feat: Phase 7 — Node v2 (multi-group, Sender Keys, 0-RTT, chat, denylist)

Implements all 8 milestones (7.0-7.7):

- 7.0: JWT carries `groups` claim; node verifies group membership at
  MNP handshake (QUIC + TCP+TLS). Resolves security review C2.
- 7.1: QUIC 0-RTT session resumption via stored session tickets
  (17-21ms reconnect vs 47ms cold).
- 7.2: Hub→node WebSocket signaling for NAT punch coordination
  (`client_incoming`/`punch_ready`) + jti denylist push. Denylist
  class blocks revoked users/jtis at handshake.
- 7.3: Multi-group daemon — one QUIC port serves N groups with
  per-group GEK, shared_root, and index routing.
- 7.4: HLS streaming via QUIC (STREAM_SEGMENT message type, ffmpeg
  segment extraction).
- 7.5: Sender Keys protocol for group chat (Signal Groups approach).
  Each member has own sending chain key, HKDF chain ratchet, AES-256-GCM
  encryption, Ed25519 signing. Resolves security review C1.
- 7.6: Chat store (SQLite via aiosqlite), CHAT_MESSAGE MNP wire type
  with peer broadcast, web UI with WebSocket push.
- 7.7: Argon2id calibration CLI.

First security review included (first-review.md). 109 tests, demo-v3
validated against meshbay.org production hub.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

2 files changed, 288 insertions, 1 deletions

diff --git a/packages/meshbay-common/src/meshbay_common/crypto.py b/packages/meshbay-common/src/meshbay_common/crypto.py
index 2104469..6066f5f 100644
--- a/packages/meshbay-common/src/meshbay_common/crypto.py
+++ b/packages/meshbay-common/src/meshbay_common/crypto.py
@@ -145,7 +145,7 @@ def derive_keystore_key(password: str, salt: bytes) -> bytes:
 
 def encrypt_keystore(plaintext: bytes, key: bytes) -> tuple[bytes, bytes, bytes]:
     """Encrypt keystore blob with AES-256-GCM. Returns (iv, ciphertext, tag)."""
-    iv = os.urandom(16)
+    iv = os.urandom(12)
     enc = Cipher(algorithms.AES(key), modes.GCM(iv)).encryptor()
     ct  = enc.update(plaintext) + enc.finalize()
     return iv, ct, enc.tag
diff --git a/packages/meshbay-common/src/meshbay_common/senderkeys.py b/packages/meshbay-common/src/meshbay_common/senderkeys.py
new file mode 100644
index 0000000..932e2e6
--- /dev/null
+++ b/packages/meshbay-common/src/meshbay_common/senderkeys.py
@@ -0,0 +1,287 @@
+"""
+MeshBay — Sender Keys protocol for group messaging.
+
+Signal Groups approach: each member maintains their own sending chain.
+Advantages over shared Double Ratchet:
+  - O(N) state per group (one chain per member) vs O(N^2) pairwise
+  - Single encrypt per message (not N encryptions)
+  - No key/nonce reuse — each sender has an independent chain
+
+Key components:
+  - Chain key ratchet: HKDF per message, provides forward secrecy
+  - Message key derivation: separate HKDF from chain key
+  - Ed25519 signing: each sender signs their ciphertext
+  - AES-256-GCM encryption: browser-compatible symmetric cipher
+
+Key distribution:
+  - On join: admin wraps each sender's SenderKeyDistribution with GEK
+  - On leave: all remaining members rotate their chain keys
+"""
+
+import os
+import struct
+from dataclasses import dataclass, field
+
+from cryptography.hazmat.primitives.asymmetric.ed25519 import (
+    Ed25519PrivateKey,
+    Ed25519PublicKey,
+)
+from cryptography.hazmat.primitives.ciphers.aead import AESGCM
+from cryptography.hazmat.primitives.kdf.hkdf import HKDF
+from cryptography.hazmat.primitives import hashes, serialization
+
+
+CHAIN_INFO = b"meshbay:sk:chain:v1"
+MSG_KEY_INFO = b"meshbay:sk:msg:v1"
+CHAIN_KEY_LEN = 32
+MSG_KEY_LEN = 32
+MAX_SKIP = 256
+
+
+def _hkdf(ikm: bytes, info: bytes, length: int = 32) -> bytes:
+    return HKDF(
+        algorithm=hashes.SHA256(), length=length, salt=None, info=info,
+    ).derive(ikm)
+
+
+def _ratchet_chain(chain_key: bytes) -> tuple[bytes, bytes]:
+    """Advance chain key → (new_chain_key, message_key)."""
+    new_ck = _hkdf(chain_key, CHAIN_INFO, CHAIN_KEY_LEN)
+    mk = _hkdf(chain_key, MSG_KEY_INFO, MSG_KEY_LEN)
+    return new_ck, mk
+
+
+# ── Data structures ──────────────────────────────────────────────────────────
+
+@dataclass
+class SenderKeyDistribution:
+    """Sent to group members when a sender joins or rotates."""
+    sender_id: str
+    chain_key: bytes       # 32-byte initial chain key
+    iteration: int         # current message counter
+    signing_pk: bytes      # 32-byte raw Ed25519 public key
+
+    def serialize(self) -> bytes:
+        sender_bytes = self.sender_id.encode()
+        return (
+            struct.pack(">H", len(sender_bytes))
+            + sender_bytes
+            + self.chain_key
+            + struct.pack(">I", self.iteration)
+            + self.signing_pk
+        )
+
+    @classmethod
+    def deserialize(cls, data: bytes) -> "SenderKeyDistribution":
+        sender_len = struct.unpack(">H", data[:2])[0]
+        offset = 2
+        sender_id = data[offset:offset + sender_len].decode()
+        offset += sender_len
+        chain_key = data[offset:offset + 32]
+        offset += 32
+        iteration = struct.unpack(">I", data[offset:offset + 4])[0]
+        offset += 4
+        signing_pk = data[offset:offset + 32]
+        return cls(sender_id=sender_id, chain_key=chain_key,
+                   iteration=iteration, signing_pk=signing_pk)
+
+
+@dataclass
+class SenderKeyState:
+    """One sender's chain state as seen by any group member."""
+    sender_id: str
+    chain_key: bytes
+    iteration: int
+    signing_key: Ed25519PublicKey
+    _skipped_keys: dict[int, bytes] = field(default_factory=dict)
+
+    @classmethod
+    def from_distribution(cls, dist: SenderKeyDistribution) -> "SenderKeyState":
+        pk = Ed25519PublicKey.from_public_bytes(dist.signing_pk)
+        return cls(
+            sender_id=dist.sender_id,
+            chain_key=dist.chain_key,
+            iteration=dist.iteration,
+            signing_key=pk,
+        )
+
+    def advance_to(self, target: int) -> bytes:
+        """Advance chain to target iteration, caching skipped keys. Returns message key."""
+        if target < self.iteration:
+            mk = self._skipped_keys.pop(target, None)
+            if mk is None:
+                raise ValueError(f"Message key {target} already consumed or too old")
+            return mk
+
+        skip_count = target - self.iteration
+        if skip_count > MAX_SKIP:
+            raise ValueError(f"Too many skipped messages: {skip_count}")
+
+        for i in range(skip_count):
+            new_ck, mk = _ratchet_chain(self.chain_key)
+            self._skipped_keys[self.iteration] = mk
+            self.chain_key = new_ck
+            self.iteration += 1
+
+        new_ck, mk = _ratchet_chain(self.chain_key)
+        self.chain_key = new_ck
+        self.iteration += 1
+        return mk
+
+
+@dataclass
+class SenderKeyRecord:
+    """Sender's own key state (includes signing private key)."""
+    sender_id: str
+    chain_key: bytes
+    iteration: int
+    signing_sk: Ed25519PrivateKey
+
+    @classmethod
+    def create(cls, sender_id: str) -> "SenderKeyRecord":
+        return cls(
+            sender_id=sender_id,
+            chain_key=os.urandom(CHAIN_KEY_LEN),
+            iteration=0,
+            signing_sk=Ed25519PrivateKey.generate(),
+        )
+
+    def distribution(self) -> SenderKeyDistribution:
+        pk_raw = self.signing_sk.public_key().public_bytes(
+            serialization.Encoding.Raw, serialization.PublicFormat.Raw)
+        return SenderKeyDistribution(
+            sender_id=self.sender_id,
+            chain_key=self.chain_key,
+            iteration=self.iteration,
+            signing_pk=pk_raw,
+        )
+
+    def rotate(self) -> "SenderKeyRecord":
+        """Create a new record with fresh chain key (call on member removal)."""
+        return SenderKeyRecord(
+            sender_id=self.sender_id,
+            chain_key=os.urandom(CHAIN_KEY_LEN),
+            iteration=0,
+            signing_sk=Ed25519PrivateKey.generate(),
+        )
+
+
+# ── Group store ──────────────────────────────────────────────────────────────
+
+class GroupSenderKeyStore:
+    """All sender key states for one group, held by one member."""
+
+    def __init__(self, group_id: str):
+        self.group_id = group_id
+        self._states: dict[str, SenderKeyState] = {}
+
+    def add_sender(self, dist: SenderKeyDistribution) -> None:
+        self._states[dist.sender_id] = SenderKeyState.from_distribution(dist)
+
+    def remove_sender(self, sender_id: str) -> None:
+        self._states.pop(sender_id, None)
+
+    def get_state(self, sender_id: str) -> SenderKeyState | None:
+        return self._states.get(sender_id)
+
+    @property
+    def sender_count(self) -> int:
+        return len(self._states)
+
+
+# ── Encrypt / Decrypt ────────────────────────────────────────────────────────
+
+@dataclass
+class SenderKeyMessage:
+    """Wire format for a Sender Keys encrypted message."""
+    sender_id: str
+    iteration: int
+    ciphertext: bytes
+    nonce: bytes
+    signature: bytes
+
+    def serialize(self) -> bytes:
+        sender_bytes = self.sender_id.encode()
+        return (
+            struct.pack(">H", len(sender_bytes))
+            + sender_bytes
+            + struct.pack(">I", self.iteration)
+            + struct.pack(">I", len(self.ciphertext))
+            + self.ciphertext
+            + self.nonce
+            + self.signature
+        )
+
+    @classmethod
+    def deserialize(cls, data: bytes) -> "SenderKeyMessage":
+        offset = 0
+        sender_len = struct.unpack(">H", data[offset:offset + 2])[0]
+        offset += 2
+        sender_id = data[offset:offset + sender_len].decode()
+        offset += sender_len
+        iteration = struct.unpack(">I", data[offset:offset + 4])[0]
+        offset += 4
+        ct_len = struct.unpack(">I", data[offset:offset + 4])[0]
+        offset += 4
+        ciphertext = data[offset:offset + ct_len]
+        offset += ct_len
+        nonce = data[offset:offset + 12]
+        offset += 12
+        signature = data[offset:offset + 64]
+        return cls(sender_id=sender_id, iteration=iteration,
+                   ciphertext=ciphertext, nonce=nonce, signature=signature)
+
+
+def encrypt_message(
+    record: SenderKeyRecord,
+    plaintext: bytes,
+    aad: bytes = b"",
+) -> tuple[SenderKeyMessage, SenderKeyRecord]:
+    """
+    Encrypt a message with the sender's chain key.
+    Returns (message, updated_record).
+    """
+    new_ck, mk = _ratchet_chain(record.chain_key)
+    iteration = record.iteration
+
+    nonce = os.urandom(12)
+    ct = AESGCM(mk).encrypt(nonce, plaintext, aad or None)
+
+    sig_payload = struct.pack(">I", iteration) + nonce + ct
+    signature = record.signing_sk.sign(sig_payload)
+
+    msg = SenderKeyMessage(
+        sender_id=record.sender_id,
+        iteration=iteration,
+        ciphertext=ct,
+        nonce=nonce,
+        signature=signature,
+    )
+
+    updated = SenderKeyRecord(
+        sender_id=record.sender_id,
+        chain_key=new_ck,
+        iteration=iteration + 1,
+        signing_sk=record.signing_sk,
+    )
+    return msg, updated
+
+
+def decrypt_message(
+    store: GroupSenderKeyStore,
+    msg: SenderKeyMessage,
+    aad: bytes = b"",
+) -> bytes:
+    """
+    Decrypt and verify a Sender Keys message.
+    Advances the sender's chain state in the store.
+    """
+    state = store.get_state(msg.sender_id)
+    if state is None:
+        raise ValueError(f"Unknown sender: {msg.sender_id}")
+
+    sig_payload = struct.pack(">I", msg.iteration) + msg.nonce + msg.ciphertext
+    state.signing_key.verify(msg.signature, sig_payload)
+
+    mk = state.advance_to(msg.iteration)
+    return AESGCM(mk).decrypt(msg.nonce, msg.ciphertext, aad or None)