diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/concord/cord03Channels/ChannelChat.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/concord/cord03Channels/ChannelChat.kt
new file mode 100644
index 0000000000..2aa2eec960
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/concord/cord03Channels/ChannelChat.kt
@@ -0,0 +1,86 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.concord.cord03Channels
+
+import com.vitorpamplona.quartz.concord.events.ConcordKinds
+import com.vitorpamplona.quartz.nip01Core.core.Event
+import com.vitorpamplona.quartz.nip01Core.core.HexKey
+import com.vitorpamplona.quartz.nip01Core.core.firstTagValue
+import com.vitorpamplona.quartz.nip59Giftwrap.rumors.RumorAssembler
+
+/**
+ * Chat Plane message binding (CORD-03).
+ *
+ * Every Chat Plane rumor — a message, reply, reaction, edit, or delete — commits
+ * to the channel and epoch it belongs to via `["channel", <id>]` and
+ * `["epoch", <n>]` tags inside the author-signed rumor. Recipients enforce this
+ * binding ([isBoundTo]) so an event lifted from one channel/epoch can't be
+ * replayed into another.
+ */
+object ChannelChat {
+    const val TAG_CHANNEL = "channel"
+    const val TAG_EPOCH = "epoch"
+
+    /** Builds the channel/epoch binding tags shared by every Chat Plane rumor. */
+    fun bindingTags(
+        channelId: HexKey,
+        epoch: Long,
+    ): Array<Array<String>> = arrayOf(arrayOf(TAG_CHANNEL, channelId), arrayOf(TAG_EPOCH, epoch.toString()))
+
+    /**
+     * Builds an unsigned kind-9 chat message rumor bound to [channelId]/[epoch].
+     * Wrap it for the channel plane with
+     * [com.vitorpamplona.quartz.concord.envelope.ConcordStreamEnvelope] (encrypted
+     * seal) to publish.
+     */
+    fun message(
+        authorPubKey: HexKey,
+        channelId: HexKey,
+        epoch: Long,
+        text: String,
+        createdAt: Long,
+        extraTags: Array<Array<String>> = emptyArray(),
+    ): Event =
+        RumorAssembler.assembleRumor(
+            pubKey = authorPubKey,
+            createdAt = createdAt,
+            kind = ConcordKinds.MESSAGE,
+            tags = bindingTags(channelId, epoch) + extraTags,
+            content = text,
+        )
+
+    /** The channel id a Chat Plane [rumor] is bound to, or null if unbound. */
+    fun channelOf(rumor: Event): HexKey? = rumor.tags.firstTagValue(TAG_CHANNEL)
+
+    /** The epoch a Chat Plane [rumor] is bound to, or null if unbound/malformed. */
+    fun epochOf(rumor: Event): Long? = rumor.tags.firstTagValue(TAG_EPOCH)?.toLongOrNull()
+
+    /**
+     * True when [rumor] is bound to exactly [channelId] and [epoch]. Recipients
+     * must reject any Chat Plane event whose binding does not match the plane it
+     * arrived on.
+     */
+    fun isBoundTo(
+        rumor: Event,
+        channelId: HexKey,
+        epoch: Long,
+    ): Boolean = channelOf(rumor) == channelId && epochOf(rumor) == epoch
+}
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/concord/cord03Channels/ConcordChannelKeys.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/concord/cord03Channels/ConcordChannelKeys.kt
new file mode 100644
index 0000000000..65dd178f9c
--- /dev/null
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/concord/cord03Channels/ConcordChannelKeys.kt
@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.concord.cord03Channels
+
+import com.vitorpamplona.quartz.concord.crypto.ConcordKeyDerivation
+import com.vitorpamplona.quartz.concord.crypto.ConcordLabels
+import com.vitorpamplona.quartz.concord.crypto.GroupKey
+
+/**
+ * Channel Chat Plane key derivation (CORD-03).
+ *
+ * Both channel types use the same `group_key("concord/channel", secret,
+ * channel_id, epoch)` derivation; only the secret and epoch differ:
+ *  - **Public** channels derive from the shared `community_root` at the current
+ *    root epoch — every member can derive them, so no key is distributed.
+ *  - **Private** channels derive from their own random `channel_key` at the
+ *    channel's own epoch — the key is delivered on role grant and rotated on
+ *    revocation.
+ *
+ * The `channel_id` is folded into the derivation so every channel gets a distinct
+ * address regardless of the secret source, and it stays constant across
+ * visibility conversions and epoch rotations.
+ */
+object ConcordChannelKeys {
+    /** Public channel: derived from the community root at the root epoch. */
+    fun publicChannel(
+        communityRoot: ByteArray,
+        channelId: ByteArray,
+        rootEpoch: Long,
+    ): GroupKey = ConcordKeyDerivation.groupKey(ConcordLabels.CHANNEL, communityRoot, channelId, rootEpoch)
+
+    /** Private channel: derived from its own channel key at the channel epoch. */
+    fun privateChannel(
+        channelKey: ByteArray,
+        channelId: ByteArray,
+        channelEpoch: Long,
+    ): GroupKey = ConcordKeyDerivation.groupKey(ConcordLabels.CHANNEL, channelKey, channelId, channelEpoch)
+}
diff --git a/quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/concord/cord03Channels/ChannelChatEndToEndTest.kt b/quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/concord/cord03Channels/ChannelChatEndToEndTest.kt
new file mode 100644
index 0000000000..ac202549c6
--- /dev/null
+++ b/quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/concord/cord03Channels/ChannelChatEndToEndTest.kt
@@ -0,0 +1,110 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.concord.cord03Channels
+
+import com.vitorpamplona.quartz.concord.envelope.ConcordStreamEnvelope
+import com.vitorpamplona.quartz.nip01Core.core.toHexKey
+import com.vitorpamplona.quartz.nip01Core.crypto.KeyPair
+import com.vitorpamplona.quartz.nip01Core.signers.NostrSignerInternal
+import kotlinx.coroutines.test.runTest
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFalse
+import kotlin.test.assertNull
+import kotlin.test.assertTrue
+
+/**
+ * Full CORD-01+03 vertical slice: two members holding the same community_root
+ * independently derive a public channel key, and one reads the other's message
+ * off the shared plane — no key distribution required.
+ */
+class ChannelChatEndToEndTest {
+    private val communityRoot = ByteArray(32) { 0x5A }
+    private val channelId = ByteArray(32) { 0x42 }
+    private val channelIdHex = channelId.toHexKey()
+    private val rootEpoch = 0L
+
+    @Test
+    fun twoMembersShareAPublicChannelWithoutKeyDistribution() =
+        runTest {
+            val alice = NostrSignerInternal(KeyPair())
+
+            // Alice derives the public channel plane and sends a message.
+            val aliceChannel = ConcordChannelKeys.publicChannel(communityRoot, channelId, rootEpoch)
+            val rumor = ChannelChat.message(alice.pubKey, channelIdHex, rootEpoch, "gm #general", createdAt = 1_700_000_000L)
+            val wrap = ConcordStreamEnvelope.wrap(rumor, aliceChannel, alice, encrypted = true)
+
+            // Bob, holding the same community_root, derives the identical plane and reads it.
+            val bobChannel = ConcordChannelKeys.publicChannel(communityRoot, channelId, rootEpoch)
+            assertEquals(aliceChannel.publicKeyHex, bobChannel.publicKeyHex)
+
+            val opened = ConcordStreamEnvelope.open(wrap, bobChannel)
+            assertEquals("gm #general", opened.rumor.content)
+            assertEquals(alice.pubKey, opened.author)
+            assertTrue(ChannelChat.isBoundTo(opened.rumor, channelIdHex, rootEpoch))
+        }
+
+    @Test
+    fun bindingRejectsCrossChannelAndCrossEpochReplay() {
+        val rumor =
+            ChannelChat.message(
+                authorPubKey = KeyPair().pubKey.toHexKey(),
+                channelId = channelIdHex,
+                epoch = 0L,
+                text = "hi",
+                createdAt = 1L,
+            )
+        assertTrue(ChannelChat.isBoundTo(rumor, channelIdHex, 0L))
+        assertFalse(ChannelChat.isBoundTo(rumor, channelIdHex, 1L)) // wrong epoch
+        assertFalse(ChannelChat.isBoundTo(rumor, "00".repeat(32), 0L)) // wrong channel
+        assertEquals(channelIdHex, ChannelChat.channelOf(rumor))
+        assertEquals(0L, ChannelChat.epochOf(rumor))
+    }
+
+    @Test
+    fun nonMembersCannotDeriveThePlane() =
+        runTest {
+            val alice = NostrSignerInternal(KeyPair())
+            val channel = ConcordChannelKeys.publicChannel(communityRoot, channelId, rootEpoch)
+            val wrap =
+                ConcordStreamEnvelope.wrap(
+                    ChannelChat.message(alice.pubKey, channelIdHex, rootEpoch, "secret", 1L),
+                    channel,
+                    alice,
+                    encrypted = true,
+                )
+
+            // A different community_root derives a different plane key ⇒ cannot open.
+            val outsiderPlane = ConcordChannelKeys.publicChannel(ByteArray(32) { 0x01 }, channelId, rootEpoch)
+            assertNull(ConcordStreamEnvelope.openOrNull(wrap, outsiderPlane))
+        }
+
+    @Test
+    fun epochRotationRotatesTheChannelAddress() {
+        val e0 = ConcordChannelKeys.publicChannel(communityRoot, channelId, 0)
+        val e1 = ConcordChannelKeys.publicChannel(communityRoot, channelId, 1)
+        assertFalse(e0.publicKeyHex == e1.publicKeyHex)
+
+        // A private channel with its own key is distinct from the public one at the same id.
+        val priv = ConcordChannelKeys.privateChannel(ByteArray(32) { 0x77 }, channelId, 0)
+        assertFalse(priv.publicKeyHex == e0.publicKeyHex)
+    }
+}