msg.data.size(), 0);
+        std::copy(msg.m_type.begin(), msg.m_type.end(), contents.data() + 1);
+        std::copy(msg.data.begin(), msg.data.end(), contents.begin() + 1 + CMessageHeader::COMMAND_SIZE);
+    }
+    // Construct ciphertext in send buffer.
+    m_send_buffer.resize(contents.size() + BIP324Cipher::EXPANSION);
+    m_cipher.Encrypt(MakeByteSpan(contents), {}, false, MakeWritableByteSpan(m_send_buffer));
+    m_send_type = msg.m_type;
+    // Release memory
+    msg.data = {};
+    return true;
+}
+
+Transport::BytesToSend V2Transport::GetBytesToSend(bool have_next_message) const noexcept
+{
+    AssertLockNotHeld(m_send_mutex);
+    LOCK(m_send_mutex);
+    if (m_send_state == SendState::V1) return m_v1_fallback.GetBytesToSend(have_next_message);
+
+    // We do not send anything in MAYBE_V1 state (as we don't know if the peer is v1 or v2),
+    // despite there being data in the send buffer in that state.
+    if (m_send_state == SendState::MAYBE_V1) return {{}, false, m_send_type};
+    Assume(m_send_pos <= m_send_buffer.size());
+    return {
+        Span{m_send_buffer}.subspan(m_send_pos),
+        // We only have more to send after the current m_send_buffer if there is a (next)
+        // message to be sent, and we're capable of sending packets. */
+        have_next_message && m_send_state == SendState::READY,
+        m_send_type
+    };
+}
+
+void V2Transport::MarkBytesSent(size_t bytes_sent) noexcept
+{
+    AssertLockNotHeld(m_send_mutex);
+    LOCK(m_send_mutex);
+    if (m_send_state == SendState::V1) return m_v1_fallback.MarkBytesSent(bytes_sent);
+
+    m_send_pos += bytes_sent;
+    Assume(m_send_pos <= m_send_buffer.size());
+    // Only wipe the buffer when everything is sent in the READY state. In the AWAITING_KEY state
+    // we still need the garbage that's in the send buffer to construct the garbage authentication
+    // packet.
+    if (m_send_state == SendState::READY && m_send_pos == m_send_buffer.size()) {
+        m_send_pos = 0;
+        m_send_buffer = {};
+    }
+}
+
+size_t V2Transport::GetSendMemoryUsage() const noexcept
+{
+    AssertLockNotHeld(m_send_mutex);
+    LOCK(m_send_mutex);
+    if (m_send_state == SendState::V1) return m_v1_fallback.GetSendMemoryUsage();
+
+    return sizeof(m_send_buffer) + memusage::DynamicUsage(m_send_buffer);
+}
+
 std::pair<size_t, bool> CConnman::SocketSendData(CNode& node) const
 {
     auto it = node.vSendMsg.begin();
     size_t nSentSize = 0;
     bool data_left{false}; //!< second return value (whether unsent data remains)
+    std::optional<bool> expected_more;
 
     while (true) {
         if (it != node.vSendMsg.end()) {
             // If possible, move one message from the send queue to the transport. This fails when
-            // there is an existing message still being sent.
+            // there is an existing message still being sent, or (for v2 transports) when the
+            // handshake has not yet completed.
             size_t memusage = it->GetMemoryUsage();
             if (node.m_transport->SetMessageToSend(*it)) {
                 // Update memory usage of send buffer (as *it will be deleted).
@@ -928,7 +1590,12 @@ std::pair<size_t, bool> CConnman::SocketSendData(CNode& node) const
                 ++it;
             }
         }
-        const auto& [data, more, msg_type] = node.m_transport->GetBytesToSend();
+        const auto& [data, more, msg_type] = node.m_transport->GetBytesToSend(it != node.vSendMsg.end());
+        // We rely on the 'more' value returned by GetBytesToSend to correctly predict whether more
+        // bytes are still to be sent, to correctly set the MSG_MORE flag. As a sanity check,
+        // verify that the previously returned 'more' was correct.
+        if (expected_more.has_value()) Assume(!data.empty() == *expected_more);
+        expected_more = more;
         data_left = !data.empty(); // will be overwritten on next loop if all of data gets sent
         int nBytes = 0;
         if (!data.empty()) {
@@ -941,9 +1608,7 @@ std::pair<size_t, bool> CConnman::SocketSendData(CNode& node) const
             }
             int flags = MSG_NOSIGNAL | MSG_DONTWAIT;
 #ifdef MSG_MORE
-            // We have more to send if either the transport itself has more, or if we have more
-            // messages to send.
-            if (more || it != node.vSendMsg.end()) {
+            if (more) {
                 flags |= MSG_MORE;
             }
 #endif
@@ -1323,9 +1988,10 @@ Sock::EventsPerSock CConnman::GenerateWaitSockets(Span<CNode* const> nodes)
         {
             LOCK(pnode->cs_vSend);
             // Sending is possible if either there are bytes to send right now, or if there will be
-            // once a potential message from vSendMsg is handed to the transport.
-            const auto& [to_send, _more, _msg_type] = pnode->m_transport->GetBytesToSend();
-            select_send = !to_send.empty() || !pnode->vSendMsg.empty();
+            // once a potential message from vSendMsg is handed to the transport. GetBytesToSend
+            // determines both of these in a single call.
+            const auto& [to_send, more, _msg_type] = pnode->m_transport->GetBytesToSend(!pnode->vSendMsg.empty());
+            select_send = !to_send.empty() || more;
         }
         if (!select_recv && !select_send) continue;
 
@@ -3007,7 +3673,10 @@ void CConnman::PushMessage(CNode* pnode, CSerializedNetMsg&& msg)
     size_t nBytesSent = 0;
     {
         LOCK(pnode->cs_vSend);
-        const auto& [to_send, _more, _msg_type] = pnode->m_transport->GetBytesToSend();
+        // Check if the transport still has unsent bytes, and indicate to it that we're about to
+        // give it a message to send.
+        const auto& [to_send, more, _msg_type] =
+            pnode->m_transport->GetBytesToSend(/*have_next_message=*/true);
         const bool queue_was_empty{to_send.empty() && pnode->vSendMsg.empty()};
 
         // Update memory usage of send buffer.
@@ -3016,10 +3685,14 @@ void CConnman::PushMessage(CNode* pnode, CSerializedNetMsg&& msg)
         // Move message to vSendMsg queue.
         pnode->vSendMsg.push_back(std::move(msg));
 
-        // If there was nothing to send before, attempt "optimistic write":
+        // If there was nothing to send before, and there is now (predicted by the "more" value
+        // returned by the GetBytesToSend call above), attempt "optimistic write":
         // because the poll/select loop may pause for SELECT_TIMEOUT_MILLISECONDS before actually
         // doing a send, try sending from the calling thread if the queue was empty before.
-        if (queue_was_empty) {
+        // With a V1Transport, more will always be true here, because adding a message always
+        // results in sendable bytes there, but with V2Transport this is not the case (it may
+        // still be in the handshake).
+        if (queue_was_empty && more) {
             std::tie(nBytesSent, std::ignore) = SocketSendData(*pnode);
         }
     }
diff --git a/src/net.h b/src/net.h
index 60a15fea55..cf7a240202 100644
--- a/src/net.h
+++ b/src/net.h
@@ -6,6 +6,7 @@
 #ifndef BITCOIN_NET_H
 #define BITCOIN_NET_H
 
+#include <bip324.h>
 #include <chainparams.h>
 #include <common/bloom.h>
 #include <compat/compat.h>
@@ -266,8 +267,6 @@ public:
 
     /** Returns true if the current message is complete (so GetReceivedMessage can be called). */
     virtual bool ReceivedMessageComplete() const = 0;
-    /** Set the deserialization context version for objects returned by GetReceivedMessage. */
-    virtual void SetReceiveVersion(int version) = 0;
 
     /** Feed wire bytes to the transport.
      *
@@ -300,7 +299,8 @@ public:
      *  - Span<const uint8_t> to_send: span of bytes to be sent over the wire (possibly empty).
      *  - bool more: whether there will be more bytes to be sent after the ones in to_send are
      *    all sent (as signaled by MarkBytesSent()).
-     *  - const std::string& m_type: message type on behalf of which this is being sent.
+     *  - const std::string& m_type: message type on behalf of which this is being sent
+     *    ("" for bytes that are not on behalf of any message).
      */
     using BytesToSend = std::tuple<
         Span<const uint8_t> /*to_send*/,
@@ -308,19 +308,42 @@ public:
         const std::string& /*m_type*/
     >;
 
-    /** Get bytes to send on the wire.
+    /** Get bytes to send on the wire, if any, along with other information about it.
      *
      * As a const function, it does not modify the transport's observable state, and is thus safe
      * to be called multiple times.
      *
-     * The bytes returned by this function act as a stream which can only be appended to. This
-     * means that with the exception of MarkBytesSent, operations on the transport can only append
-     * to what is being returned.
+     * @param[in] have_next_message If true, the "more" return value reports whether more will
+     *            be sendable after a SetMessageToSend call. It is set by the caller when they know
+     *            they have another message ready to send, and only care about what happens
+     *            after that. The have_next_message argument only affects this "more" return value
+     *            and nothing else.
      *
-     * Note that m_type and to_send refer to data that is internal to the transport, and calling
-     * any non-const function on this object may invalidate them.
+     *            Effectively, there are three possible outcomes about whether there are more bytes
+     *            to send:
+     *            - Yes:     the transport itself has more bytes to send later. For example, for
+     *                       V1Transport this happens during the sending of the header of a
+     *                       message, when there is a non-empty payload that follows.
+     *            - No:      the transport itself has no more bytes to send, but will have bytes to
+     *                       send if handed a message through SetMessageToSend. In V1Transport this
+     *                       happens when sending the payload of a message.
+     *            - Blocked: the transport itself has no more bytes to send, and is also incapable
+     *                       of sending anything more at all now, if it were handed another
+     *                       message to send. This occurs in V2Transport before the handshake is
+     *                       complete, as the encryption ciphers are not set up for sending
+     *                       messages before that point.
+     *
+     *            The boolean 'more' is true for Yes, false for Blocked, and have_next_message
+     *            controls what is returned for No.
+     *
+     * @return a BytesToSend object. The to_send member returned acts as a stream which is only
+     *         ever appended to. This means that with the exception of MarkBytesSent (which pops
+     *         bytes off the front of later to_sends), operations on the transport can only append
+     *         to what is being returned. Also note that m_type and to_send refer to data that is
+     *         internal to the transport, and calling any non-const function on this object may
+     *         invalidate them.
      */
-    virtual BytesToSend GetBytesToSend() const noexcept = 0;
+    virtual BytesToSend GetBytesToSend(bool have_next_message) const noexcept = 0;
 
     /** Report how many bytes returned by the last GetBytesToSend() have been sent.
      *
@@ -392,14 +415,6 @@ public:
         return WITH_LOCK(m_recv_mutex, return CompleteInternal());
     }
 
-    void SetReceiveVersion(int nVersionIn) override EXCLUSIVE_LOCKS_REQUIRED(!m_recv_mutex)
-    {
-        AssertLockNotHeld(m_recv_mutex);
-        LOCK(m_recv_mutex);
-        hdrbuf.SetVersion(nVersionIn);
-        vRecv.SetVersion(nVersionIn);
-    }
-
     bool ReceivedBytes(Span<const uint8_t>& msg_bytes) override EXCLUSIVE_LOCKS_REQUIRED(!m_recv_mutex)
     {
         AssertLockNotHeld(m_recv_mutex);
@@ -416,7 +431,221 @@ public:
     CNetMessage GetReceivedMessage(std::chrono::microseconds time, bool& reject_message) override EXCLUSIVE_LOCKS_REQUIRED(!m_recv_mutex);
 
     bool SetMessageToSend(CSerializedNetMsg& msg) noexcept override EXCLUSIVE_LOCKS_REQUIRED(!m_send_mutex);
-    BytesToSend GetBytesToSend() const noexcept override EXCLUSIVE_LOCKS_REQUIRED(!m_send_mutex);
+    BytesToSend GetBytesToSend(bool have_next_message) const noexcept override EXCLUSIVE_LOCKS_REQUIRED(!m_send_mutex);
+    void MarkBytesSent(size_t bytes_sent) noexcept override EXCLUSIVE_LOCKS_REQUIRED(!m_send_mutex);
+    size_t GetSendMemoryUsage() const noexcept override EXCLUSIVE_LOCKS_REQUIRED(!m_send_mutex);
+};
+
+class V2Transport final : public Transport
+{
+private:
+    /** Contents of the version packet to send. BIP324 stipulates that senders should leave this
+     *  empty, and receivers should ignore it. Future extensions can change what is sent as long as
+     *  an empty version packet contents is interpreted as no extensions supported. */
+    static constexpr std::array<std::byte, 0> VERSION_CONTENTS = {};
+
+    /** The length of the V1 prefix to match bytes initially received by responders with to
+     *  determine if their peer is speaking V1 or V2. */
+    static constexpr size_t V1_PREFIX_LEN = 12;
+
+    // The sender side and receiver side of V2Transport are state machines that are transitioned
+    // through, based on what has been received. The receive state corresponds to the contents of,
+    // and bytes received to, the receive buffer. The send state controls what can be appended to
+    // the send buffer and what can be sent from it.
+
+    /** State type that defines the current contents of the receive buffer and/or how the next
+     *  received bytes added to it will be interpreted.
+     *
+     * Diagram:
+     *
+     *   start(responder)
+     *        |
+     *        |  start(initiator)                                       /---------\
+     *        |          |                                              |         |
+     *        v          v                                              v         |
+     *  KEY_MAYBE_V1 -> KEY -> GARB_GARBTERM -> GARBAUTH -> VERSION -> APP -> APP_READY
+     *        |
+     *        \-------> V1
+     */
+    enum class RecvState : uint8_t {
+        /** (Responder only) either v2 public key or v1 header.
+         *
+         * This is the initial state for responders, before data has been received to distinguish
+         * v1 from v2 connections. When that happens, the state becomes either KEY (for v2) or V1
+         * (for v1). */
+        KEY_MAYBE_V1,
+
+        /** Public key.
+         *
+         * This is the initial state for initiators, during which the other side's public key is
+         * received. When that information arrives, the ciphers get initialized and the state
+         * becomes GARB_GARBTERM. */
+        KEY,
+
+        /** Garbage and garbage terminator.
+         *
+         * Whenever a byte is received, the last 16 bytes are compared with the expected garbage
+         * terminator. When that happens, the state becomes GARBAUTH. If no matching terminator is
+         * received in 4111 bytes (4095 for the maximum garbage length, and 16 bytes for the
+         * terminator), the connection aborts. */
+        GARB_GARBTERM,
+
+        /** Garbage authentication packet.
+         *
+         * A packet is received, and decrypted/verified with AAD set to the garbage received during
+         * the GARB_GARBTERM state. If that succeeds, the state becomes VERSION. If it fails the
+         * connection aborts. */
+        GARBAUTH,
+
+        /** Version packet.
+         *
+         * A packet is received, and decrypted/verified. If that succeeds, the state becomes APP,
+         * and the decrypted contents is interpreted as version negotiation (currently, that means
+         * ignoring it, but it can be used for negotiating future extensions). If it fails, the
+         * connection aborts. */
+        VERSION,
+
+        /** Application packet.
+         *
+         * A packet is received, and decrypted/verified. If that succeeds, the state becomes
+         * APP_READY and the decrypted contents is kept in m_recv_decode_buffer until it is
+         * retrieved as a message by GetMessage(). */
+        APP,
+
+        /** Nothing (an application packet is available for GetMessage()).
+         *
+         * Nothing can be received in this state. When the message is retrieved by GetMessage,
+         * the state becomes APP again. */
+        APP_READY,
+
+        /** Nothing (this transport is using v1 fallback).
+         *
+         * All receive operations are redirected to m_v1_fallback. */
+        V1,
+    };
+
+    /** State type that controls the sender side.
+     *
+     * Diagram:
+     *
+     *  start(responder)
+     *      |
+     *      |      start(initiator)
+     *      |            |
+     *      v            v
+     *  MAYBE_V1 -> AWAITING_KEY -> READY
+     *      |
+     *      \-----> V1
+     */
+    enum class SendState : uint8_t {
+        /** (Responder only) Not sending until v1 or v2 is detected.
+         *
+         * This is the initial state for responders. The send buffer contains the public key to
+         * send, but nothing is sent in this state yet. When the receiver determines whether this
+         * is a V1 or V2 connection, the sender state becomes AWAITING_KEY (for v2) or V1 (for v1).
+         */
+        MAYBE_V1,
+
+        /** Waiting for the other side's public key.
+         *
+         * This is the initial state for initiators. The public key is sent out. When the receiver
+         * receives the other side's public key and transitions to GARB_GARBTERM, the sender state
+         * becomes READY. */
+        AWAITING_KEY,
+
+        /** Normal sending state.
+         *
+         * In this state, the ciphers are initialized, so packets can be sent. When this state is
+         * entered, the garbage, garbage terminator, garbage authentication packet, and version
+         * packet are appended to the send buffer (in addition to the key which may still be
+         * there). In this state a message can be provided if the send buffer is empty. */
+        READY,
+
+        /** This transport is using v1 fallback.
+         *
+         * All send operations are redirected to m_v1_fallback. */
+        V1,
+    };
+
+    /** Cipher state. */
+    BIP324Cipher m_cipher;
+    /** Whether we are the initiator side. */
+    const bool m_initiating;
+    /** NodeId (for debug logging). */
+    const NodeId m_nodeid;
+    /** Encapsulate a V1Transport to fall back to. */
+    V1Transport m_v1_fallback;
+
+    /** Lock for receiver-side fields. */
+    mutable Mutex m_recv_mutex ACQUIRED_BEFORE(m_send_mutex);
+    /** In {GARBAUTH, VERSION, APP}, the decrypted packet length, if m_recv_buffer.size() >=
+     *  BIP324Cipher::LENGTH_LEN. Unspecified otherwise. */
+    uint32_t m_recv_len GUARDED_BY(m_recv_mutex) {0};
+    /** Receive buffer; meaning is determined by m_recv_state. */
+    std::vector<uint8_t> m_recv_buffer GUARDED_BY(m_recv_mutex);
+    /** During GARBAUTH, the garbage received during GARB_GARBTERM. */
+    std::vector<uint8_t> m_recv_garbage GUARDED_BY(m_recv_mutex);
+    /** Buffer to put decrypted contents in, for converting to CNetMessage. */
+    std::vector<uint8_t> m_recv_decode_buffer GUARDED_BY(m_recv_mutex);
+    /** Deserialization type. */
+    const int m_recv_type;
+    /** Deserialization version number. */
+    const int m_recv_version;
+    /** Current receiver state. */
+    RecvState m_recv_state GUARDED_BY(m_recv_mutex);
+
+    /** Lock for sending-side fields. If both sending and receiving fields are accessed,
+     *  m_recv_mutex must be acquired before m_send_mutex. */
+    mutable Mutex m_send_mutex ACQUIRED_AFTER(m_recv_mutex);
+    /** The send buffer; meaning is determined by m_send_state. */
+    std::vector<uint8_t> m_send_buffer GUARDED_BY(m_send_mutex);
+    /** How many bytes from the send buffer have been sent so far. */
+    uint32_t m_send_pos GUARDED_BY(m_send_mutex) {0};
+    /** Type of the message being sent. */
+    std::string m_send_type GUARDED_BY(m_send_mutex);
+    /** Current sender state. */
+    SendState m_send_state GUARDED_BY(m_send_mutex);
+
+    /** Change the receive state. */
+    void SetReceiveState(RecvState recv_state) noexcept EXCLUSIVE_LOCKS_REQUIRED(m_recv_mutex);
+    /** Change the send state. */
+    void SetSendState(SendState send_state) noexcept EXCLUSIVE_LOCKS_REQUIRED(m_send_mutex);
+    /** Given a packet's contents, find the message type (if valid), and strip it from contents. */
+    static std::optional<std::string> GetMessageType(Span<const uint8_t>& contents) noexcept;
+    /** Determine how many received bytes can be processed in one go (not allowed in V1 state). */
+    size_t GetMaxBytesToProcess() noexcept EXCLUSIVE_LOCKS_REQUIRED(m_recv_mutex);
+    /** Process bytes in m_recv_buffer, while in KEY_MAYBE_V1 state. */
+    void ProcessReceivedMaybeV1Bytes() noexcept EXCLUSIVE_LOCKS_REQUIRED(m_recv_mutex, !m_send_mutex);
+    /** Process bytes in m_recv_buffer, while in KEY state. */
+    bool ProcessReceivedKeyBytes() noexcept EXCLUSIVE_LOCKS_REQUIRED(m_recv_mutex, !m_send_mutex);
+    /** Process bytes in m_recv_buffer, while in GARB_GARBTERM state. */
+    bool ProcessReceivedGarbageBytes() noexcept EXCLUSIVE_LOCKS_REQUIRED(m_recv_mutex);
+    /** Process bytes in m_recv_buffer, while in GARBAUTH/VERSION/APP state. */
+    bool ProcessReceivedPacketBytes() noexcept EXCLUSIVE_LOCKS_REQUIRED(m_recv_mutex);
+
+public:
+    static constexpr uint32_t MAX_GARBAGE_LEN = 4095;
+
+    /** Construct a V2 transport with securely generated random keys.
+     *
+     * @param[in] nodeid      the node's NodeId (only for debug log output).
+     * @param[in] initiating  whether we are the initiator side.
+     * @param[in] type_in     the serialization type of returned CNetMessages.
+     * @param[in] version_in  the serialization version of returned CNetMessages.
+     */
+    V2Transport(NodeId nodeid, bool initiating, int type_in, int version_in) noexcept;
+
+    /** Construct a V2 transport with specified keys and garbage (test use only). */
+    V2Transport(NodeId nodeid, bool initiating, int type_in, int version_in, const CKey& key, Span<const std::byte> ent32, Span<const uint8_t> garbage) noexcept;
+
+    // Receive side functions.
+    bool ReceivedMessageComplete() const noexcept override EXCLUSIVE_LOCKS_REQUIRED(!m_recv_mutex);
+    bool ReceivedBytes(Span<const uint8_t>& msg_bytes) noexcept override EXCLUSIVE_LOCKS_REQUIRED(!m_recv_mutex, !m_send_mutex);
+    CNetMessage GetReceivedMessage(std::chrono::microseconds time, bool& reject_message) noexcept override EXCLUSIVE_LOCKS_REQUIRED(!m_recv_mutex);
+
+    // Send side functions.
+    bool SetMessageToSend(CSerializedNetMsg& msg) noexcept override EXCLUSIVE_LOCKS_REQUIRED(!m_send_mutex);
+    BytesToSend GetBytesToSend(bool have_next_message) const noexcept override EXCLUSIVE_LOCKS_REQUIRED(!m_send_mutex);
     void MarkBytesSent(size_t bytes_sent) noexcept override EXCLUSIVE_LOCKS_REQUIRED(!m_send_mutex);
     size_t GetSendMemoryUsage() const noexcept override EXCLUSIVE_LOCKS_REQUIRED(!m_send_mutex);
 };
diff --git a/src/pubkey.cpp b/src/pubkey.cpp
index 4866feed67..05808e4c22 100644
--- a/src/pubkey.cpp
+++ b/src/pubkey.cpp
@@ -336,6 +336,12 @@ bool CPubKey::Derive(CPubKey& pubkeyChild, ChainCode &ccChild, unsigned int nChi
     return true;
 }
 
+EllSwiftPubKey::EllSwiftPubKey(Span<const std::byte> ellswift) noexcept
+{
+    assert(ellswift.size() == SIZE);
+    std::copy(ellswift.begin(), ellswift.end(), m_pubkey.begin());
+}
+
 CPubKey EllSwiftPubKey::Decode() const
 {
     secp256k1_pubkey pubkey;
diff --git a/src/pubkey.h b/src/pubkey.h
index 00defa25a0..274779f9a4 100644
--- a/src/pubkey.h
+++ b/src/pubkey.h
@@ -303,8 +303,7 @@ public:
     EllSwiftPubKey() noexcept = default;
 
     /** Construct a new ellswift public key from a given serialization. */
-    EllSwiftPubKey(const std::array<std::byte, SIZE>& ellswift) :
-        m_pubkey(ellswift) {}
+    EllSwiftPubKey(Span<const std::byte> ellswift) noexcept;
 
     /** Decode to normal compressed CPubKey (for debugging purposes). */
     CPubKey Decode() const;
diff --git a/src/test/bip324_tests.cpp b/src/test/bip324_tests.cpp
index 04472611ec..1ed7e23bcf 100644
--- a/src/test/bip324_tests.cpp
+++ b/src/test/bip324_tests.cpp
@@ -38,14 +38,8 @@ void TestBIP324PacketVector(
 {
     // Convert input from hex to char/byte vectors/arrays.
     const auto in_priv_ours = ParseHex(in_priv_ours_hex);
-    const auto in_ellswift_ours_vec = ParseHex<std::byte>(in_ellswift_ours_hex);
-    assert(in_ellswift_ours_vec.size() == 64);
-    std::array<std::byte, 64> in_ellswift_ours;
-    std::copy(in_ellswift_ours_vec.begin(), in_ellswift_ours_vec.end(), in_ellswift_ours.begin());
-    const auto in_ellswift_theirs_vec = ParseHex<std::byte>(in_ellswift_theirs_hex);
-    assert(in_ellswift_theirs_vec.size() == 64);
-    std::array<std::byte, 64> in_ellswift_theirs;
-    std::copy(in_ellswift_theirs_vec.begin(), in_ellswift_theirs_vec.end(), in_ellswift_theirs.begin());
+    const auto in_ellswift_ours = ParseHex<std::byte>(in_ellswift_ours_hex);
+    const auto in_ellswift_theirs = ParseHex<std::byte>(in_ellswift_theirs_hex);
     const auto in_contents = ParseHex<std::byte>(in_contents_hex);
     const auto in_aad = ParseHex<std::byte>(in_aad_hex);
     const auto mid_send_garbage = ParseHex<std::byte>(mid_send_garbage_hex);
diff --git a/src/test/denialofservice_tests.cpp b/src/test/denialofservice_tests.cpp
index 7f5d587cf6..8c1182b5e1 100644
--- a/src/test/denialofservice_tests.cpp
+++ b/src/test/denialofservice_tests.cpp
@@ -86,7 +86,7 @@ BOOST_AUTO_TEST_CASE(outbound_slow_chain_eviction)
 
     {
         LOCK(dummyNode1.cs_vSend);
-        const auto& [to_send, _more, _msg_type] = dummyNode1.m_transport->GetBytesToSend();
+        const auto& [to_send, _more, _msg_type] = dummyNode1.m_transport->GetBytesToSend(false);
         BOOST_CHECK(!to_send.empty());
     }
     connman.FlushSendBuffer(dummyNode1);
@@ -97,7 +97,7 @@ BOOST_AUTO_TEST_CASE(outbound_slow_chain_eviction)
     BOOST_CHECK(peerman.SendMessages(&dummyNode1)); // should result in getheaders
     {
         LOCK(dummyNode1.cs_vSend);
-        const auto& [to_send, _more, _msg_type] = dummyNode1.m_transport->GetBytesToSend();
+        const auto& [to_send, _more, _msg_type] = dummyNode1.m_transport->GetBytesToSend(false);
         BOOST_CHECK(!to_send.empty());
     }
     // Wait 3 more minutes
diff --git a/src/test/fuzz/p2p_transport_serialization.cpp b/src/test/fuzz/p2p_transport_serialization.cpp
index 2fa5de5008..6e3ef2a707 100644
--- a/src/test/fuzz/p2p_transport_serialization.cpp
+++ b/src/test/fuzz/p2p_transport_serialization.cpp
@@ -25,6 +25,7 @@ std::vector<std::string> g_all_messages;
 
 void initialize_p2p_transport_serialization()
 {
+    ECC_Start();
     SelectParams(ChainType::REGTEST);
     g_all_messages = getAllNetMessageTypes();
     std::sort(g_all_messages.begin(), g_all_messages.end());
@@ -92,7 +93,7 @@ FUZZ_TARGET(p2p_transport_serialization, .init = initialize_p2p_transport_serial
             assert(queued);
             std::optional<bool> known_more;
             while (true) {
-                const auto& [to_send, more, _msg_type] = send_transport.GetBytesToSend();
+                const auto& [to_send, more, _msg_type] = send_transport.GetBytesToSend(false);
                 if (known_more) assert(!to_send.empty() == *known_more);
                 if (to_send.empty()) break;
                 send_transport.MarkBytesSent(to_send.size());
@@ -124,11 +125,13 @@ void SimulationTest(Transport& initiator, Transport& responder, R& rng, FuzzedDa
     // Vectors with bytes last returned by GetBytesToSend() on transport[i].
     std::array<std::vector<uint8_t>, 2> to_send;
 
-    // Last returned 'more' values (if still relevant) by transport[i]->GetBytesToSend().
-    std::array<std::optional<bool>, 2> last_more;
+    // Last returned 'more' values (if still relevant) by transport[i]->GetBytesToSend(), for
+    // both have_next_message false and true.
+    std::array<std::optional<bool>, 2> last_more, last_more_next;
 
-    // Whether more bytes to be sent are expected on transport[i].
-    std::array<std::optional<bool>, 2> expect_more;
+    // Whether more bytes to be sent are expected on transport[i], before and after
+    // SetMessageToSend().
+    std::array<std::optional<bool>, 2> expect_more, expect_more_next;
 
     // Function to consume a message type.
     auto msg_type_fn = [&]() {
@@ -177,18 +180,27 @@ void SimulationTest(Transport& initiator, Transport& responder, R& rng, FuzzedDa
 
     // Wrapper around transport[i]->GetBytesToSend() that performs sanity checks.
     auto bytes_to_send_fn = [&](int side) -> Transport::BytesToSend {
-        const auto& [bytes, more, msg_type] = transports[side]->GetBytesToSend();
+        // Invoke GetBytesToSend twice (for have_next_message = {false, true}). This function does
+        // not modify state (it's const), and only the "more" return value should differ between
+        // the calls.
+        const auto& [bytes, more_nonext, msg_type] = transports[side]->GetBytesToSend(false);
+        const auto& [bytes_next, more_next, msg_type_next] = transports[side]->GetBytesToSend(true);
         // Compare with expected more.
         if (expect_more[side].has_value()) assert(!bytes.empty() == *expect_more[side]);
+        // Verify consistency between the two results.
+        assert(bytes == bytes_next);
+        assert(msg_type == msg_type_next);
+        if (more_nonext) assert(more_next);
         // Compare with previously reported output.
         assert(to_send[side].size() <= bytes.size());
         assert(to_send[side] == Span{bytes}.first(to_send[side].size()));
         to_send[side].resize(bytes.size());
         std::copy(bytes.begin(), bytes.end(), to_send[side].begin());
-        // Remember 'more' result.
-        last_more[side] = {more};
+        // Remember 'more' results.
+        last_more[side] = {more_nonext};
+        last_more_next[side] = {more_next};
         // Return.
-        return {bytes, more, msg_type};
+        return {bytes, more_nonext, msg_type};
     };
 
     // Function to make side send a new message.
@@ -199,7 +211,8 @@ void SimulationTest(Transport& initiator, Transport& responder, R& rng, FuzzedDa
         CSerializedNetMsg msg = next_msg[side].Copy();
         bool queued = transports[side]->SetMessageToSend(msg);
         // Update expected more data.
-        expect_more[side] = std::nullopt;
+        expect_more[side] = expect_more_next[side];
+        expect_more_next[side] = std::nullopt;
         // Verify consistency of GetBytesToSend after SetMessageToSend
         bytes_to_send_fn(/*side=*/side);
         if (queued) {
@@ -223,6 +236,7 @@ void SimulationTest(Transport& initiator, Transport& responder, R& rng, FuzzedDa
         // If all to-be-sent bytes were sent, move last_more data to expect_more data.
         if (send_now == bytes.size()) {
             expect_more[side] = last_more[side];
+            expect_more_next[side] = last_more_next[side];
         }
         // Remove the bytes from the last reported to-be-sent vector.
         assert(to_send[side].size() >= send_now);
@@ -251,6 +265,7 @@ void SimulationTest(Transport& initiator, Transport& responder, R& rng, FuzzedDa
             // Clear cached expected 'more' information: if certainly no more data was to be sent
             // before, receiving bytes makes this uncertain.
             if (expect_more[!side] == false) expect_more[!side] = std::nullopt;
+            if (expect_more_next[!side] == false) expect_more_next[!side] = std::nullopt;
             // Verify consistency of GetBytesToSend after ReceivedBytes
             bytes_to_send_fn(/*side=*/!side);
             bool progress = to_recv.size() < old_len;
@@ -320,6 +335,40 @@ std::unique_ptr<Transport> MakeV1Transport(NodeId nodeid) noexcept
     return std::make_unique<V1Transport>(nodeid, SER_NETWORK, INIT_PROTO_VERSION);
 }
 
+template<typename RNG>
+std::unique_ptr<Transport> MakeV2Transport(NodeId nodeid, bool initiator, RNG& rng, FuzzedDataProvider& provider)
+{
+    // Retrieve key
+    auto key = ConsumePrivateKey(provider);
+    if (!key.IsValid()) return {};
+    // Construct garbage
+    size_t garb_len = provider.ConsumeIntegralInRange<size_t>(0, V2Transport::MAX_GARBAGE_LEN);
+    std::vector<uint8_t> garb;
+    if (garb_len <= 64) {
+        // When the garbage length is up to 64 bytes, read it directly from the fuzzer input.
+        garb = provider.ConsumeBytes<uint8_t>(garb_len);
+        garb.resize(garb_len);
+    } else {
+        // If it's longer, generate it from the RNG. This avoids having large amounts of
+        // (hopefully) irrelevant data needing to be stored in the fuzzer data.
+        for (auto& v : garb) v = uint8_t(rng());
+    }
+    // Retrieve entropy
+    auto ent = provider.ConsumeBytes<std::byte>(32);
+    ent.resize(32);
+    // Use as entropy SHA256(ent || garbage). This prevents a situation where the fuzzer manages to
+    // include the garbage terminator (which is a function of both ellswift keys) in the garbage.
+    // This is extremely unlikely (~2^-116) with random keys/garbage, but the fuzzer can choose
+    // both non-randomly and dependently. Since the entropy is hashed anyway inside the ellswift
+    // computation, no coverage should be lost by using a hash as entropy, and it removes the
+    // possibility of garbage that happens to contain what is effectively a hash of the keys.
+    CSHA256().Write(UCharCast(ent.data()), ent.size())
+             .Write(garb.data(), garb.size())
+             .Finalize(UCharCast(ent.data()));
+
+    return std::make_unique<V2Transport>(nodeid, initiator, SER_NETWORK, INIT_PROTO_VERSION, key, ent, garb);
+}
+
 } // namespace
 
 FUZZ_TARGET(p2p_transport_bidirectional, .init = initialize_p2p_transport_serialization)
@@ -332,3 +381,25 @@ FUZZ_TARGET(p2p_transport_bidirectional, .init = initialize_p2p_transport_serial
     if (!t1 || !t2) return;
     SimulationTest(*t1, *t2, rng, provider);
 }
+
+FUZZ_TARGET(p2p_transport_bidirectional_v2, .init = initialize_p2p_transport_serialization)
+{
+    // Test with two V2 transports talking to each other.
+    FuzzedDataProvider provider{buffer.data(), buffer.size()};
+    XoRoShiRo128PlusPlus rng(provider.ConsumeIntegral<uint64_t>());
+    auto t1 = MakeV2Transport(NodeId{0}, true, rng, provider);
+    auto t2 = MakeV2Transport(NodeId{1}, false, rng, provider);
+    if (!t1 || !t2) return;
+    SimulationTest(*t1, *t2, rng, provider);
+}
+
+FUZZ_TARGET(p2p_transport_bidirectional_v1v2, .init = initialize_p2p_transport_serialization)
+{
+    // Test with a V1 initiator talking to a V2 responder.
+    FuzzedDataProvider provider{buffer.data(), buffer.size()};
+    XoRoShiRo128PlusPlus rng(provider.ConsumeIntegral<uint64_t>());
+    auto t1 = MakeV1Transport(NodeId{0});
+    auto t2 = MakeV2Transport(NodeId{1}, false, rng, provider);
+    if (!t1 || !t2) return;
+    SimulationTest(*t1, *t2, rng, provider);
+}
diff --git a/src/test/net_tests.cpp b/src/test/net_tests.cpp
index 295cb78b36..900e311d22 100644
--- a/src/test/net_tests.cpp
+++ b/src/test/net_tests.cpp
@@ -15,6 +15,7 @@
 #include <serialize.h>
 #include <span.h>
 #include <streams.h>
+#include <test/util/random.h>
 #include <test/util/setup_common.h>
 #include <test/util/validation.h>
 #include <timedata.h>
@@ -1005,4 +1006,530 @@ BOOST_AUTO_TEST_CASE(advertise_local_address)
     RemoveLocal(addr_cjdns);
 }
 
+namespace {
+
+/** A class for scenario-based tests of V2Transport
+ *
+ * Each V2TransportTester encapsulates a V2Transport (the one being tested), and can be told to
+ * interact with it. To do so, it also encapsulates a BIP324Cipher to act as the other side. A
+ * second V2Transport is not used, as doing so would not permit scenarios that involve sending
+ * invalid data, or ones scenarios using BIP324 features that are not implemented on the sending
+ * side (like decoy packets).
+ */
+class V2TransportTester
+{
+    V2Transport m_transport; //!< V2Transport being tested
+    BIP324Cipher m_cipher; //!< Cipher to help with the other side
+    bool m_test_initiator; //!< Whether m_transport is the initiator (true) or responder (false)
+
+    std::vector<uint8_t> m_sent_garbage; //!< The garbage we've sent to m_transport.
+    std::vector<uint8_t> m_to_send; //!< Bytes we have queued up to send to m_transport.
+    std::vector<uint8_t> m_received; //!< Bytes we have received from m_transport.
+    std::deque<CSerializedNetMsg> m_msg_to_send; //!< Messages to be sent *by* m_transport to us.
+
+public:
+    /** Construct a tester object. test_initiator: whether the tested transport is initiator. */
+    V2TransportTester(bool test_initiator) :
+        m_transport(0, test_initiator, SER_NETWORK, INIT_PROTO_VERSION),
+        m_test_initiator(test_initiator) {}
+
+    /** Data type returned by Interact:
+     *
+     * - std::nullopt: transport error occurred
+     * - otherwise: a vector of
+     *   - std::nullopt: invalid message received
+     *   - otherwise: a CNetMessage retrieved
+     */
+    using InteractResult = std::optional<std::vector<std::optional<CNetMessage>>>;
+
+    /** Send/receive scheduled/available bytes and messages.
+     *
+     * This is the only function that interacts with the transport being tested; everything else is
+     * scheduling things done by Interact(), or processing things learned by it.
+     */
+    InteractResult Interact()
+    {
+        std::vector<std::optional<CNetMessage>> ret;
+        while (true) {
+            bool progress{false};
+            // Send bytes from m_to_send to the transport.
+            if (!m_to_send.empty()) {
+                Span<const uint8_t> to_send = Span{m_to_send}.first(1 + InsecureRandRange(m_to_send.size()));
+                size_t old_len = to_send.size();
+                if (!m_transport.ReceivedBytes(to_send)) {
+                    return std::nullopt; // transport error occurred
+                }
+                if (old_len != to_send.size()) {
+                    progress = true;
+                    m_to_send.erase(m_to_send.begin(), m_to_send.begin() + (old_len - to_send.size()));
+                }
+            }
+            // Retrieve messages received by the transport.
+            if (m_transport.ReceivedMessageComplete() && (!progress || InsecureRandBool())) {
+                bool reject{false};
+                auto msg = m_transport.GetReceivedMessage({}, reject);
+                if (reject) {
+                    ret.push_back(std::nullopt);
+                } else {
+                    ret.push_back(std::move(msg));
+                }
+                progress = true;
+            }
+            // Enqueue a message to be sent by the transport to us.
+            if (!m_msg_to_send.empty() && (!progress || InsecureRandBool())) {
+                if (m_transport.SetMessageToSend(m_msg_to_send.front())) {
+                    m_msg_to_send.pop_front();
+                    progress = true;
+                }
+            }
+            // Receive bytes from the transport.
+            const auto& [recv_bytes, _more, _msg_type] = m_transport.GetBytesToSend(!m_msg_to_send.empty());
+            if (!recv_bytes.empty() && (!progress || InsecureRandBool())) {
+                size_t to_receive = 1 + InsecureRandRange(recv_bytes.size());
+                m_received.insert(m_received.end(), recv_bytes.begin(), recv_bytes.begin() + to_receive);
+                progress = true;
+                m_transport.MarkBytesSent(to_receive);
+            }
+            if (!progress) break;
+        }
+        return ret;
+    }
+
+    /** Expose the cipher. */
+    BIP324Cipher& GetCipher() { return m_cipher; }
+
+    /** Schedule bytes to be sent to the transport. */
+    void Send(Span<const uint8_t> data)
+    {
+        m_to_send.insert(m_to_send.end(), data.begin(), data.end());
+    }
+
+    /** Send V1 version message header to the transport. */
+    void SendV1Version(const CMessageHeader::MessageStartChars& magic)
+    {
+        CMessageHeader hdr(magic, "version", 126 + InsecureRandRange(11));
+        CDataStream ser(SER_NETWORK, CLIENT_VERSION);
+        ser << hdr;
+        m_to_send.insert(m_to_send.end(), UCharCast(ser.data()), UCharCast(ser.data() + ser.size()));
+    }
+
+    /** Schedule bytes to be sent to the transport. */
+    void Send(Span<const std::byte> data) { Send(MakeUCharSpan(data)); }
+
+    /** Schedule our ellswift key to be sent to the transport. */
+    void SendKey() { Send(m_cipher.GetOurPubKey()); }
+
+    /** Schedule specified garbage to be sent to the transport. */
+    void SendGarbage(Span<const uint8_t> garbage)
+    {
+        // Remember the specified garbage (so we can use it for constructing the garbage
+        // authentication packet).
+        m_sent_garbage.assign(garbage.begin(), garbage.end());
+        // Schedule it for sending.
+        Send(m_sent_garbage);
+    }
+
+    /** Schedule garbage (of specified length) to be sent to the transport. */
+    void SendGarbage(size_t garbage_len)
+    {
+        // Generate random garbage and send it.
+        SendGarbage(g_insecure_rand_ctx.randbytes<uint8_t>(garbage_len));
+    }
+
+    /** Schedule garbage (with valid random length) to be sent to the transport. */
+    void SendGarbage()
+    {
+         SendGarbage(InsecureRandRange(V2Transport::MAX_GARBAGE_LEN + 1));
+    }
+
+    /** Schedule a message to be sent to us by the transport. */
+    void AddMessage(std::string m_type, std::vector<uint8_t> payload)
+    {
+        CSerializedNetMsg msg;
+        msg.m_type = std::move(m_type);
+        msg.data = std::move(payload);
+        m_msg_to_send.push_back(std::move(msg));
+    }
+
+    /** Expect ellswift key to have been received from transport and process it.
+     *
+     * Many other V2TransportTester functions cannot be called until after ReceiveKey() has been
+     * called, as no encryption keys are set up before that point.
+     */
+    void ReceiveKey()
+    {
+        // When processing a key, enough bytes need to have been received already.
+        BOOST_REQUIRE(m_received.size() >= EllSwiftPubKey::size());
+        // Initialize the cipher using it (acting as the opposite side of the tested transport).
+        m_cipher.Initialize(MakeByteSpan(m_received).first(EllSwiftPubKey::size()), !m_test_initiator);
+        // Strip the processed bytes off the front of the receive buffer.
+        m_received.erase(m_received.begin(), m_received.begin() + EllSwiftPubKey::size());
+    }
+
+    /** Schedule an encrypted packet with specified content/aad/ignore to be sent to transport
+     *  (only after ReceiveKey). */
+    void SendPacket(Span<const uint8_t> content, Span<const uint8_t> aad = {}, bool ignore = false)
+    {
+        // Use cipher to construct ciphertext.
+        std::vector<std::byte> ciphertext;
+        ciphertext.resize(content.size() + BIP324Cipher::EXPANSION);
+        m_cipher.Encrypt(
+            /*contents=*/MakeByteSpan(content),
+            /*aad=*/MakeByteSpan(aad),
+            /*ignore=*/ignore,
+            /*output=*/ciphertext);
+        // Schedule it for sending.
+        Send(ciphertext);
+    }
+
+    /** Schedule garbage terminator and authentication packet to be sent to the transport (only
+     *  after ReceiveKey). */
+    void SendGarbageTermAuth(size_t garb_auth_data_len = 0, bool garb_auth_ignore = false)
+    {
+        // Generate random data to include in the garbage authentication packet (ignored by peer).
+        auto garb_auth_data = g_insecure_rand_ctx.randbytes<uint8_t>(garb_auth_data_len);
+        // Schedule the garbage terminator to be sent.
+        Send(m_cipher.GetSendGarbageTerminator());
+        // Schedule the garbage authentication packet to be sent.
+        SendPacket(/*content=*/garb_auth_data, /*aad=*/m_sent_garbage, /*ignore=*/garb_auth_ignore);
+    }
+
+    /** Schedule version packet to be sent to the transport (only after ReceiveKey). */
+    void SendVersion(Span<const uint8_t> version_data = {}, bool vers_ignore = false)
+    {
+        SendPacket(/*content=*/version_data, /*aad=*/{}, /*ignore=*/vers_ignore);
+    }
+
+    /** Expect a packet to have been received from transport, process it, and return its contents
+     *  (only after ReceiveKey). By default, decoys are skipped. */
+    std::vector<uint8_t> ReceivePacket(Span<const std::byte> aad = {}, bool skip_decoy = true)
+    {
+        std::vector<uint8_t> contents;
+        // Loop as long as there are ignored packets that are to be skipped.
+        while (true) {
+            // When processing a packet, at least enough bytes for its length descriptor must be received.
+            BOOST_REQUIRE(m_received.size() >= BIP324Cipher::LENGTH_LEN);
+            // Decrypt the content length.
+            size_t size = m_cipher.DecryptLength(MakeByteSpan(Span{m_received}.first(BIP324Cipher::LENGTH_LEN)));
+            // Check that the full packet is in the receive buffer.
+            BOOST_REQUIRE(m_received.size() >= size + BIP324Cipher::EXPANSION);
+            // Decrypt the packet contents.
+            contents.resize(size);
+            bool ignore{false};
+            bool ret = m_cipher.Decrypt(
+                /*input=*/MakeByteSpan(
+                    Span{m_received}.first(size + BIP324Cipher::EXPANSION).subspan(BIP324Cipher::LENGTH_LEN)),
+                /*aad=*/aad,
+                /*ignore=*/ignore,
+                /*contents=*/MakeWritableByteSpan(contents));
+            BOOST_CHECK(ret);
+            // Strip the processed packet's bytes off the front of the receive buffer.
+            m_received.erase(m_received.begin(), m_received.begin() + size + BIP324Cipher::EXPANSION);
+            // Stop if the ignore bit is not set on this packet, or if we choose to not honor it.
+            if (!ignore || !skip_decoy) break;
+        }
+        return contents;
+    }
+
+    /** Expect garbage, garbage terminator, and garbage auth packet to have been received, and
+     *  process them (only after ReceiveKey). */
+    void ReceiveGarbage()
+    {
+        // Figure out the garbage length.
+        size_t garblen;
+        for (garblen = 0; garblen <= V2Transport::MAX_GARBAGE_LEN; ++garblen) {
+            BOOST_REQUIRE(m_received.size() >= garblen + BIP324Cipher::GARBAGE_TERMINATOR_LEN);
+            auto term_span = MakeByteSpan(Span{m_received}.subspan(garblen, BIP324Cipher::GARBAGE_TERMINATOR_LEN));
+            if (term_span == m_cipher.GetReceiveGarbageTerminator()) break;
+        }
+        // Copy the garbage to a buffer.
+        std::vector<uint8_t> garbage(m_received.begin(), m_received.begin() + garblen);
+        // Strip garbage + garbage terminator off the front of the receive buffer.
+        m_received.erase(m_received.begin(), m_received.begin() + garblen + BIP324Cipher::GARBAGE_TERMINATOR_LEN);
+        // Process the expected garbage authentication packet. Such a packet still functions as one
+        // even when its ignore bit is set to true, so we do not skip decoy packets here.
+        ReceivePacket(/*aad=*/MakeByteSpan(garbage), /*skip_decoy=*/false);
+    }
+
+    /** Expect version packet to have been received, and process it (only after ReceiveKey). */
+    void ReceiveVersion()
+    {
+        auto contents = ReceivePacket();
+        // Version packets from real BIP324 peers are expected to be empty, despite the fact that
+        // this class supports *sending* non-empty version packets (to test that BIP324 peers
+        // correctly ignore version packet contents).
+        BOOST_CHECK(contents.empty());
+    }
+
+    /** Expect application packet to have been received, with specified short id and payload.
+     *  (only after ReceiveKey). */
+    void ReceiveMessage(uint8_t short_id, Span<const uint8_t> payload)
+    {
+        auto ret = ReceivePacket();
+        BOOST_CHECK(ret.size() == payload.size() + 1);
+        BOOST_CHECK(ret[0] == short_id);
+        BOOST_CHECK(Span{ret}.subspan(1) == payload);
+    }
+
+    /** Expect application packet to have been received, with specified 12-char message type and
+     *  payload (only after ReceiveKey). */
+    void ReceiveMessage(const std::string& m_type, Span<const uint8_t> payload)
+    {
+        auto ret = ReceivePacket();
+        BOOST_REQUIRE(ret.size() == payload.size() + 1 + CMessageHeader::COMMAND_SIZE);
+        BOOST_CHECK(ret[0] == 0);
+        for (unsigned i = 0; i < 12; ++i) {
+            if (i < m_type.size()) {
+                BOOST_CHECK(ret[1 + i] == m_type[i]);
+            } else {
+                BOOST_CHECK(ret[1 + i] == 0);
+            }
+        }
+        BOOST_CHECK(Span{ret}.subspan(1 + CMessageHeader::COMMAND_SIZE) == payload);
+    }
+
+    /** Schedule an encrypted packet with specified message type and payload to be sent to
+     *  transport (only after ReceiveKey). */
+    void SendMessage(std::string mtype, Span<const uint8_t> payload)
+    {
+        // Construct contents consisting of 0x00 + 12-byte message type + payload.
+        std::vector<uint8_t> contents(1 + CMessageHeader::COMMAND_SIZE + payload.size());
+        std::copy(mtype.begin(), mtype.end(), reinterpret_cast<char*>(contents.data() + 1));
+        std::copy(payload.begin(), payload.end(), contents.begin() + 1 + CMessageHeader::COMMAND_SIZE);
+        // Send a packet with that as contents.
+        SendPacket(contents);
+    }
+
+    /** Schedule an encrypted packet with specified short message id and payload to be sent to
+     *  transport (only after ReceiveKey). */
+    void SendMessage(uint8_t short_id, Span<const uint8_t> payload)
+    {
+        // Construct contents consisting of short_id + payload.
+        std::vector<uint8_t> contents(1 + payload.size());
+        contents[0] = short_id;
+        std::copy(payload.begin(), payload.end(), contents.begin() + 1);
+        // Send a packet with that as contents.
+        SendPacket(contents);
+    }
+
+    /** Introduce a bit error in the data scheduled to be sent. */
+    void Damage()
+    {
+        m_to_send[InsecureRandRange(m_to_send.size())] ^= (uint8_t{1} << InsecureRandRange(8));
+    }
+};
+
+} // namespace
+
+BOOST_AUTO_TEST_CASE(v2transport_test)
+{
+    // A mostly normal scenario, testing a transport in initiator mode.
+    for (int i = 0; i < 10; ++i) {
+        V2TransportTester tester(true);
+        auto ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->empty());
+        tester.SendKey();
+        tester.SendGarbage();
+        tester.ReceiveKey();
+        tester.SendGarbageTermAuth();
+        tester.SendVersion();
+        ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->empty());
+        tester.ReceiveGarbage();
+        tester.ReceiveVersion();
+        auto msg_data_1 = g_insecure_rand_ctx.randbytes<uint8_t>(InsecureRandRange(100000));
+        auto msg_data_2 = g_insecure_rand_ctx.randbytes<uint8_t>(InsecureRandRange(1000));
+        tester.SendMessage(uint8_t(4), msg_data_1); // cmpctblock short id
+        tester.SendMessage(0, {}); // Invalidly encoded message
+        tester.SendMessage("tx", msg_data_2); // 12-character encoded message type
+        ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->size() == 3);
+        BOOST_CHECK((*ret)[0] && (*ret)[0]->m_type == "cmpctblock" && Span{(*ret)[0]->m_recv} == MakeByteSpan(msg_data_1));
+        BOOST_CHECK(!(*ret)[1]);
+        BOOST_CHECK((*ret)[2] && (*ret)[2]->m_type == "tx" && Span{(*ret)[2]->m_recv} == MakeByteSpan(msg_data_2));
+
+        // Then send a message with a bit error, expecting failure.
+        tester.SendMessage("bad", msg_data_1);
+        tester.Damage();
+        ret = tester.Interact();
+        BOOST_CHECK(!ret);
+    }
+
+    // Normal scenario, with a transport in responder node.
+    for (int i = 0; i < 10; ++i) {
+        V2TransportTester tester(false);
+        tester.SendKey();
+        tester.SendGarbage();
+        auto ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->empty());
+        tester.ReceiveKey();
+        tester.SendGarbageTermAuth();
+        tester.SendVersion();
+        ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->empty());
+        tester.ReceiveGarbage();
+        tester.ReceiveVersion();
+        auto msg_data_1 = g_insecure_rand_ctx.randbytes<uint8_t>(InsecureRandRange(100000));
+        auto msg_data_2 = g_insecure_rand_ctx.randbytes<uint8_t>(InsecureRandRange(1000));
+        tester.SendMessage(uint8_t(14), msg_data_1); // inv short id
+        tester.SendMessage(uint8_t(19), msg_data_2); // pong short id
+        ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->size() == 2);
+        BOOST_CHECK((*ret)[0] && (*ret)[0]->m_type == "inv" && Span{(*ret)[0]->m_recv} == MakeByteSpan(msg_data_1));
+        BOOST_CHECK((*ret)[1] && (*ret)[1]->m_type == "pong" && Span{(*ret)[1]->m_recv} == MakeByteSpan(msg_data_2));
+
+        // Then send a too-large message.
+        auto msg_data_3 = g_insecure_rand_ctx.randbytes<uint8_t>(4005000);
+        tester.SendMessage(uint8_t(11), msg_data_3); // getdata short id
+        ret = tester.Interact();
+        BOOST_CHECK(!ret);
+    }
+
+    // Various valid but unusual scenarios.
+    for (int i = 0; i < 50; ++i) {
+        /** Whether an initiator or responder is being tested. */
+        bool initiator = InsecureRandBool();
+        /** Use either 0 bytes or the maximum possible (4095 bytes) garbage length. */
+        size_t garb_len = InsecureRandBool() ? 0 : V2Transport::MAX_GARBAGE_LEN;
+        /** Sometimes, use non-empty contents in the garbage authentication packet (which is to be ignored). */
+        size_t garb_auth_data_len = InsecureRandBool() ? 0 : InsecureRandRange(100000);
+        /** Whether to set the ignore bit on the garbage authentication packet (it still functions as garbage authentication). */
+        bool garb_ignore = InsecureRandBool();
+        /** How many decoy packets to send before the version packet. */
+        unsigned num_ignore_version = InsecureRandRange(10);
+        /** What data to send in the version packet (ignored by BIP324 peers, but reserved for future extensions). */
+        auto ver_data = g_insecure_rand_ctx.randbytes<uint8_t>(InsecureRandBool() ? 0 : InsecureRandRange(1000));
+        /** Whether to immediately send key and garbage out (required for responders, optional otherwise). */
+        bool send_immediately = !initiator || InsecureRandBool();
+        /** How many decoy packets to send before the first and second real message. */
+        unsigned num_decoys_1 = InsecureRandRange(1000), num_decoys_2 = InsecureRandRange(1000);
+        V2TransportTester tester(initiator);
+        if (send_immediately) {
+            tester.SendKey();
+            tester.SendGarbage(garb_len);
+        }
+        auto ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->empty());
+        if (!send_immediately) {
+            tester.SendKey();
+            tester.SendGarbage(garb_len);
+        }
+        tester.ReceiveKey();
+        tester.SendGarbageTermAuth(garb_auth_data_len, garb_ignore);
+        for (unsigned v = 0; v < num_ignore_version; ++v) {
+            size_t ver_ign_data_len = InsecureRandBool() ? 0 : InsecureRandRange(1000);
+            auto ver_ign_data = g_insecure_rand_ctx.randbytes<uint8_t>(ver_ign_data_len);
+            tester.SendVersion(ver_ign_data, true);
+        }
+        tester.SendVersion(ver_data, false);
+        ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->empty());
+        tester.ReceiveGarbage();
+        tester.ReceiveVersion();
+        for (unsigned d = 0; d < num_decoys_1; ++d) {
+            auto decoy_data = g_insecure_rand_ctx.randbytes<uint8_t>(InsecureRandRange(1000));
+            tester.SendPacket(/*content=*/decoy_data, /*aad=*/{}, /*ignore=*/true);
+        }
+        auto msg_data_1 = g_insecure_rand_ctx.randbytes<uint8_t>(InsecureRandRange(4000000));
+        tester.SendMessage(uint8_t(28), msg_data_1);
+        for (unsigned d = 0; d < num_decoys_2; ++d) {
+            auto decoy_data = g_insecure_rand_ctx.randbytes<uint8_t>(InsecureRandRange(1000));
+            tester.SendPacket(/*content=*/decoy_data, /*aad=*/{}, /*ignore=*/true);
+        }
+        auto msg_data_2 = g_insecure_rand_ctx.randbytes<uint8_t>(InsecureRandRange(1000));
+        tester.SendMessage(uint8_t(13), msg_data_2); // headers short id
+        // Send invalidly-encoded message
+        tester.SendMessage(std::string("blocktxn\x00\x00\x00a", CMessageHeader::COMMAND_SIZE), {});
+        tester.SendMessage("foobar", {}); // test receiving unknown message type
+        tester.AddMessage("barfoo", {}); // test sending unknown message type
+        ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->size() == 4);
+        BOOST_CHECK((*ret)[0] && (*ret)[0]->m_type == "addrv2" && Span{(*ret)[0]->m_recv} == MakeByteSpan(msg_data_1));
+        BOOST_CHECK((*ret)[1] && (*ret)[1]->m_type == "headers" && Span{(*ret)[1]->m_recv} == MakeByteSpan(msg_data_2));
+        BOOST_CHECK(!(*ret)[2]);
+        BOOST_CHECK((*ret)[3] && (*ret)[3]->m_type == "foobar" && (*ret)[3]->m_recv.empty());
+        tester.ReceiveMessage("barfoo", {});
+    }
+
+    // Too long garbage (initiator).
+    {
+        V2TransportTester tester(true);
+        auto ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->empty());
+        tester.SendKey();
+        tester.SendGarbage(V2Transport::MAX_GARBAGE_LEN + 1);
+        tester.ReceiveKey();
+        tester.SendGarbageTermAuth();
+        ret = tester.Interact();
+        BOOST_CHECK(!ret);
+    }
+
+    // Too long garbage (responder).
+    {
+        V2TransportTester tester(false);
+        tester.SendKey();
+        tester.SendGarbage(V2Transport::MAX_GARBAGE_LEN + 1);
+        auto ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->empty());
+        tester.ReceiveKey();
+        tester.SendGarbageTermAuth();
+        ret = tester.Interact();
+        BOOST_CHECK(!ret);
+    }
+
+    // Send garbage that includes the first 15 garbage terminator bytes somewhere.
+    {
+        V2TransportTester tester(true);
+        auto ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->empty());
+        tester.SendKey();
+        tester.ReceiveKey();
+        /** The number of random garbage bytes before the included first 15 bytes of terminator. */
+        size_t len_before = InsecureRandRange(V2Transport::MAX_GARBAGE_LEN - 16 + 1);
+        /** The number of random garbage bytes after it. */
+        size_t len_after = InsecureRandRange(V2Transport::MAX_GARBAGE_LEN - 16 - len_before + 1);
+        // Construct len_before + 16 + len_after random bytes.
+        auto garbage = g_insecure_rand_ctx.randbytes<uint8_t>(len_before + 16 + len_after);
+        // Replace the designed 16 bytes in the middle with the to-be-sent garbage terminator.
+        auto garb_term = MakeUCharSpan(tester.GetCipher().GetSendGarbageTerminator());
+        std::copy(garb_term.begin(), garb_term.begin() + 16, garbage.begin() + len_before);
+        // Introduce a bit error in the last byte of that copied garbage terminator, making only
+        // the first 15 of them match.
+        garbage[len_before + 15] ^= (uint8_t(1) << InsecureRandRange(8));
+        tester.SendGarbage(garbage);
+        tester.SendGarbageTermAuth();
+        tester.SendVersion();
+        ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->empty());
+        tester.ReceiveGarbage();
+        tester.ReceiveVersion();
+        auto msg_data_1 = g_insecure_rand_ctx.randbytes<uint8_t>(4000000); // test that receiving 4M payload works
+        auto msg_data_2 = g_insecure_rand_ctx.randbytes<uint8_t>(4000000); // test that sending 4M payload works
+        tester.SendMessage(uint8_t(InsecureRandRange(223) + 33), {}); // unknown short id
+        tester.SendMessage(uint8_t(2), msg_data_1); // "block" short id
+        tester.AddMessage("blocktxn", msg_data_2); // schedule blocktxn to be sent to us
+        ret = tester.Interact();
+        BOOST_REQUIRE(ret && ret->size() == 2);
+        BOOST_CHECK(!(*ret)[0]);
+        BOOST_CHECK((*ret)[1] && (*ret)[1]->m_type == "block" && Span{(*ret)[1]->m_recv} == MakeByteSpan(msg_data_1));
+        tester.ReceiveMessage(uint8_t(3), msg_data_2); // "blocktxn" short id
+    }
+
+    // Send correct network's V1 header
+    {
+        V2TransportTester tester(false);
+        tester.SendV1Version(Params().MessageStart());
+        auto ret = tester.Interact();
+        BOOST_CHECK(ret);
+    }
+
+    // Send wrong network's V1 header
+    {
+        V2TransportTester tester(false);
+        tester.SendV1Version(CChainParams::Main()->MessageStart());
+        auto ret = tester.Interact();
+        BOOST_CHECK(!ret);
+    }
+}
+
 BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/test/util/net.cpp b/src/test/util/net.cpp
index 5696f8d13c..dc64c0b4c1 100644
--- a/src/test/util/net.cpp
+++ b/src/test/util/net.cpp
@@ -78,7 +78,7 @@ void ConnmanTestMsg::FlushSendBuffer(CNode& node) const
     node.vSendMsg.clear();
     node.m_send_memusage = 0;
     while (true) {
-        const auto& [to_send, _more, _msg_type] = node.m_transport->GetBytesToSend();
+        const auto& [to_send, _more, _msg_type] = node.m_transport->GetBytesToSend(false);
         if (to_send.empty()) break;
         node.m_transport->MarkBytesSent(to_send.size());
     }
@@ -90,7 +90,7 @@ bool ConnmanTestMsg::ReceiveMsgFrom(CNode& node, CSerializedNetMsg&& ser_msg) co
     assert(queued);
     bool complete{false};
     while (true) {
-        const auto& [to_send, _more, _msg_type] = node.m_transport->GetBytesToSend();
+        const auto& [to_send, _more, _msg_type] = node.m_transport->GetBytesToSend(false);
         if (to_send.empty()) break;
         NodeReceiveMsgBytes(node, to_send, complete);
         node.m_transport->MarkBytesSent(to_send.size());