Carry post-quantum through every ratchet step, not just the handshake

iOS mixed a fresh ML-KEM secret into every ratchet step; Android used it only in X3DH. A conversation gets the property only if both ends do, so nobody had it — which is what the comparison page now says, and this is what would let that row change. The gap is not academic. v2 already protects a whole session against harvest-now-decrypt-later, because the root chain never recovers once an adversary has missed the start. What v2 lacks is post-quantum HEALING: after a compromise, recovery rests entirely on X25519 steps, which a quantum adversary breaks. The session never heals against that attacker. Implemented to §13 rather than by reading the Swift: PQs/PQr/PQpending on the session, KDF_RK_v3 over `dh_out || ss` with its own info string, and the two optional header fields. 🔴 Decapsulation happens BEFORE adopting the peer's new PQ key. They encapsulated to the key we held going into the step, and reversing those two derives a different root key with no error until an AEAD open fails somewhere later. ⚠️ The PQ block repeats on every message of a sending chain until the peer answers. Sending it once would mean losing that one message strands the chain: the receiver never gets the material to advance its root key and nothing later in the chain can be opened. It is replaced by the next ratchet step, not cleared when the peer replies — it belongs to the sending chain. Seven tests. Six drive conversations; the seventh pins the canonical header as bytes, because the header is the AAD and three engines must produce it identically. A conversation test cannot catch a reordered field — both sides would drift together. It also pins that half a PQ step falls back to the v2 shape rather than emitting a malformed header. A peer with no PQ material still runs v2 and emits no PQ fields (§13.4), and an archive written by an older build simply has no PQ state and stays on v2. Closes #2 Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
2026-08-31 11:17:04 -04:00
parent 9ae7b5473d
commit 98342dd608
3 changed files with 334 additions and 16 deletions
@@ -47,6 +47,24 @@ class DoubleRatchet private constructor(
    private var received: Int,
    /** Length of the PREVIOUS sending chain, so the far side knows how many to skip. */
    private var previousChainLength: Int,
+    /**
+     * Our current ML-KEM keypair (spec §13.2 `PQs`). Null for a v1/v2 session,
+     * which is what a peer that does not advertise v3 gets.
+     */
+    private var pqSelf: PQKEM.KeyPair? = null,
+    /** The peer's current ML-KEM public key (`PQr`). */
+    private var pqRemote: ByteArray? = null,
+    /**
+     * `{pub, ct}` to attach to outgoing messages (`PQpending`).
+     *
+     * ⚠️ **Repeated on every message of the sending chain, not just the first
+     * (§13.3).** Sending it once would mean losing that single message strands
+     * the whole chain: the receiver never gets the material to advance its root
+     * key, and nothing later in the chain can be decrypted. It is replaced by
+     * the next ratchet step, and is NOT cleared when the peer answers — it
+     * belongs to the sending chain.
+     */
+    private var pqPending: Pair<ByteArray, ByteArray>? = null,
 ) {

    /** Keys for messages that arrived out of order, or have not arrived yet. */
@@ -54,7 +72,10 @@ class DoubleRatchet private constructor(

    data class SkippedKey(val ratchetPublic: String, val index: Int)

-    data class Header(val dh: ByteArray, val pn: Int, val n: Int) {
+    data class Header(
+        val dh: ByteArray, val pn: Int, val n: Int,
+        val pqdh: ByteArray? = null, val pqct: ByteArray? = null,
+    ) {
        /**
         * The exact bytes that go into the AAD.
         *
@@ -63,15 +84,32 @@ class DoubleRatchet private constructor(
         * free to reorder keys or add spaces between releases. Key order is
         * `dh, pn, n` and there are no spaces — both are part of the protocol.
         */
-        fun canonicalJson(): ByteArray =
-            ("{\"dh\":\"" + Bytes.b64(dh) + "\",\"pn\":" + pn + ",\"n\":" + n + "}")
-                .toByteArray(Charsets.UTF_8)
+        fun canonicalJson(): ByteArray {
+            val sb = StringBuilder("{\"dh\":\"").append(Bytes.b64(dh))
+                .append("\",\"pn\":").append(pn).append(",\"n\":").append(n)
+            // 🔴 Omitted ENTIRELY when absent, never emitted as null (§13.3).
+            // The AAD is reproduced byte-for-byte by three implementations; a
+            // header that spelled absence differently would fail every open.
+            if (pqdh != null && pqct != null) {
+                sb.append(",\"pqdh\":\"").append(Bytes.b64(pqdh))
+                  .append("\",\"pqct\":\"").append(Bytes.b64(pqct)).append("\"")
+            }
+            return sb.append("}").toString().toByteArray(Charsets.UTF_8)
+        }
    }

    data class Message(val header: Header, val ciphertext: ByteArray)

    companion object {
        private val RATCHET_INFO = "FlamenetE2E_Ratchet_v1".toByteArray(Charsets.UTF_8)
+
+        /**
+         * 🔴 **The info string MUST change for v3 (spec §13.2).** A v3 IKM is
+         * `dh_out || ss` — 64 bytes where v1 is 32 — so the two cannot collide
+         * by length, but the version string is what makes the separation
+         * explicit rather than incidental.
+         */
+        private val RATCHET_INFO_V3 = "FlamenetE2E_Ratchet_v3".toByteArray(Charsets.UTF_8)
        private val MSGKEY_INFO = "FlamenetE2E_MsgKey_v1".toByteArray(Charsets.UTF_8)
        private val ZERO_SALT = ByteArray(32)

@@ -99,13 +137,32 @@ class DoubleRatchet private constructor(
        private const val NONCE_LEN = 12

        /** Alice, who has just completed X3DH and knows Bob's signed prekey. */
-        fun initiator(sharedSecret: ByteArray, remoteSignedPreKey: ByteArray): DoubleRatchet {
+        fun initiator(
+            sharedSecret: ByteArray,
+            remoteSignedPreKey: ByteArray,
+            /**
+             * The PQ prekey X3DH encapsulated to (§12.5) — the SAME key, which
+             * is what makes it a valid `PQr` to start from. Null runs v2.
+             */
+            remotePQPreKey: ByteArray? = null,
+        ): DoubleRatchet {
            val self = X25519.generate()
-            val (rk, ck) = rootStep(sharedSecret, X25519.agree(self.private, remoteSignedPreKey))
+            var pqSelf: PQKEM.KeyPair? = null
+            var pqPending: Pair<ByteArray, ByteArray>? = null
+            var ss: ByteArray? = null
+            if (remotePQPreKey != null) {
+                pqSelf = PQKEM.generate()
+                val enc = PQKEM.encapsulate(remotePQPreKey)
+                ss = enc.sharedSecret
+                pqPending = pqSelf.publicKey to enc.ciphertext
+            }
+            val (rk, ck) = rootStep(
+                sharedSecret, X25519.agree(self.private, remoteSignedPreKey), ss)
            return DoubleRatchet(
                rootKey = rk, sendingChain = ck, receivingChain = null,
                selfRatchet = self, remoteRatchet = remoteSignedPreKey,
                sent = 0, received = 0, previousChainLength = 0,
+                pqSelf = pqSelf, pqRemote = remotePQPreKey, pqPending = pqPending,
            )
        }

@@ -115,11 +172,21 @@ class DoubleRatchet private constructor(
         * He does not step the root yet: he has nothing to step it with until a
         * message from Alice tells him her ratchet public key.
         */
-        fun responder(sharedSecret: ByteArray, signedPreKey: java.security.KeyPair): DoubleRatchet =
+        fun responder(
+            sharedSecret: ByteArray,
+            signedPreKey: java.security.KeyPair,
+            /**
+             * The seed of the PQ prekey the initiator encapsulated to, so the
+             * first inbound ratchet step can decapsulate. `PQr` is adopted from
+             * that first header (§13.2).
+             */
+            pqPreKeySeed: ByteArray? = null,
+        ): DoubleRatchet =
            DoubleRatchet(
                rootKey = sharedSecret, sendingChain = null, receivingChain = null,
                selfRatchet = signedPreKey, remoteRatchet = null,
                sent = 0, received = 0, previousChainLength = 0,
+                pqSelf = pqPreKeySeed?.let(PQKEM::fromSeed),
            )

        /** The other half of [archive]. */
@@ -141,6 +208,13 @@ class DoubleRatchet private constructor(
                sent = archive.getInt("ns"),
                received = archive.getInt("nr"),
                previousChainLength = archive.getInt("pn"),
+                // Rebuilt from the seed, so an archive written by an older
+                // build simply has none and the session runs v2.
+                pqSelf = opt("pqs")?.let(PQKEM::fromSeed),
+                pqRemote = opt("pqr"),
+                pqPending = opt("pqPendingPub")?.let { pub ->
+                    opt("pqPendingCt")?.let { ct -> pub to ct }
+                },
            )
            val skipped = archive.optJSONArray("skipped") ?: org.json.JSONArray()
            for (i in 0 until skipped.length()) {
@@ -151,8 +225,12 @@ class DoubleRatchet private constructor(
            return ratchet
        }

-        private fun rootStep(rootKey: ByteArray, dhOutput: ByteArray): Pair<ByteArray, ByteArray> {
-            val out = Hkdf.derive(dhOutput, rootKey, RATCHET_INFO, 64)
+        private fun rootStep(
+            rootKey: ByteArray, dhOutput: ByteArray, ss: ByteArray? = null,
+        ): Pair<ByteArray, ByteArray> {
+            val ikm = if (ss == null) dhOutput else dhOutput + ss
+            val info = if (ss == null) RATCHET_INFO else RATCHET_INFO_V3
+            val out = Hkdf.derive(ikm, rootKey, info, 64)
            return out.copyOfRange(0, 32) to out.copyOfRange(32, 64)
        }

@@ -184,6 +262,8 @@ class DoubleRatchet private constructor(
            dh = X25519.rawFromPublicKey(selfRatchet!!.public),
            pn = previousChainLength,
            n = sent,
+            pqdh = pqPending?.first,
+            pqct = pqPending?.second,
        )
        sent += 1

@@ -224,7 +304,7 @@ class DoubleRatchet private constructor(
        // then turn the DH ratchet.
        if (remoteRatchet == null || !message.header.dh.contentEquals(remoteRatchet)) {
            skipTo(message.header.pn)
-            dhRatchet(message.header.dh)
+            dhRatchet(message.header.dh, message.header.pqdh, message.header.pqct)
        }
        skipTo(message.header.n)

@@ -265,18 +345,40 @@ class DoubleRatchet private constructor(
     * their new key, a fresh keypair of our own, and a second root step to open
     * the sending chain they will decrypt with.
     */
-    private fun dhRatchet(theirRatchet: ByteArray) {
+    private fun dhRatchet(
+        theirRatchet: ByteArray, pqdh: ByteArray? = null, pqct: ByteArray? = null,
+    ) {
        previousChainLength = sent
        sent = 0
        received = 0
+
+        // 🔴 **Decapsulate BEFORE adopting their new PQ key, and getting this
+        // order wrong is SILENT.** The peer encapsulated to the key we held
+        // going into this step, so `pqSelf` here must still be the old one
+        // (§13.2). Reversing these derives a different root key with no error
+        // until an AEAD open fails somewhere later.
+        val ssRecv = if (pqct != null) pqSelf?.let { PQKEM.decapsulate(pqct, it.seed) } else null
        remoteRatchet = theirRatchet

-        var (rk, ck) = rootStep(rootKey, X25519.agree(selfRatchet!!.private, theirRatchet))
+        val (rk, ck) = rootStep(
+            rootKey, X25519.agree(selfRatchet!!.private, theirRatchet), ssRecv)
        rootKey = rk
        receivingChain = ck

+        // Now adopt theirs and encapsulate to it for the sending chain.
+        var ssSend: ByteArray? = null
+        if (pqdh != null) {
+            pqRemote = pqdh
+            val mine = PQKEM.generate()
+            pqSelf = mine
+            val enc = PQKEM.encapsulate(pqdh)
+            ssSend = enc.sharedSecret
+            pqPending = mine.publicKey to enc.ciphertext
+        }
+
        selfRatchet = X25519.generate()
-        val stepped = rootStep(rootKey, X25519.agree(selfRatchet!!.private, theirRatchet))
+        val stepped = rootStep(
+            rootKey, X25519.agree(selfRatchet!!.private, theirRatchet), ssSend)
        rootKey = stepped.first
        sendingChain = stepped.second
    }
@@ -312,6 +414,14 @@ class DoubleRatchet private constructor(
            .put("ns", sent)
            .put("nr", received)
            .put("pn", previousChainLength)
+            // ⚠️ A v3 session that lost these on a restart would silently drop
+            // to X25519-only healing: the ratchet keeps working, so nothing
+            // fails, and the post-quantum property is just gone.
+            .put("pqs", pqSelf?.let { Bytes.b64(it.seed) })
+            .put("pqsPub", pqSelf?.let { Bytes.b64(it.publicKey) })
+            .put("pqr", pqRemote?.let { Bytes.b64(it) })
+            .put("pqPendingPub", pqPending?.let { Bytes.b64(it.first) })
+            .put("pqPendingCt", pqPending?.let { Bytes.b64(it.second) })
            .put("skipped", skippedJson)
    }
 }
@@ -91,7 +91,11 @@ class E2ESession private constructor(
                .put("opk_id", bundle.oneTimePreKeyId)

            return E2ESession(
-                ratchet = DoubleRatchet.initiator(initiated.sharedSecret, bundle.signedPreKey),
+                // 🔴 `chosen?.publicKey` is the PQ prekey X3DH just encapsulated
+                // to, and the ratchet has to start from that exact key or the
+                // responder cannot decapsulate the first step (§13.2).
+                ratchet = DoubleRatchet.initiator(
+                    initiated.sharedSecret, bundle.signedPreKey, chosen?.publicKey),
                associatedData = X3DH.associatedData(
                    identity.identitySigningPublic, bundle.identitySigning,
                    initiated.pqCiphertext),
@@ -150,7 +154,9 @@ class E2ESession private constructor(
            )

            val session = E2ESession(
-                ratchet = DoubleRatchet.responder(sharedSecret, signedPreKey),
+                // The seed of the PQ prekey the initiator used, so the first
+                // inbound ratchet step can decapsulate to it.
+                ratchet = DoubleRatchet.responder(sharedSecret, signedPreKey, pq?.first),
                associatedData = X3DH.associatedData(
                    theirIdentitySigning, identity.identitySigningPublic, pq?.second),
                pendingHandshake = null,
@@ -373,7 +379,11 @@ data class Envelope(
        .put("header", JSONObject()
            .put("dh", Bytes.b64(header.dh))
            .put("pn", header.pn)
-            .put("n", header.n))
+            .put("n", header.n)
+            // §13.3 — present only on a PQ step. `JSONObject.put(String, null)`
+            // omits the key, which is exactly what the canonical form needs.
+            .put("pqdh", header.pqdh?.let(Bytes::b64))
+            .put("pqct", header.pqct?.let(Bytes::b64)))
        .put("x3dh", handshake)
        .put("pq", pq?.let {
            JSONObject().put("kind", it.kind).put("id", it.id)
@@ -397,6 +407,8 @@ data class Envelope(
                    dh = Bytes.unb64(header.getString("dh")),
                    pn = header.getInt("pn"),
                    n = header.getInt("n"),
+                    pqdh = header.optString("pqdh").ifEmpty { null }?.let(Bytes::unb64),
+                    pqct = header.optString("pqct").ifEmpty { null }?.let(Bytes::unb64),
                ),
                handshake = json.optJSONObject("x3dh"),
                pq = json.optJSONObject("pq")?.let {
@@ -47,6 +47,202 @@ class DoubleRatchetTest {
        return alice to bob
    }

+    /**
+     * The same pair, running protocol v3 — a fresh ML-KEM secret mixed into
+     * every ratchet step (§13).
+     *
+     * Bob's PQ prekey is the shared starting point: Alice encapsulates to it,
+     * and Bob holds its seed so he can decapsulate the first step.
+     */
+    private fun pqPair(): Triple<DoubleRatchet, DoubleRatchet, PQKEM.KeyPair> {
+        val bobSigned = X25519.generate()
+        val aliceIdentity = X25519.generate()
+        val bobIdentity = X25519.generate()
+        val bobPQ = PQKEM.generate()
+
+        val initiated = X3DH.initiate(
+            aliceIdentity.private,
+            X3DH.Bundle(
+                identityKeyDH = X25519.rawFromPublicKey(bobIdentity.public),
+                signedPreKey = X25519.rawFromPublicKey(bobSigned.public),
+                oneTimePreKey = null,
+            ),
+            bobPQ = bobPQ.publicKey,
+        )
+        val bobSecret = X3DH.respond(
+            identityPrivateDH = bobIdentity.private,
+            signedPreKeyPrivate = bobSigned.private,
+            oneTimePrivate = null,
+            initiatorIdentityDH = X25519.rawFromPublicKey(aliceIdentity.public),
+            initiatorEphemeral = initiated.ephemeralPublic,
+            pq = bobPQ.seed to initiated.pqCiphertext!!,
+        )
+        assertContentEquals(initiated.sharedSecret, bobSecret, "PQ X3DH disagreed")
+
+        val alice = DoubleRatchet.initiator(
+            initiated.sharedSecret, X25519.rawFromPublicKey(bobSigned.public), bobPQ.publicKey)
+        val bob = DoubleRatchet.responder(bobSecret, bobSigned, bobPQ.seed)
+        return Triple(alice, bob, bobPQ)
+    }
+
+    // MARK: - Protocol v3, the post-quantum ratchet (§13)
+
+    /**
+     * 🔴 **The cross-engine contract, pinned as bytes.**
+     *
+     * The header goes into the AAD, so three implementations must produce
+     * byte-identical JSON or nothing opens across engines. Key order is
+     * `dh, pn, n, pqdh, pqct`, there are no spaces, and absent PQ fields are
+     * omitted ENTIRELY rather than written as null (§13.3).
+     *
+     * This is the assertion that would catch a JSON library being introduced
+     * here, or a field being reordered — neither of which any conversation
+     * test can see, because both sides would drift together.
+     */
+    @Test
+    fun `the canonical header is exactly the bytes the spec names`() {
+        val dh = ByteArray(32) { it.toByte() }
+        val pqdh = ByteArray(8) { (it + 100).toByte() }
+        val pqct = ByteArray(8) { (it + 200).toByte() }
+
+        assertEquals(
+            "{\"dh\":\"" + Bytes.b64(dh) + "\",\"pn\":3,\"n\":7}",
+            String(DoubleRatchet.Header(dh, 3, 7).canonicalJson(), Charsets.UTF_8),
+            "the v2 header changed shape",
+        )
+        assertEquals(
+            "{\"dh\":\"" + Bytes.b64(dh) + "\",\"pn\":3,\"n\":7" +
+                ",\"pqdh\":\"" + Bytes.b64(pqdh) + "\",\"pqct\":\"" + Bytes.b64(pqct) + "\"}",
+            String(DoubleRatchet.Header(dh, 3, 7, pqdh, pqct).canonicalJson(), Charsets.UTF_8),
+            "the v3 header changed shape",
+        )
+        // ⚠️ Half a PQ step is not a PQ step. One field without the other must
+        // fall back to the v2 shape rather than emit a malformed header.
+        assertEquals(
+            "{\"dh\":\"" + Bytes.b64(dh) + "\",\"pn\":0,\"n\":0}",
+            String(DoubleRatchet.Header(dh, 0, 0, pqdh, null).canonicalJson(), Charsets.UTF_8),
+            "a half-populated PQ step leaked into the header",
+        )
+    }
+
+    @Test
+    fun `a v3 conversation going both ways stays in step`() {
+        val (alice, bob, _) = pqPair()
+        // Four direction changes: each one turns the DH ratchet AND the PQ
+        // ratchet in lockstep, so an ordering mistake in either compounds.
+        assertContentEquals("a1".toByteArray(), bob.decrypt(alice.encrypt("a1".toByteArray(), AD), AD))
+        assertContentEquals("b1".toByteArray(), alice.decrypt(bob.encrypt("b1".toByteArray(), AD), AD))
+        assertContentEquals("a2".toByteArray(), bob.decrypt(alice.encrypt("a2".toByteArray(), AD), AD))
+        assertContentEquals("b2".toByteArray(), alice.decrypt(bob.encrypt("b2".toByteArray(), AD), AD))
+    }
+
+    /**
+     * 🔴 **The property v3 exists for.** v2 already protects a whole session
+     * against harvest-now-decrypt-later, because the root chain never recovers
+     * once the adversary has missed the start. What v2 lacks is post-quantum
+     * *healing*: after a compromise, recovery rests entirely on X25519 steps,
+     * which a quantum adversary breaks. So every step must carry NEW ML-KEM
+     * material, not merely the handshake's.
+     */
+    @Test
+    fun `every ratchet step carries a fresh post-quantum key`() {
+        val (alice, bob, bobPQ) = pqPair()
+
+        val first = alice.encrypt("a1".toByteArray(), AD)
+        assertTrue(first.header.pqdh != null && first.header.pqct != null,
+            "the first sending chain carried no PQ step at all")
+        // Encapsulated to Bob's PREKEY, which is what he can decapsulate with.
+        val seen = mutableListOf(Bytes.b64(first.header.pqdh!!))
+        bob.decrypt(first, AD)
+
+        // Bob answers: a ratchet step, so a PQ key of his own.
+        val reply = bob.encrypt("b1".toByteArray(), AD)
+        assertTrue(reply.header.pqdh != null, "Bob's ratchet step carried no PQ key")
+        seen += Bytes.b64(reply.header.pqdh!!)
+        alice.decrypt(reply, AD)
+
+        val third = alice.encrypt("a2".toByteArray(), AD)
+        seen += Bytes.b64(third.header.pqdh!!)
+        bob.decrypt(third, AD)
+
+        assertEquals(seen.size, seen.toSet().size,
+            "a PQ key was reused across ratchet steps: $seen")
+        assertTrue(seen.none { it == Bytes.b64(bobPQ.publicKey) },
+            "a ratchet step reused the handshake prekey instead of a fresh key")
+    }
+
+    /**
+     * ⚠️ **Repeated on every message of a chain, not just the first (§13.3).**
+     * Sending it once would mean losing that one message strands the entire
+     * chain: the receiver never gets the material to advance its root key, and
+     * nothing later in the chain can ever be decrypted.
+     */
+    @Test
+    fun `the post-quantum step repeats until the peer answers`() {
+        val (alice, bob, _) = pqPair()
+        val first = alice.encrypt("a1".toByteArray(), AD)
+        val second = alice.encrypt("a2".toByteArray(), AD)
+        val third = alice.encrypt("a3".toByteArray(), AD)
+
+        for ((i, m) in listOf(first, second, third).withIndex()) {
+            assertTrue(m.header.pqct != null, "message $i of the chain dropped the PQ step")
+        }
+        assertContentEquals(first.header.pqdh, third.header.pqdh,
+            "the PQ key changed inside one sending chain")
+
+        // 🔴 And the whole chain still opens if the FIRST message is the one
+        // that goes missing — which is the loss the repetition exists for.
+        assertContentEquals("a2".toByteArray(), bob.decrypt(second, AD))
+        assertContentEquals("a3".toByteArray(), bob.decrypt(third, AD))
+    }
+
+    /**
+     * A stripped PQ step must not silently downgrade to a v1 root step. The
+     * header is bound into the AAD, so removing it fails the open — which is
+     * the difference between a downgrade attack and a broken message.
+     */
+    @Test
+    fun `a stripped post-quantum step does not open`() {
+        val (alice, bob, _) = pqPair()
+        val message = alice.encrypt("a1".toByteArray(), AD)
+        val stripped = DoubleRatchet.Message(
+            DoubleRatchet.Header(message.header.dh, message.header.pn, message.header.n),
+            message.ciphertext,
+        )
+        assertFailsWith<Exception> { bob.decrypt(stripped, AD) }
+    }
+
+    /**
+     * ⚠️ A v3 session that lost its PQ state on a restart would keep working
+     * and silently fall back to X25519-only healing — nothing fails, and the
+     * property is simply gone.
+     */
+    @Test
+    fun `a v3 session survives being archived`() {
+        val (alice, bob, _) = pqPair()
+        bob.decrypt(alice.encrypt("a1".toByteArray(), AD), AD)
+
+        val restoredBob = DoubleRatchet.fromArchive(bob.archive())
+        val restoredAlice = DoubleRatchet.fromArchive(alice.archive())
+
+        val reply = restoredBob.encrypt("b1".toByteArray(), AD)
+        assertTrue(reply.header.pqdh != null, "the restored session lost its PQ step")
+        assertContentEquals("b1".toByteArray(), restoredAlice.decrypt(reply, AD))
+    }
+
+    /**
+     * 🔴 A v3 device must still speak v2 to a peer that does not advertise it
+     * (§13.4), and the header then carries no PQ fields at all.
+     */
+    @Test
+    fun `a session without post-quantum material still runs v2`() {
+        val (alice, bob) = pair()
+        val message = alice.encrypt("hello".toByteArray(), AD)
+        assertTrue(message.header.pqdh == null && message.header.pqct == null,
+            "a v2 session emitted PQ header fields")
+        assertContentEquals("hello".toByteArray(), bob.decrypt(message, AD))
+    }
+
    @Test
    fun `a message survives the trip`() {
        val (alice, bob) = pair()