diff --git a/e2ee/omemo/backend_test.go b/e2ee/omemo/backend_test.go
new file mode 100644
index 0000000..9164b38
--- /dev/null
+++ b/e2ee/omemo/backend_test.go
@@ -0,0 +1,126 @@
+package omemo_test
+
+import (
+	"bytes"
+	"testing"
+
+	"dev.narayana.im/narayana/telegabber/e2ee"
+	"dev.narayana.im/narayana/telegabber/e2ee/omemo"
+	"dev.narayana.im/narayana/telegabber/e2ee/omemo/libsignal"
+	"dev.narayana.im/narayana/telegabber/e2ee/store/badgerstore"
+)
+
+func newTestBackend(t *testing.T, version omemo.Version) *omemo.Backend {
+	t.Helper()
+
+	ctx, err := libsignal.NewContext()
+	if err != nil {
+		t.Fatalf("NewContext: %v", err)
+	}
+	t.Cleanup(ctx.Close)
+
+	db, err := badgerstore.Open(t.TempDir(), nil)
+	if err != nil {
+		t.Fatalf("badgerstore.Open: %v", err)
+	}
+	t.Cleanup(func() { db.Close() })
+
+	return omemo.New(ctx, db, version)
+}
+
+// TestBackendRoundTrip exercises the full e2ee.Backend interface, not just
+// the underlying libsignal primitives (already covered by libsignal's own
+// round-trip test): EnsureIdentity, publishing a device-list/bundle,
+// ingesting them on the other side, and Encrypt/Decrypt in both
+// directions. This simulates two independent parties - "gateway" (the
+// bridged chat pseudo-JID's owned identity) and "client" (the real XMPP
+// user's own device) - each with their own Backend/store, exchanging
+// PublishedIdentity/PublishedBundle results directly in place of the real
+// PEP fetch (e2ee/fetch.go's actual IQ round trip isn't built yet).
+func TestBackendRoundTrip(t *testing.T) {
+	const gatewayLogin = "telegram-login-1"
+	const gatewayBareJID = "12345@transport.example"
+	const realUserBareJID = "alice@real.example"
+
+	for _, tc := range []struct {
+		name    string
+		version omemo.Version
+	}{
+		{"omemo0", omemo.Omemo0},
+		{"omemo1", omemo.Omemo1},
+		{"omemo2", omemo.Omemo2},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			if tc.version != omemo.Omemo0 && !libsignal.ProtocolV4Supported {
+				t.Skip("protocol v4 (omemo1/omemo2) not supported by this build (signal_legacy tag)")
+			}
+
+			gateway := newTestBackend(t, tc.version)
+			client := newTestBackend(t, tc.version)
+
+			gatewayOwned := e2ee.OwnedPeer(gatewayLogin, gatewayBareJID)
+			clientOwned := e2ee.OwnedPeer("n/a", realUserBareJID)
+			realUser := e2ee.PeerID(realUserBareJID)     // how the gateway refers to the real user
+			gatewayAsPeer := e2ee.PeerID(gatewayBareJID) // how the client refers to the gateway
+
+			if err := gateway.EnsureIdentity(gatewayOwned); err != nil {
+				t.Fatalf("gateway.EnsureIdentity: %v", err)
+			}
+			if err := client.EnsureIdentity(clientOwned); err != nil {
+				t.Fatalf("client.EnsureIdentity: %v", err)
+			}
+
+			// Gateway "fetches" the client's device list + bundle.
+			clientDeviceList, err := client.PublishedIdentity(clientOwned)
+			if err != nil {
+				t.Fatalf("client.PublishedIdentity: %v", err)
+			}
+			if err := gateway.IngestRemoteDeviceList(gatewayOwned, realUser, clientDeviceList); err != nil {
+				t.Fatalf("gateway.IngestRemoteDeviceList: %v", err)
+			}
+			clientBundle, err := client.PublishedBundle(clientOwned, omemo.OwnDeviceID)
+			if err != nil {
+				t.Fatalf("client.PublishedBundle: %v", err)
+			}
+			if err := gateway.IngestRemoteBundle(gatewayOwned, realUser, omemo.OwnDeviceID, clientBundle); err != nil {
+				t.Fatalf("gateway.IngestRemoteBundle: %v", err)
+			}
+
+			// Gateway encrypts a message originating from Telegram.
+			plaintext1 := []byte("hello from telegram")
+			env1, err := gateway.Encrypt(gatewayOwned, []e2ee.PeerID{realUser}, plaintext1)
+			if err != nil {
+				t.Fatalf("gateway.Encrypt: %v", err)
+			}
+
+			// Client decrypts it - this establishes its side of the
+			// session as a side effect, with no prior bundle fetch needed
+			// on its end (matching the prekey-message responder flow
+			// already validated at the libsignal level).
+			got1, err := client.Decrypt(gatewayAsPeer, clientOwned, env1)
+			if err != nil {
+				t.Fatalf("client.Decrypt: %v", err)
+			}
+			if !bytes.Equal(got1, plaintext1) {
+				t.Fatalf("round trip mismatch: got %q, want %q", got1, plaintext1)
+			}
+
+			// Client replies. It needs no explicit IngestRemoteBundle call
+			// for the gateway's device - the session established during
+			// the decrypt above already covers it.
+			plaintext2 := []byte("hi telegram")
+			env2, err := client.Encrypt(clientOwned, []e2ee.PeerID{gatewayAsPeer}, plaintext2)
+			if err != nil {
+				t.Fatalf("client.Encrypt: %v", err)
+			}
+
+			got2, err := gateway.Decrypt(realUser, gatewayOwned, env2)
+			if err != nil {
+				t.Fatalf("gateway.Decrypt: %v", err)
+			}
+			if !bytes.Equal(got2, plaintext2) {
+				t.Fatalf("round trip mismatch: got %q, want %q", got2, plaintext2)
+			}
+		})
+	}
+}
diff --git a/e2ee/omemo/bundle.go b/e2ee/omemo/bundle.go
new file mode 100644
index 0000000..e9c336c
--- /dev/null
+++ b/e2ee/omemo/bundle.go
@@ -0,0 +1,198 @@
+package omemo
+
+import (
+	"encoding/base64"
+	"encoding/xml"
+	"fmt"
+
+	"dev.narayana.im/narayana/telegabber/e2ee/omemo/libsignal"
+)
+
+// Bundle document parsing/encoding for all three OMEMO versions. As with
+// devicelist.go, Omemo1 and Omemo2 share an identical element shape (spk/
+// spks/ik/prekeys/pk) - only the namespace differs, per the same
+// changelog-silence reasoning - so one xepBundle struct serves both,
+// namespace-agnostic on Unmarshal, explicit on Marshal.
+//
+// Omemo0's element names (signedPreKeyPublic/signedPreKeySignature/
+// identityKey/prekeys/preKeyPublic) do NOT carry over to Omemo1/Omemo2 -
+// confirmed against the XEP-0384 XML schema directly, not assumed.
+//
+// None of the three bundle formats carry a registration id (unlike
+// vanilla Signal protocol's own prekey-bundle API, which does) - OMEMO
+// simply doesn't use it (XEP-0384's OMEMOKeyExchange has no such field
+// either). parseBundle leaves RegistrationID as 0; callers pass that
+// straight through to libsignal.RemoteBundle, which is the standard,
+// harmless practice for this field in OMEMO implementations - device_id
+// already does what registration_id would otherwise disambiguate.
+
+type omemo0Bundle struct {
+	XMLName               xml.Name         `xml:"eu.siacs.conversations.axolotl bundle"`
+	SignedPreKeyPublic     omemo0IDText     `xml:"signedPreKeyPublic"`
+	SignedPreKeySignature  string           `xml:"signedPreKeySignature"`
+	IdentityKey            string           `xml:"identityKey"`
+	PreKeys                omemo0PreKeysWrap `xml:"prekeys"`
+}
+
+type omemo0IDText struct {
+	ID   uint32 `xml:"signedPreKeyId,attr"`
+	Text string `xml:",chardata"`
+}
+
+type omemo0PreKeysWrap struct {
+	List []omemo0PreKey `xml:"preKeyPublic"`
+}
+
+type omemo0PreKey struct {
+	ID   uint32 `xml:"preKeyId,attr"`
+	Text string `xml:",chardata"`
+}
+
+type xepBundle struct {
+	XMLName xml.Name       `xml:"bundle"`
+	SPK     xepIDText      `xml:"spk"`
+	SPKS    string         `xml:"spks"`
+	IK      string         `xml:"ik"`
+	PreKeys xepPreKeysWrap `xml:"prekeys"`
+}
+
+type xepIDText struct {
+	ID   uint32 `xml:"id,attr"`
+	Text string `xml:",chardata"`
+}
+
+type xepPreKeysWrap struct {
+	List []xepIDText `xml:"pk"`
+}
+
+// parsedBundle is the common, namespace-agnostic result of parsing any of
+// the three bundle formats, in the raw (still base64-decoded) bytes
+// libsignal expects.
+type parsedBundle struct {
+	SignedPreKeyID        uint32
+	SignedPreKeyPublic    []byte
+	SignedPreKeySignature []byte
+	IdentityKeyPublic     []byte
+	PreKeys               []parsedPreKey
+}
+
+type parsedPreKey struct {
+	ID     uint32
+	Public []byte
+}
+
+func parseBundle(raw []byte) (*parsedBundle, error) {
+	var omemo0 omemo0Bundle
+	if err := xml.Unmarshal(raw, &omemo0); err == nil {
+		return decodeOmemo0Bundle(omemo0)
+	}
+
+	var xep xepBundle
+	if err := xml.Unmarshal(raw, &xep); err != nil {
+		return nil, err
+	}
+	return decodeXepBundle(xep)
+}
+
+func decodeOmemo0Bundle(b omemo0Bundle) (*parsedBundle, error) {
+	spk, err := b64Decode(b.SignedPreKeyPublic.Text)
+	if err != nil {
+		return nil, fmt.Errorf("omemo: omemo0 bundle signedPreKeyPublic: %w", err)
+	}
+	sig, err := b64Decode(b.SignedPreKeySignature)
+	if err != nil {
+		return nil, fmt.Errorf("omemo: omemo0 bundle signedPreKeySignature: %w", err)
+	}
+	ik, err := b64Decode(b.IdentityKey)
+	if err != nil {
+		return nil, fmt.Errorf("omemo: omemo0 bundle identityKey: %w", err)
+	}
+
+	result := &parsedBundle{
+		SignedPreKeyID:        b.SignedPreKeyPublic.ID,
+		SignedPreKeyPublic:    spk,
+		SignedPreKeySignature: sig,
+		IdentityKeyPublic:     ik,
+	}
+	for _, pk := range b.PreKeys.List {
+		data, err := b64Decode(pk.Text)
+		if err != nil {
+			return nil, fmt.Errorf("omemo: omemo0 bundle preKeyPublic %d: %w", pk.ID, err)
+		}
+		result.PreKeys = append(result.PreKeys, parsedPreKey{ID: pk.ID, Public: data})
+	}
+	return result, nil
+}
+
+func decodeXepBundle(b xepBundle) (*parsedBundle, error) {
+	spk, err := b64Decode(b.SPK.Text)
+	if err != nil {
+		return nil, fmt.Errorf("omemo: bundle spk: %w", err)
+	}
+	sig, err := b64Decode(b.SPKS)
+	if err != nil {
+		return nil, fmt.Errorf("omemo: bundle spks: %w", err)
+	}
+	ik, err := b64Decode(b.IK)
+	if err != nil {
+		return nil, fmt.Errorf("omemo: bundle ik: %w", err)
+	}
+
+	result := &parsedBundle{
+		SignedPreKeyID:        b.SPK.ID,
+		SignedPreKeyPublic:    spk,
+		SignedPreKeySignature: sig,
+		IdentityKeyPublic:     ik,
+	}
+	for _, pk := range b.PreKeys.List {
+		data, err := b64Decode(pk.Text)
+		if err != nil {
+			return nil, fmt.Errorf("omemo: bundle pk %d: %w", pk.ID, err)
+		}
+		result.PreKeys = append(result.PreKeys, parsedPreKey{ID: pk.ID, Public: data})
+	}
+	return result, nil
+}
+
+// encodeBundle produces a bundle document for one of this gateway's own
+// published devices, in the namespace matching version. Public keys are
+// expected in the form DecodeSignedPreKey/DecodePreKey/
+// DecodeIdentityPublicKey return - i.e. already picking the signature form
+// (Omemo0 vs Omemo1/Omemo2) matching version, per the rule documented on
+// libsignal.SignedPreKeyInfo.
+func encodeBundle(version Version, identityKeyPublic []byte, spkID uint32, spkPublic, spkSignature []byte, preKeys []libsignal.PreKeyInfo) ([]byte, error) {
+	if version == Omemo0 {
+		b := omemo0Bundle{
+			SignedPreKeyPublic:    omemo0IDText{ID: spkID, Text: b64Encode(spkPublic)},
+			SignedPreKeySignature: b64Encode(spkSignature),
+			IdentityKey:           b64Encode(identityKeyPublic),
+		}
+		for _, pk := range preKeys {
+			b.PreKeys.List = append(b.PreKeys.List, omemo0PreKey{ID: pk.ID, Text: b64Encode(pk.PublicKey)})
+		}
+		return xml.Marshal(b)
+	}
+
+	ns := omemo1NS
+	if version == Omemo2 {
+		ns = omemo2NS
+	}
+	b := xepBundle{
+		XMLName: xml.Name{Space: ns, Local: "bundle"},
+		SPK:     xepIDText{ID: spkID, Text: b64Encode(spkPublic)},
+		SPKS:    b64Encode(spkSignature),
+		IK:      b64Encode(identityKeyPublic),
+	}
+	for _, pk := range preKeys {
+		b.PreKeys.List = append(b.PreKeys.List, xepIDText{ID: pk.ID, Text: b64Encode(pk.PublicKey)})
+	}
+	return xml.Marshal(b)
+}
+
+func b64Encode(b []byte) string {
+	return base64.StdEncoding.EncodeToString(b)
+}
+
+func b64Decode(s string) ([]byte, error) {
+	return base64.StdEncoding.DecodeString(s)
+}
diff --git a/e2ee/omemo/devicelist.go b/e2ee/omemo/devicelist.go
new file mode 100644
index 0000000..48600d3
--- /dev/null
+++ b/e2ee/omemo/devicelist.go
@@ -0,0 +1,83 @@
+package omemo
+
+import "encoding/xml"
+
+// Device-list document parsing/encoding for all three OMEMO versions.
+// Omemo1 and Omemo2 share an identical element shape (only the namespace
+// differs - the XEP-0384 0.8.0 changelog only calls out SCE-related and
+// namespace renames, nothing about the device-list/bundle schema itself,
+// and 0.4.0/Omemo1's own changelog entry that introduced this schema
+// doesn't mention it changing again later), so one xepDeviceList struct
+// serves both: its XMLName tag omits the namespace, which makes Go's
+// encoding/xml match on local name only during Unmarshal (so it accepts
+// either namespace), while Marshal gets the right namespace by having the
+// caller set XMLName explicitly before encoding.
+
+// Namespace strings shared by both the device-list and bundle documents
+// (see bundle.go) - both document kinds are just different local element
+// names within the same OMEMO namespace.
+const (
+	omemo0NS = "eu.siacs.conversations.axolotl"
+	omemo1NS = "urn:xmpp:omemo:1"
+	omemo2NS = "urn:xmpp:omemo:2"
+)
+
+type omemo0DeviceList struct {
+	XMLName xml.Name       `xml:"eu.siacs.conversations.axolotl list"`
+	Devices []omemo0Device `xml:"device"`
+}
+
+type omemo0Device struct {
+	ID uint32 `xml:"id,attr"`
+}
+
+type xepDeviceList struct {
+	XMLName xml.Name    `xml:"devices"`
+	Devices []xepDevice `xml:"device"`
+}
+
+type xepDevice struct {
+	ID       uint32 `xml:"id,attr"`
+	Label    string `xml:"label,attr,omitempty"`
+	LabelSig string `xml:"labelsig,attr,omitempty"`
+}
+
+// parseDeviceList extracts device ids from a device-list document, in
+// whichever of the three known namespaces it's in.
+func parseDeviceList(raw []byte) ([]uint32, error) {
+	var omemo0 omemo0DeviceList
+	if err := xml.Unmarshal(raw, &omemo0); err == nil {
+		ids := make([]uint32, len(omemo0.Devices))
+		for i, d := range omemo0.Devices {
+			ids[i] = d.ID
+		}
+		return ids, nil
+	}
+
+	var xep xepDeviceList
+	if err := xml.Unmarshal(raw, &xep); err != nil {
+		return nil, err
+	}
+	ids := make([]uint32, len(xep.Devices))
+	for i, d := range xep.Devices {
+		ids[i] = d.ID
+	}
+	return ids, nil
+}
+
+// encodeDeviceList produces a device-list document for this gateway's own
+// published identity, in the namespace matching version.
+func encodeDeviceList(version Version, deviceID uint32) ([]byte, error) {
+	if version == Omemo0 {
+		return xml.Marshal(omemo0DeviceList{Devices: []omemo0Device{{ID: deviceID}}})
+	}
+	ns := omemo1NS
+	if version == Omemo2 {
+		ns = omemo2NS
+	}
+	list := xepDeviceList{
+		XMLName: xml.Name{Space: ns, Local: "devices"},
+		Devices: []xepDevice{{ID: deviceID}},
+	}
+	return xml.Marshal(list)
+}
diff --git a/e2ee/omemo/encrypt.go b/e2ee/omemo/encrypt.go
new file mode 100644
index 0000000..2b860ac
--- /dev/null
+++ b/e2ee/omemo/encrypt.go
@@ -0,0 +1,149 @@
+package omemo
+
+import (
+	"errors"
+	"fmt"
+
+	"dev.narayana.im/narayana/telegabber/e2ee"
+	"dev.narayana.im/narayana/telegabber/e2ee/omemo/libsignal"
+)
+
+func (b *Backend) Encrypt(from e2ee.PeerID, to []e2ee.PeerID, plaintext []byte) (e2ee.Envelope, error) {
+	login, owner, ok := e2ee.SplitOwnedPeer(from)
+	if !ok {
+		return e2ee.Envelope{}, fmt.Errorf("omemo: Encrypt: %q is not an OwnedPeer", from)
+	}
+
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	store := b.db.Store(b.libctx, login, owner)
+
+	// Outer content encryption happens once per message, regardless of how
+	// many recipient devices it ends up going to - see envelope.go.
+	payload, err := EncryptOuter(b.version, plaintext, owner)
+	if err != nil {
+		return e2ee.Envelope{}, fmt.Errorf("omemo: EncryptOuter: %w", err)
+	}
+
+	storeCtx, err := libsignal.NewStoreContext(b.libctx, store)
+	if err != nil {
+		return e2ee.Envelope{}, fmt.Errorf("omemo: NewStoreContext: %w", err)
+	}
+	defer storeCtx.Close()
+
+	env := &WireEnvelope{
+		Version:   b.version,
+		SenderSID: ownDeviceIDNum,
+		IV:        payload.IV,
+		Payload:   payload.Ciphertext,
+	}
+
+	for _, peer := range to {
+		deviceIDs, err := store.GetSubDeviceSessions(string(peer))
+		if err != nil {
+			return e2ee.Envelope{}, fmt.Errorf("omemo: GetSubDeviceSessions(%s): %w", peer, err)
+		}
+		if len(deviceIDs) == 0 {
+			return e2ee.Envelope{}, fmt.Errorf("omemo: Encrypt: no known devices/sessions for %s (IngestRemoteBundle was not called)", peer)
+		}
+
+		for _, deviceID := range deviceIDs {
+			cipher, err := libsignal.NewSessionCipher(b.libctx, storeCtx, libsignal.Address{Name: string(peer), DeviceID: deviceID}, b.version.libsignalVersion())
+			if err != nil {
+				return e2ee.Envelope{}, fmt.Errorf("omemo: NewSessionCipher(%s:%d): %w", peer, deviceID, err)
+			}
+			ct, err := cipher.Encrypt(payload.ContentKey)
+			cipher.Close()
+			if err != nil {
+				return e2ee.Envelope{}, fmt.Errorf("omemo: Encrypt(%s:%d): %w", peer, deviceID, err)
+			}
+			env.Keys = append(env.Keys, WireKey{
+				RecipientJID: string(peer),
+				DeviceID:     deviceID,
+				IsPreKey:     ct.Type == libsignal.CiphertextPreKeyType,
+				Ciphertext:   ct.Serialized,
+			})
+		}
+	}
+
+	raw, err := env.Encode()
+	if err != nil {
+		return e2ee.Envelope{}, err
+	}
+	return e2ee.Envelope{Backend: Name, Raw: raw}, nil
+}
+
+func (b *Backend) Decrypt(from, to e2ee.PeerID, env e2ee.Envelope) ([]byte, error) {
+	if env.Backend != Name {
+		return nil, fmt.Errorf("omemo: Decrypt: envelope backend %q, want %q", env.Backend, Name)
+	}
+	login, owner, ok := e2ee.SplitOwnedPeer(to)
+	if !ok {
+		return nil, fmt.Errorf("omemo: Decrypt: %q is not an OwnedPeer", to)
+	}
+
+	wireEnv, err := DecodeWireEnvelope(env.Raw)
+	if err != nil {
+		return nil, fmt.Errorf("omemo: DecodeWireEnvelope: %w", err)
+	}
+
+	var keyForUs *WireKey
+	for i := range wireEnv.Keys {
+		if wireEnv.Keys[i].DeviceID == ownDeviceIDNum {
+			keyForUs = &wireEnv.Keys[i]
+			break
+		}
+	}
+	if keyForUs == nil {
+		return nil, errors.New("omemo: Decrypt: no <key> addressed to our own device in this envelope")
+	}
+
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	store := b.db.Store(b.libctx, login, owner)
+	regID, err := store.GetLocalRegistrationID()
+	if err != nil {
+		return nil, fmt.Errorf("omemo: GetLocalRegistrationID: %w", err)
+	}
+
+	storeCtx, err := libsignal.NewStoreContext(b.libctx, store)
+	if err != nil {
+		return nil, fmt.Errorf("omemo: NewStoreContext: %w", err)
+	}
+	defer storeCtx.Close()
+
+	// The cipher's configured version must match what the SENDER actually
+	// used (wireEnv.Version, derived from which XML namespace their
+	// stanza used), not this backend's own outgoing preference (b.version)
+	// - a remote peer's message version isn't guaranteed to match ours,
+	// and this is the one case telegabber's own M1 round-trip test never
+	// exercised (both sides there always shared the same configured
+	// version), so this is deliberately conservative rather than assumed.
+	cipher, err := libsignal.NewSessionCipher(b.libctx, storeCtx, libsignal.Address{Name: string(from), DeviceID: wireEnv.SenderSID}, wireEnv.Version.libsignalVersion())
+	if err != nil {
+		return nil, fmt.Errorf("omemo: NewSessionCipher: %w", err)
+	}
+	defer cipher.Close()
+
+	msgType := libsignal.CiphertextSignalType
+	if keyForUs.IsPreKey {
+		msgType = libsignal.CiphertextPreKeyType
+	}
+	// Omemo1 and Omemo2 both use the OMEMOKeyExchange/OMEMOAuthenticatedMessage
+	// wire framing (libsignal.ProtocolVersionV4); only Omemo0 uses the
+	// original WhisperTextProtocol framing.
+	useOmemoFraming := wireEnv.Version != Omemo0
+
+	contentKey, err := cipher.Decrypt(b.libctx, libsignal.CiphertextMessage{Type: msgType, Serialized: keyForUs.Ciphertext}, useOmemoFraming, regID)
+	if err != nil {
+		return nil, fmt.Errorf("omemo: SessionCipher.Decrypt: %w", err)
+	}
+
+	plaintext, err := DecryptOuter(wireEnv.Version, &EncryptedPayload{Ciphertext: wireEnv.Payload, IV: wireEnv.IV, ContentKey: contentKey})
+	if err != nil {
+		return nil, fmt.Errorf("omemo: DecryptOuter: %w", err)
+	}
+	return plaintext, nil
+}
diff --git a/e2ee/omemo/envelope.go b/e2ee/omemo/envelope.go
index fefe233..b0eb325 100644
--- a/e2ee/omemo/envelope.go
+++ b/e2ee/omemo/envelope.go
@@ -14,8 +14,6 @@ import (
 	"io"
 
 	"golang.org/x/crypto/hkdf"
-
-	"dev.narayana.im/narayana/telegabber/e2ee/omemo/libsignal"
 )
 
 // This file is OMEMO's outer, non-Signal content encryption: the layer
@@ -24,94 +22,115 @@ import (
 // blob (not the body itself) is what gets Signal/Double-Ratchet-encrypted
 // once per recipient device (see libsignal.SessionCipher.Encrypt) - that
 // per-device wrapping, and the device/session bookkeeping around it, is
-// omemo.go's job, built on top of EncryptOuter/DecryptOuter here.
+// identity.go/encrypt.go's job, built on top of EncryptOuter/DecryptOuter
+// here.
 //
-// Two schemes, matching the two wire versions (confirmed against the
-// XEP-0384 spec text directly, not memory - see the M2 kickoff discussion):
+// Three versions (see version.go), confirmed against the XEP-0384 spec
+// text and its version history directly, not memory:
 //
-//   - Legacy (ProtocolVersionLegacy, eu.siacs.conversations.axolotl):
-//     AES-128-GCM directly on the plaintext body. The GCM tag is stripped
-//     from the ciphertext and instead appended to the key (key||tag, 32
-//     bytes) before that blob is Signal-encrypted per recipient; <payload>
-//     carries ciphertext only, <iv> carries the GCM nonce.
+//   - Omemo0 (eu.siacs.conversations.axolotl): AES-128-GCM directly on the
+//     plaintext body. The GCM tag is stripped from the ciphertext and
+//     instead appended to the key (key||tag, 32 bytes) before that blob is
+//     Signal-encrypted per recipient; <payload> carries ciphertext only,
+//     <iv> carries the GCM nonce.
 //
-//   - Modern (ProtocolVersionModern, OMEMO 1/2): the plaintext is first
-//     wrapped in a minimal XEP-0420 SCE envelope (sce.go). That envelope is
-//     encrypted with AES-256-CBC using a key/authKey/iv derived via
+//   - Omemo1 and Omemo2 share the same payload crypto (introduced at
+//     Omemo1/XEP-0384 0.4.0, unchanged since): the plaintext is first
+//     wrapped in an XEP-0420 SCE envelope - Omemo1 and Omemo2 use two
+//     different SCE dialects, see sce.go - then that envelope is encrypted
+//     with AES-256-CBC using a key/authKey/iv derived via
 //     HKDF-SHA256(random 32-byte key, salt=32 zero bytes, info="OMEMO
 //     Payload") -> 80 bytes, split 32/32/16. The ciphertext is
-//     HMAC-SHA256'd with authKey and truncated to 16 bytes. key||tag (48
-//     bytes) is the per-recipient blob; <payload> carries ciphertext only;
-//     there is no <iv> element - the receiver re-derives it from the key
-//     via the same HKDF.
+//     HMAC-SHA256'd with authKey and truncated to 16 bytes (confirmed
+//     against the current spec text directly - an earlier spec version
+//     had a 32-vs-16-byte inconsistency here, fixed in 0.8.2). key||tag
+//     (48 bytes) is the per-recipient blob; <payload> carries ciphertext
+//     only; there is no <iv> element - the receiver re-derives it from the
+//     key via the same HKDF.
 
 const (
-	legacyKeyLen = 16 // AES-128
-	legacyIVLen  = 12 // GCM nonce, standard size (crypto/cipher.NewGCM's default)
-	legacyTagLen = 16
+	omemo0KeyLen = 16 // AES-128
+	omemo0IVLen  = 12 // GCM nonce, standard size (crypto/cipher.NewGCM's default)
+	omemo0TagLen = 16
 
-	modernKeyLen     = 32 // AES-256
-	modernAuthKeyLen = 32
-	modernIVLen      = 16 // AES block size
-	modernTagLen     = 16
-	modernHKDFInfo   = "OMEMO Payload"
+	sceKeyLen     = 32 // AES-256 (Omemo1/Omemo2 shared payload crypto)
+	sceAuthKeyLen = 32
+	sceIVLen      = 16 // AES block size
+	sceTagLen     = 16
+	sceHKDFInfo   = "OMEMO Payload"
 )
 
 // EncryptedPayload is the result of OMEMO's outer content-encryption step.
 type EncryptedPayload struct {
 	Ciphertext []byte
-	IV         []byte // legacy only; nil for modern
+	IV         []byte // Omemo0 only; nil for Omemo1/Omemo2
 	ContentKey []byte // the blob to Signal-encrypt once per recipient device
 }
 
 // EncryptOuter performs OMEMO's outer content encryption for plaintext,
-// per version (ProtocolVersionLegacy or ProtocolVersionModern). fromJID is
-// used as the SCE <from> binding for modern; ignored for legacy.
-func EncryptOuter(version int, plaintext []byte, fromJID string) (*EncryptedPayload, error) {
+// per version. fromJID is used as the SCE <from> binding for Omemo1/
+// Omemo2; ignored for Omemo0 (which has no SCE wrapping at all).
+func EncryptOuter(version Version, plaintext []byte, fromJID string) (*EncryptedPayload, error) {
 	switch version {
-	case libsignal.ProtocolVersionLegacy:
-		ciphertext, iv, contentKey, err := encryptPayloadLegacy(plaintext)
+	case Omemo0:
+		ciphertext, iv, contentKey, err := encryptPayloadOmemo0(plaintext)
 		if err != nil {
 			return nil, err
 		}
 		return &EncryptedPayload{Ciphertext: ciphertext, IV: iv, ContentKey: contentKey}, nil
-	case libsignal.ProtocolVersionModern:
-		sceXML, err := sceEncode(plaintext, fromJID)
+	case Omemo1:
+		sceXML, err := sceEncodeV0(plaintext, fromJID)
 		if err != nil {
 			return nil, err
 		}
-		ciphertext, contentKey, err := encryptPayloadModern(sceXML)
+		ciphertext, contentKey, err := encryptPayloadSCE(sceXML)
+		if err != nil {
+			return nil, err
+		}
+		return &EncryptedPayload{Ciphertext: ciphertext, ContentKey: contentKey}, nil
+	case Omemo2:
+		sceXML, err := sceEncodeV1(plaintext, fromJID)
+		if err != nil {
+			return nil, err
+		}
+		ciphertext, contentKey, err := encryptPayloadSCE(sceXML)
 		if err != nil {
 			return nil, err
 		}
 		return &EncryptedPayload{Ciphertext: ciphertext, ContentKey: contentKey}, nil
 	default:
-		return nil, fmt.Errorf("omemo: unsupported protocol version %d", version)
+		return nil, fmt.Errorf("omemo: EncryptOuter: unsupported version %v", version)
 	}
 }
 
 // DecryptOuter reverses EncryptOuter.
-func DecryptOuter(version int, payload *EncryptedPayload) ([]byte, error) {
+func DecryptOuter(version Version, payload *EncryptedPayload) ([]byte, error) {
 	switch version {
-	case libsignal.ProtocolVersionLegacy:
-		return decryptPayloadLegacy(payload.Ciphertext, payload.IV, payload.ContentKey)
-	case libsignal.ProtocolVersionModern:
-		sceXML, err := decryptPayloadModern(payload.Ciphertext, payload.ContentKey)
+	case Omemo0:
+		return decryptPayloadOmemo0(payload.Ciphertext, payload.IV, payload.ContentKey)
+	case Omemo1:
+		sceXML, err := decryptPayloadSCE(payload.Ciphertext, payload.ContentKey)
 		if err != nil {
 			return nil, err
 		}
-		return sceDecode(sceXML)
+		return sceDecodeV0(sceXML)
+	case Omemo2:
+		sceXML, err := decryptPayloadSCE(payload.Ciphertext, payload.ContentKey)
+		if err != nil {
+			return nil, err
+		}
+		return sceDecodeV1(sceXML)
 	default:
-		return nil, fmt.Errorf("omemo: unsupported protocol version %d", version)
+		return nil, fmt.Errorf("omemo: DecryptOuter: unsupported version %v", version)
 	}
 }
 
-func encryptPayloadLegacy(plaintext []byte) (ciphertext, iv, contentKey []byte, err error) {
-	key := make([]byte, legacyKeyLen)
+func encryptPayloadOmemo0(plaintext []byte) (ciphertext, iv, contentKey []byte, err error) {
+	key := make([]byte, omemo0KeyLen)
 	if _, err := rand.Read(key); err != nil {
 		return nil, nil, nil, err
 	}
-	iv = make([]byte, legacyIVLen)
+	iv = make([]byte, omemo0IVLen)
 	if _, err := rand.Read(iv); err != nil {
 		return nil, nil, nil, err
 	}
@@ -124,21 +143,21 @@ func encryptPayloadLegacy(plaintext []byte) (ciphertext, iv, contentKey []byte,
 		return nil, nil, nil, err
 	}
 	sealed := gcm.Seal(nil, iv, plaintext, nil)
-	if len(sealed) < legacyTagLen {
+	if len(sealed) < omemo0TagLen {
 		return nil, nil, nil, errors.New("omemo: GCM output shorter than its own tag")
 	}
-	ciphertext = sealed[:len(sealed)-legacyTagLen]
-	tag := sealed[len(sealed)-legacyTagLen:]
+	ciphertext = sealed[:len(sealed)-omemo0TagLen]
+	tag := sealed[len(sealed)-omemo0TagLen:]
 	contentKey = append(append([]byte{}, key...), tag...)
 	return ciphertext, iv, contentKey, nil
 }
 
-func decryptPayloadLegacy(ciphertext, iv, contentKey []byte) ([]byte, error) {
-	if len(contentKey) != legacyKeyLen+legacyTagLen {
-		return nil, fmt.Errorf("omemo: legacy content key must be %d bytes, got %d", legacyKeyLen+legacyTagLen, len(contentKey))
+func decryptPayloadOmemo0(ciphertext, iv, contentKey []byte) ([]byte, error) {
+	if len(contentKey) != omemo0KeyLen+omemo0TagLen {
+		return nil, fmt.Errorf("omemo: omemo0 content key must be %d bytes, got %d", omemo0KeyLen+omemo0TagLen, len(contentKey))
 	}
-	key := contentKey[:legacyKeyLen]
-	tag := contentKey[legacyKeyLen:]
+	key := contentKey[:omemo0KeyLen]
+	tag := contentKey[omemo0KeyLen:]
 	block, err := aes.NewCipher(key)
 	if err != nil {
 		return nil, err
@@ -152,16 +171,19 @@ func decryptPayloadLegacy(ciphertext, iv, contentKey []byte) ([]byte, error) {
 }
 
 func hkdfSplit(key []byte) (encKey, authKey, iv []byte, err error) {
-	h := hkdf.New(sha256.New, key, make([]byte, sha256.Size), []byte(modernHKDFInfo))
-	out := make([]byte, modernKeyLen+modernAuthKeyLen+modernIVLen)
+	h := hkdf.New(sha256.New, key, make([]byte, sha256.Size), []byte(sceHKDFInfo))
+	out := make([]byte, sceKeyLen+sceAuthKeyLen+sceIVLen)
 	if _, err := io.ReadFull(h, out); err != nil {
 		return nil, nil, nil, err
 	}
-	return out[:modernKeyLen], out[modernKeyLen : modernKeyLen+modernAuthKeyLen], out[modernKeyLen+modernAuthKeyLen:], nil
+	return out[:sceKeyLen], out[sceKeyLen : sceKeyLen+sceAuthKeyLen], out[sceKeyLen+sceAuthKeyLen:], nil
 }
 
-func encryptPayloadModern(plaintext []byte) (ciphertext, contentKey []byte, err error) {
-	key := make([]byte, modernKeyLen)
+// encryptPayloadSCE/decryptPayloadSCE are the payload crypto shared by
+// Omemo1 and Omemo2 - only the SCE dialect wrapping plaintext before this
+// runs differs between them (see sce.go).
+func encryptPayloadSCE(plaintext []byte) (ciphertext, contentKey []byte, err error) {
+	key := make([]byte, sceKeyLen)
 	if _, err := rand.Read(key); err != nil {
 		return nil, nil, err
 	}
@@ -179,18 +201,18 @@ func encryptPayloadModern(plaintext []byte) (ciphertext, contentKey []byte, err
 
 	mac := hmac.New(sha256.New, authKey)
 	mac.Write(ciphertext)
-	tag := mac.Sum(nil)[:modernTagLen]
+	tag := mac.Sum(nil)[:sceTagLen]
 
 	contentKey = append(append([]byte{}, key...), tag...)
 	return ciphertext, contentKey, nil
 }
 
-func decryptPayloadModern(ciphertext, contentKey []byte) ([]byte, error) {
-	if len(contentKey) != modernKeyLen+modernTagLen {
-		return nil, fmt.Errorf("omemo: modern content key must be %d bytes, got %d", modernKeyLen+modernTagLen, len(contentKey))
+func decryptPayloadSCE(ciphertext, contentKey []byte) ([]byte, error) {
+	if len(contentKey) != sceKeyLen+sceTagLen {
+		return nil, fmt.Errorf("omemo: SCE content key must be %d bytes, got %d", sceKeyLen+sceTagLen, len(contentKey))
 	}
-	key := contentKey[:modernKeyLen]
-	tag := contentKey[modernKeyLen:]
+	key := contentKey[:sceKeyLen]
+	tag := contentKey[sceKeyLen:]
 
 	encKey, authKey, iv, err := hkdfSplit(key)
 	if err != nil {
@@ -199,7 +221,7 @@ func decryptPayloadModern(ciphertext, contentKey []byte) ([]byte, error) {
 
 	mac := hmac.New(sha256.New, authKey)
 	mac.Write(ciphertext)
-	expectedTag := mac.Sum(nil)[:modernTagLen]
+	expectedTag := mac.Sum(nil)[:sceTagLen]
 	if !hmac.Equal(expectedTag, tag) {
 		return nil, errors.New("omemo: payload authentication failed")
 	}
@@ -245,31 +267,31 @@ func pkcs7Unpad(data []byte) ([]byte, error) {
 
 // WireKey is one recipient device's Signal-encrypted content-key blob,
 // version-and-namespace-agnostic - the xmpp-layer glue turns this into a
-// <key rid=".." prekey=".."/> (legacy) or <key rid=".." kex=".."/>
-// (modern) element; both attributes mean the same thing (IsPreKey).
+// <key rid=".." prekey=".."/> (Omemo0) or <key rid=".." kex=".."/>
+// (Omemo1/Omemo2) element; both attributes mean the same thing (IsPreKey).
 type WireKey struct {
 	RecipientJID string
 	DeviceID     uint32
 	IsPreKey     bool
-	Ciphertext   []byte // libsignal.CiphertextMessage.Serialized - already in the correct per-version wire format (see the M2 architecture note: libomemo-c itself produces OMEMOKeyExchange/OMEMOAuthenticatedMessage protobuf bytes for protocol v4, no extra wrapping needed here)
+	Ciphertext   []byte // libsignal.CiphertextMessage.Serialized - already in the correct per-version wire format (libomemo-c itself produces the OMEMOKeyExchange/OMEMOAuthenticatedMessage protobuf bytes for protocol v4, no extra wrapping needed here)
 }
 
 // WireEnvelope is the decoded structure of an OMEMO <encrypted> element,
-// independent of which XML shape (legacy vs OMEMO 2/SCE) it came from or
-// will be marshaled to - that XML-specific glue lives in the xmpp package,
-// which imports this package explicitly when the omemo backend is active.
+// independent of which XML shape it came from or will be marshaled to -
+// that XML-specific glue lives in the xmpp package, which imports this
+// package explicitly when the omemo backend is active.
 type WireEnvelope struct {
-	Version   int
+	Version   Version
 	SenderSID uint32
-	IV        []byte // legacy only
+	IV        []byte // Omemo0 only
 	Payload   []byte // outer ciphertext
 	Keys      []WireKey
 }
 
 // Encode serializes a WireEnvelope for e2ee.Envelope.Raw. This is
-// Go-to-Go-only plumbing (produced by omemo.go's Encrypt, consumed by the
-// xmpp-layer glue in the same process), not a wire format in its own
-// right, so gob is a fine fit - no schema stability concerns.
+// Go-to-Go-only plumbing (produced by identity.go/encrypt.go's Encrypt,
+// consumed by the xmpp-layer glue in the same process), not a wire format
+// in its own right, so gob is a fine fit - no schema stability concerns.
 func (e *WireEnvelope) Encode() ([]byte, error) {
 	var buf bytes.Buffer
 	if err := gob.NewEncoder(&buf).Encode(e); err != nil {
diff --git a/e2ee/omemo/envelope_test.go b/e2ee/omemo/envelope_test.go
index 5f9c22c..9995a40 100644
--- a/e2ee/omemo/envelope_test.go
+++ b/e2ee/omemo/envelope_test.go
@@ -4,17 +4,16 @@ import (
 	"bytes"
 	"reflect"
 	"testing"
-
-	"dev.narayana.im/narayana/telegabber/e2ee/omemo/libsignal"
 )
 
 func TestEncryptOuterRoundTrip(t *testing.T) {
 	for _, tc := range []struct {
 		name    string
-		version int
+		version Version
 	}{
-		{"legacy", libsignal.ProtocolVersionLegacy},
-		{"modern", libsignal.ProtocolVersionModern},
+		{"omemo0", Omemo0},
+		{"omemo1", Omemo1},
+		{"omemo2", Omemo2},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			plaintext := []byte("hello from telegram")
@@ -26,11 +25,11 @@ func TestEncryptOuterRoundTrip(t *testing.T) {
 			if len(payload.Ciphertext) == 0 {
 				t.Fatalf("expected non-empty ciphertext")
 			}
-			if tc.version == libsignal.ProtocolVersionLegacy && len(payload.IV) != legacyIVLen {
-				t.Fatalf("expected a %d-byte IV for legacy, got %d", legacyIVLen, len(payload.IV))
+			if tc.version == Omemo0 && len(payload.IV) != omemo0IVLen {
+				t.Fatalf("expected a %d-byte IV for omemo0, got %d", omemo0IVLen, len(payload.IV))
 			}
-			if tc.version == libsignal.ProtocolVersionModern && payload.IV != nil {
-				t.Fatalf("expected no IV for modern (receiver re-derives it), got %d bytes", len(payload.IV))
+			if tc.version != Omemo0 && payload.IV != nil {
+				t.Fatalf("expected no IV for %s (receiver re-derives it), got %d bytes", tc.name, len(payload.IV))
 			}
 
 			got, err := DecryptOuter(tc.version, payload)
@@ -47,10 +46,11 @@ func TestEncryptOuterRoundTrip(t *testing.T) {
 func TestEncryptOuterTamperDetection(t *testing.T) {
 	for _, tc := range []struct {
 		name    string
-		version int
+		version Version
 	}{
-		{"legacy", libsignal.ProtocolVersionLegacy},
-		{"modern", libsignal.ProtocolVersionModern},
+		{"omemo0", Omemo0},
+		{"omemo1", Omemo1},
+		{"omemo2", Omemo2},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			payload, err := EncryptOuter(tc.version, []byte("secret"), "gateway@example.com")
@@ -71,22 +71,39 @@ func TestEncryptOuterTamperDetection(t *testing.T) {
 
 func TestSCERoundTrip(t *testing.T) {
 	plaintext := []byte("hello sce")
-	xmlBytes, err := sceEncode(plaintext, "alice@example.com")
-	if err != nil {
-		t.Fatalf("sceEncode: %v", err)
-	}
-	got, err := sceDecode(xmlBytes)
-	if err != nil {
-		t.Fatalf("sceDecode: %v", err)
-	}
-	if !bytes.Equal(got, plaintext) {
-		t.Fatalf("got %q, want %q", got, plaintext)
-	}
+
+	t.Run("v0", func(t *testing.T) {
+		xmlBytes, err := sceEncodeV0(plaintext, "alice@example.com")
+		if err != nil {
+			t.Fatalf("sceEncodeV0: %v", err)
+		}
+		got, err := sceDecodeV0(xmlBytes)
+		if err != nil {
+			t.Fatalf("sceDecodeV0: %v", err)
+		}
+		if !bytes.Equal(got, plaintext) {
+			t.Fatalf("got %q, want %q", got, plaintext)
+		}
+	})
+
+	t.Run("v1", func(t *testing.T) {
+		xmlBytes, err := sceEncodeV1(plaintext, "alice@example.com")
+		if err != nil {
+			t.Fatalf("sceEncodeV1: %v", err)
+		}
+		got, err := sceDecodeV1(xmlBytes)
+		if err != nil {
+			t.Fatalf("sceDecodeV1: %v", err)
+		}
+		if !bytes.Equal(got, plaintext) {
+			t.Fatalf("got %q, want %q", got, plaintext)
+		}
+	})
 }
 
 func TestWireEnvelopeEncodeDecode(t *testing.T) {
 	env := &WireEnvelope{
-		Version:   libsignal.ProtocolVersionModern,
+		Version:   Omemo2,
 		SenderSID: 12345,
 		Payload:   []byte("ciphertext-bytes"),
 		Keys: []WireKey{
diff --git a/e2ee/omemo/identity.go b/e2ee/omemo/identity.go
new file mode 100644
index 0000000..85d7f3e
--- /dev/null
+++ b/e2ee/omemo/identity.go
@@ -0,0 +1,350 @@
+package omemo
+
+import (
+	"crypto/rand"
+	"errors"
+	"fmt"
+	"math/big"
+	"strconv"
+	"sync"
+
+	"dev.narayana.im/narayana/telegabber/e2ee"
+	"dev.narayana.im/narayana/telegabber/e2ee/omemo/libsignal"
+	"dev.narayana.im/narayana/telegabber/e2ee/store/badgerstore"
+)
+
+// Name identifies this backend in the e2ee registry and config.
+const Name = "omemo"
+
+// initialPreKeyCount is how many one-time prekeys EnsureIdentity generates
+// up front - a conventional pool size (matches common OMEMO implementation
+// practice) balancing bundle-refill frequency against bundle size.
+// Replenishing a depleted pool (prekeys are removed automatically as
+// they're consumed - see ListPreKeyIDs's doc comment) is a future
+// enhancement, not handled yet.
+const initialPreKeyCount = 100
+
+// ownDeviceIDNum is the sole device id this gateway ever publishes for an
+// owned identity - there is exactly one telegabber process encrypting on
+// behalf of any given bridged chat pseudo-JID, so unlike a real multi-device
+// XMPP account, no more than one device id is ever needed.
+const ownDeviceIDNum uint32 = 1
+
+// OwnDeviceID is ownDeviceIDNum in e2ee.DeviceID form, for callers that
+// need to reference "the" device of an owned identity (e.g. PublishedBundle).
+var OwnDeviceID = e2ee.DeviceID(strconv.FormatUint(uint64(ownDeviceIDNum), 10))
+
+// Backend is the OMEMO (XEP-0384) implementation of e2ee.Backend.
+type Backend struct {
+	libctx  *libsignal.Context
+	db      *badgerstore.DB
+	version Version // which version this backend produces for new outgoing content; incoming messages of any of the three versions are always accepted, decoded per-message
+
+	// libsignal is not internally thread-safe (see its package doc) - one
+	// mutex serializes every call into it, across every identity this
+	// backend manages. Simple and correct; per-identity striping would
+	// allow more concurrency but isn't needed at telegabber's message
+	// volume.
+	mu sync.Mutex
+}
+
+// New creates an OMEMO backend backed by db, producing new outgoing
+// content in the given version (Omemo0, Omemo1, or Omemo2 - see version.go).
+func New(libctx *libsignal.Context, db *badgerstore.DB, version Version) *Backend {
+	return &Backend{libctx: libctx, db: db, version: version}
+}
+
+var _ e2ee.Backend = (*Backend)(nil)
+
+func (b *Backend) Name() string { return Name }
+
+func (b *Backend) Namespaces() e2ee.Namespaces {
+	switch b.version {
+	case Omemo0:
+		return e2ee.Namespaces{
+			Disco: []string{omemo0NS + ".devicelist+notify"},
+			EME:   omemo0NS,
+		}
+	case Omemo1:
+		return e2ee.Namespaces{
+			Disco: []string{omemo1NS + ":devices+notify"},
+			EME:   omemo1NS,
+		}
+	default:
+		return e2ee.Namespaces{
+			Disco: []string{omemo2NS + ":devices+notify"},
+			EME:   omemo2NS,
+		}
+	}
+}
+
+func (b *Backend) EnsureIdentity(peer e2ee.PeerID) error {
+	login, owner, ok := e2ee.SplitOwnedPeer(peer)
+	if !ok {
+		return fmt.Errorf("omemo: EnsureIdentity: %q is not an OwnedPeer", peer)
+	}
+
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	store := b.db.Store(b.libctx, login, owner)
+
+	has, err := store.HasIdentityKeyPair()
+	if err != nil {
+		return fmt.Errorf("omemo: HasIdentityKeyPair: %w", err)
+	}
+	if has {
+		return nil // idempotent - already set up
+	}
+
+	idKeyPair, err := libsignal.GenerateIdentityKeyPair(b.libctx)
+	if err != nil {
+		return fmt.Errorf("omemo: GenerateIdentityKeyPair: %w", err)
+	}
+	if err := store.SaveIdentityKeyPair(idKeyPair.Record); err != nil {
+		return fmt.Errorf("omemo: SaveIdentityKeyPair: %w", err)
+	}
+
+	regID, err := libsignal.GenerateRegistrationID(b.libctx)
+	if err != nil {
+		return fmt.Errorf("omemo: GenerateRegistrationID: %w", err)
+	}
+	if err := store.SaveRegistrationID(regID); err != nil {
+		return fmt.Errorf("omemo: SaveRegistrationID: %w", err)
+	}
+
+	signedPreKey, err := libsignal.GenerateSignedPreKey(b.libctx, idKeyPair, 1)
+	if err != nil {
+		return fmt.Errorf("omemo: GenerateSignedPreKey: %w", err)
+	}
+	if err := store.StoreSignedPreKey(signedPreKey.ID, signedPreKey.Record); err != nil {
+		return fmt.Errorf("omemo: StoreSignedPreKey: %w", err)
+	}
+
+	start, err := store.NextPreKeyIDs(initialPreKeyCount)
+	if err != nil {
+		return fmt.Errorf("omemo: NextPreKeyIDs: %w", err)
+	}
+	preKeys, err := libsignal.GeneratePreKeys(b.libctx, start, initialPreKeyCount)
+	if err != nil {
+		return fmt.Errorf("omemo: GeneratePreKeys: %w", err)
+	}
+	for _, pk := range preKeys {
+		if err := store.StorePreKey(pk.ID, pk.Record); err != nil {
+			return fmt.Errorf("omemo: StorePreKey %d: %w", pk.ID, err)
+		}
+	}
+
+	return nil
+}
+
+func (b *Backend) PublishedIdentity(peer e2ee.PeerID) (e2ee.DeviceListDoc, error) {
+	login, owner, ok := e2ee.SplitOwnedPeer(peer)
+	if !ok {
+		return e2ee.DeviceListDoc{}, fmt.Errorf("omemo: PublishedIdentity: %q is not an OwnedPeer", peer)
+	}
+
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	store := b.db.Store(b.libctx, login, owner)
+	if has, err := store.HasIdentityKeyPair(); err != nil {
+		return e2ee.DeviceListDoc{}, err
+	} else if !has {
+		return e2ee.DeviceListDoc{}, errors.New("omemo: PublishedIdentity: EnsureIdentity was not called")
+	}
+
+	raw, err := encodeDeviceList(b.version, ownDeviceIDNum)
+	if err != nil {
+		return e2ee.DeviceListDoc{}, err
+	}
+	return e2ee.DeviceListDoc{Raw: raw}, nil
+}
+
+func (b *Backend) PublishedBundle(peer e2ee.PeerID, device e2ee.DeviceID) (e2ee.BundleDoc, error) {
+	login, owner, ok := e2ee.SplitOwnedPeer(peer)
+	if !ok {
+		return e2ee.BundleDoc{}, fmt.Errorf("omemo: PublishedBundle: %q is not an OwnedPeer", peer)
+	}
+	if device != OwnDeviceID {
+		return e2ee.BundleDoc{}, fmt.Errorf("omemo: PublishedBundle: unknown device %q", device)
+	}
+
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	store := b.db.Store(b.libctx, login, owner)
+
+	identityPublic, _, err := store.GetIdentityKeyPair()
+	if err != nil {
+		return e2ee.BundleDoc{}, fmt.Errorf("omemo: GetIdentityKeyPair: %w", err)
+	}
+
+	// EnsureIdentity always creates signed prekey id 1 and never rotates it
+	// yet (a future enhancement), so id 1 is the only one that will exist.
+	spkRecord, found, err := store.LoadSignedPreKey(1)
+	if err != nil {
+		return e2ee.BundleDoc{}, fmt.Errorf("omemo: LoadSignedPreKey: %w", err)
+	}
+	if !found {
+		return e2ee.BundleDoc{}, errors.New("omemo: PublishedBundle: EnsureIdentity was not called")
+	}
+	spkInfo, err := libsignal.DecodeSignedPreKey(b.libctx, spkRecord)
+	if err != nil {
+		return e2ee.BundleDoc{}, fmt.Errorf("omemo: DecodeSignedPreKey: %w", err)
+	}
+	// See the rule documented on libsignal.SignedPreKeyInfo: the signature
+	// must match the serialization form the verifier will re-derive, which
+	// depends on the recipient's protocol version (Omemo0 -> legacy form,
+	// Omemo1/Omemo2 -> OMEMO form, since both share libsignal.ProtocolVersionV4)
+	// - but since we only publish one bundle version at a time (this
+	// backend's own configured version), that's the one to use here too.
+	signature := spkInfo.Signature
+	if b.version != Omemo0 {
+		signature = spkInfo.SignatureOMEMO
+	}
+
+	preKeyIDs, err := store.ListPreKeyIDs()
+	if err != nil {
+		return e2ee.BundleDoc{}, fmt.Errorf("omemo: ListPreKeyIDs: %w", err)
+	}
+	preKeys := make([]libsignal.PreKeyInfo, 0, len(preKeyIDs))
+	for _, id := range preKeyIDs {
+		record, found, err := store.LoadPreKey(id)
+		if err != nil {
+			return e2ee.BundleDoc{}, fmt.Errorf("omemo: LoadPreKey %d: %w", id, err)
+		}
+		if !found {
+			continue // consumed between ListPreKeyIDs and here - fine, just skip it
+		}
+		info, err := libsignal.DecodePreKey(b.libctx, record)
+		if err != nil {
+			return e2ee.BundleDoc{}, fmt.Errorf("omemo: DecodePreKey %d: %w", id, err)
+		}
+		preKeys = append(preKeys, *info)
+	}
+
+	raw, err := encodeBundle(b.version, identityPublic, spkInfo.ID, spkInfo.PublicKey, signature, preKeys)
+	if err != nil {
+		return e2ee.BundleDoc{}, err
+	}
+	return e2ee.BundleDoc{Raw: raw}, nil
+}
+
+func (b *Backend) IngestRemoteDeviceList(owner, peer e2ee.PeerID, doc e2ee.DeviceListDoc) error {
+	login, ownerJID, ok := e2ee.SplitOwnedPeer(owner)
+	if !ok {
+		return fmt.Errorf("omemo: IngestRemoteDeviceList: %q is not an OwnedPeer", owner)
+	}
+	if _, err := parseDeviceList(doc.Raw); err != nil {
+		return fmt.Errorf("omemo: parseDeviceList: %w", err)
+	}
+
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	store := b.db.Store(b.libctx, login, ownerJID)
+	return store.SaveRemoteDeviceListCache(string(peer), doc.Raw)
+}
+
+func (b *Backend) IngestRemoteBundle(owner, peer e2ee.PeerID, device e2ee.DeviceID, doc e2ee.BundleDoc) error {
+	login, ownerJID, ok := e2ee.SplitOwnedPeer(owner)
+	if !ok {
+		return fmt.Errorf("omemo: IngestRemoteBundle: %q is not an OwnedPeer", owner)
+	}
+	deviceID, err := parseDeviceID(device)
+	if err != nil {
+		return err
+	}
+	parsed, err := parseBundle(doc.Raw)
+	if err != nil {
+		return fmt.Errorf("omemo: parseBundle: %w", err)
+	}
+
+	var preKeyID uint32
+	var preKeyPublic []byte
+	if len(parsed.PreKeys) > 0 {
+		idx, err := cryptoRandIndex(len(parsed.PreKeys))
+		if err != nil {
+			return err
+		}
+		preKeyID = parsed.PreKeys[idx].ID
+		preKeyPublic = parsed.PreKeys[idx].Public
+	}
+
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	store := b.db.Store(b.libctx, login, ownerJID)
+	storeCtx, err := libsignal.NewStoreContext(b.libctx, store)
+	if err != nil {
+		return fmt.Errorf("omemo: NewStoreContext: %w", err)
+	}
+	defer storeCtx.Close()
+
+	builder, err := libsignal.NewSessionBuilder(b.libctx, storeCtx, libsignal.Address{Name: string(peer), DeviceID: deviceID}, b.version.libsignalVersion())
+	if err != nil {
+		return fmt.Errorf("omemo: NewSessionBuilder: %w", err)
+	}
+	defer builder.Close()
+
+	err = builder.ProcessPreKeyBundle(b.libctx, libsignal.RemoteBundle{
+		// OMEMO's wire format (neither legacy nor OMEMOKeyExchange) ever
+		// transmits a registration id - device id already does what it
+		// would otherwise disambiguate. 0 is standard, harmless practice.
+		RegistrationID:        0,
+		DeviceID:              deviceID,
+		PreKeyID:              preKeyID,
+		PreKeyPublic:          preKeyPublic,
+		SignedPreKeyID:        parsed.SignedPreKeyID,
+		SignedPreKeyPublic:    parsed.SignedPreKeyPublic,
+		SignedPreKeySignature: parsed.SignedPreKeySignature,
+		IdentityKeyPublic:     parsed.IdentityKeyPublic,
+	})
+	if err != nil {
+		return fmt.Errorf("omemo: ProcessPreKeyBundle: %w", err)
+	}
+	return nil
+}
+
+func (b *Backend) Devices(owner, peer e2ee.PeerID) ([]e2ee.DeviceInfo, error) {
+	login, ownerJID, ok := e2ee.SplitOwnedPeer(owner)
+	if !ok {
+		return nil, fmt.Errorf("omemo: Devices: %q is not an OwnedPeer", owner)
+	}
+
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	store := b.db.Store(b.libctx, login, ownerJID)
+	deviceIDs, err := store.GetSubDeviceSessions(string(peer))
+	if err != nil {
+		return nil, err
+	}
+
+	infos := make([]e2ee.DeviceInfo, 0, len(deviceIDs))
+	for _, id := range deviceIDs {
+		// A stored session implies IsTrustedIdentity already accepted this
+		// device's key (TOFU) - finer-grained trust states (manually
+		// verified/revoked) are a documented future enhancement, not
+		// needed for this phase.
+		infos = append(infos, e2ee.DeviceInfo{ID: e2ee.DeviceID(strconv.FormatUint(uint64(id), 10)), Trusted: true})
+	}
+	return infos, nil
+}
+
+func parseDeviceID(device e2ee.DeviceID) (uint32, error) {
+	id, err := strconv.ParseUint(string(device), 10, 32)
+	if err != nil {
+		return 0, fmt.Errorf("omemo: invalid device id %q: %w", device, err)
+	}
+	return uint32(id), nil
+}
+
+func cryptoRandIndex(n int) (int, error) {
+	i, err := rand.Int(rand.Reader, big.NewInt(int64(n)))
+	if err != nil {
+		return 0, err
+	}
+	return int(i.Int64()), nil
+}
diff --git a/e2ee/omemo/libsignal/capability_legacy.go b/e2ee/omemo/libsignal/capability_legacy.go
index f7fac72..25be604 100644
--- a/e2ee/omemo/libsignal/capability_legacy.go
+++ b/e2ee/omemo/libsignal/capability_legacy.go
@@ -2,7 +2,7 @@
 
 package libsignal
 
-// ModernOMEMOSupported is false under the "signal_legacy" build tag: this
+// ProtocolV4Supported is false under the "signal_legacy" build tag: this
 // build links vanilla libsignal-protocol-c, which has no modern OMEMO
 // (protocol v4) support at all - only legacy/siacs OMEMO (protocol v3).
-const ModernOMEMOSupported = false
+const ProtocolV4Supported = false
diff --git a/e2ee/omemo/libsignal/capability_modern.go b/e2ee/omemo/libsignal/capability_modern.go
index cd068f5..8d26b63 100644
--- a/e2ee/omemo/libsignal/capability_modern.go
+++ b/e2ee/omemo/libsignal/capability_modern.go
@@ -2,9 +2,9 @@
 
 package libsignal
 
-// ModernOMEMOSupported reports whether this build links a library capable
+// ProtocolV4Supported reports whether this build links a library capable
 // of modern OMEMO (protocol v4, shared by OMEMO 1/urn:xmpp:omemo:1 and
 // OMEMO 2/urn:xmpp:omemo:2), as opposed to only legacy/siacs OMEMO
 // (protocol v3). Callers should check this before advertising or
 // negotiating anything beyond the legacy namespace.
-const ModernOMEMOSupported = true
+const ProtocolV4Supported = true
diff --git a/e2ee/omemo/libsignal/cipher.go b/e2ee/omemo/libsignal/cipher.go
index 5f5e697..2431062 100644
--- a/e2ee/omemo/libsignal/cipher.go
+++ b/e2ee/omemo/libsignal/cipher.go
@@ -40,7 +40,7 @@ type SessionCipher struct {
 
 // NewSessionCipher creates a session cipher for storeCtx's local identity
 // talking to remote. version selects the wire format for messages this
-// cipher *produces* (ProtocolVersionLegacy or ProtocolVersionModern);
+// cipher *produces* (ProtocolVersionV3 or ProtocolVersionV4);
 // decrypting adapts to whatever version the incoming message declares.
 func NewSessionCipher(ctx *Context, storeCtx *StoreContext, remote Address, version int) (*SessionCipher, error) {
 	addr := newCAddress(remote)
diff --git a/e2ee/omemo/libsignal/deserialize_legacy.go b/e2ee/omemo/libsignal/deserialize_legacy.go
index e9ad47e..a929924 100644
--- a/e2ee/omemo/libsignal/deserialize_legacy.go
+++ b/e2ee/omemo/libsignal/deserialize_legacy.go
@@ -11,7 +11,7 @@ import "C"
 // Vanilla libsignal-protocol-c has no OMEMO (protocol v4) wire format
 // support at all, so these always fail with ErrInvalidVersion - callers
 // (SessionCipher.Decrypt) should never reach here with useOmemoFraming set
-// under this build, since ModernOMEMOSupported is false and the caller is
+// under this build, since ProtocolV4Supported is false and the caller is
 // expected to check it first.
 
 func deserializeSignalMessageOmemo(ctx *Context, data *C.uint8_t, length C.size_t) (*C.signal_message, C.int) {
diff --git a/e2ee/omemo/libsignal/doc.go b/e2ee/omemo/libsignal/doc.go
index 7041837..6dd9d86 100644
--- a/e2ee/omemo/libsignal/doc.go
+++ b/e2ee/omemo/libsignal/doc.go
@@ -17,7 +17,7 @@
 // useful on distros that only package that one (e.g. Debian bullseye ships
 // libsignal-protocol-c-dev but not libomemo-c-dev, which arrived in
 // bookworm). That build only supports legacy/siacs OMEMO; check
-// ModernOMEMOSupported before relying on anything else. See cgo_modern.go/
+// ProtocolV4Supported before relying on anything else. See cgo_modern.go/
 // cgo_legacy.go and the version_*/deserialize_*/signedprekey_* files for
 // the resulting API-surface differences between the two libraries.
 //
diff --git a/e2ee/omemo/libsignal/keys.go b/e2ee/omemo/libsignal/keys.go
index ea7b451..4e9307a 100644
--- a/e2ee/omemo/libsignal/keys.go
+++ b/e2ee/omemo/libsignal/keys.go
@@ -245,7 +245,7 @@ func deserializeSignedPreKey(ctx *Context, record []byte) (*C.session_signed_pre
 // 33-byte DJB_TYPE-prefixed legacy form) or ec_public_key_serialize_omemo
 // (version >= 4, the raw 32-byte Montgomery form) and checks it against
 // whichever signature is supplied - so use Signature for
-// ProtocolVersionLegacy and SignatureOMEMO for ProtocolVersionModern.
+// ProtocolVersionV3 and SignatureOMEMO for ProtocolVersionV4.
 type SignedPreKeyInfo struct {
 	ID             uint32
 	PublicKey      []byte
diff --git a/e2ee/omemo/libsignal/session.go b/e2ee/omemo/libsignal/session.go
index a9e7420..592e784 100644
--- a/e2ee/omemo/libsignal/session.go
+++ b/e2ee/omemo/libsignal/session.go
@@ -19,8 +19,8 @@ import (
 // (urn:xmpp:omemo:2) - those two differ only in outer SCE stanza framing,
 // not in this underlying Double Ratchet/X3DH wire format.
 const (
-	ProtocolVersionLegacy = 3
-	ProtocolVersionModern = 4
+	ProtocolVersionV3 = 3
+	ProtocolVersionV4 = 4
 )
 
 // RemoteBundle is the decoded material from a peer's XEP-0384 device
@@ -47,7 +47,7 @@ type SessionBuilder struct {
 // NewSessionBuilder creates a session builder for storeCtx's local identity
 // talking to remote. version controls the protocol version new sessions
 // built by ProcessPreKeyBundle will use for encryption going forward
-// (ProtocolVersionLegacy or ProtocolVersionModern).
+// (ProtocolVersionV3 or ProtocolVersionV4).
 func NewSessionBuilder(ctx *Context, storeCtx *StoreContext, remote Address, version int) (*SessionBuilder, error) {
 	addr := newCAddress(remote)
 
diff --git a/e2ee/omemo/libsignal/session_test.go b/e2ee/omemo/libsignal/session_test.go
index bfb20c7..3cb0a62 100644
--- a/e2ee/omemo/libsignal/session_test.go
+++ b/e2ee/omemo/libsignal/session_test.go
@@ -118,7 +118,7 @@ func (id *identity) bundle(t *testing.T, deviceID uint32, version int) libsignal
 	}
 
 	signature := spkInfo.Signature
-	if version >= libsignal.ProtocolVersionModern {
+	if version >= libsignal.ProtocolVersionV4 {
 		signature = spkInfo.SignatureOMEMO
 	}
 
@@ -140,12 +140,12 @@ func TestSessionRoundTrip(t *testing.T) {
 		version int
 		omemo   bool
 	}{
-		{"legacy", libsignal.ProtocolVersionLegacy, false},
-		{"modern", libsignal.ProtocolVersionModern, true},
+		{"legacy", libsignal.ProtocolVersionV3, false},
+		{"modern", libsignal.ProtocolVersionV4, true},
 	} {
 		tc := tc
 		t.Run(tc.name, func(t *testing.T) {
-			if tc.version == libsignal.ProtocolVersionModern && !libsignal.ModernOMEMOSupported {
+			if tc.version == libsignal.ProtocolVersionV4 && !libsignal.ProtocolV4Supported {
 				t.Skip("modern OMEMO not supported by this build (signal_legacy tag)")
 			}
 
diff --git a/e2ee/omemo/libsignal/version_legacy.go b/e2ee/omemo/libsignal/version_legacy.go
index 5185d45..9711f09 100644
--- a/e2ee/omemo/libsignal/version_legacy.go
+++ b/e2ee/omemo/libsignal/version_legacy.go
@@ -18,19 +18,19 @@ var errModernOMEMOUnavailable = errors.New("libsignal: modern OMEMO (protocol v4
 // Vanilla libsignal-protocol-c has no session_builder_set_version /
 // session_cipher_set_version - it only ever produces/consumes
 // CIPHERTEXT_CURRENT_VERSION (protocol v3, legacy/siacs OMEMO) messages.
-// Requesting ProtocolVersionLegacy is a no-op (that's the library's only
+// Requesting ProtocolVersionV3 is a no-op (that's the library's only
 // behavior); requesting anything else is rejected rather than silently
 // ignored.
 
 func setBuilderVersion(b *C.session_builder, version int) error {
-	if version != ProtocolVersionLegacy {
+	if version != ProtocolVersionV3 {
 		return errModernOMEMOUnavailable
 	}
 	return nil
 }
 
 func setCipherVersion(c *C.session_cipher, version int) error {
-	if version != ProtocolVersionLegacy {
+	if version != ProtocolVersionV3 {
 		return errModernOMEMOUnavailable
 	}
 	return nil
diff --git a/e2ee/omemo/libsignal/version_modern.go b/e2ee/omemo/libsignal/version_modern.go
index 54ace5f..fe64460 100644
--- a/e2ee/omemo/libsignal/version_modern.go
+++ b/e2ee/omemo/libsignal/version_modern.go
@@ -10,7 +10,7 @@ package libsignal
 import "C"
 
 // setBuilderVersion and setCipherVersion select the wire protocol version
-// (ProtocolVersionLegacy or ProtocolVersionModern) libomemo-c's
+// (ProtocolVersionV3 or ProtocolVersionV4) libomemo-c's
 // session_builder_set_version/session_cipher_set_version support. See
 // version_legacy.go for the vanilla-libsignal-protocol-c build, which has
 // no such functions and only ever produces protocol v3.
diff --git a/e2ee/omemo/sce.go b/e2ee/omemo/sce.go
index 0417ca8..4ca35d0 100644
--- a/e2ee/omemo/sce.go
+++ b/e2ee/omemo/sce.go
@@ -7,22 +7,34 @@ import (
 	"math/big"
 )
 
-// Minimal XEP-0420 Stanza Content Encryption envelope, used only as the
-// plaintext OMEMO 2 (modern) content encryption operates on - just enough
-// to carry a plain message body, per the spec's requirement that the
+// Two dialects of XEP-0420 Stanza Content Encryption, matching what each
+// OMEMO version actually wraps its plaintext in - just enough of each to
+// carry a plain message body, per each dialect's requirement that the
 // envelope MUST contain <rpad/> and SHOULD contain <from/> (MUST contain
 // <to/> for MUC, which this gateway doesn't do OMEMO for - see the plan's
 // scope note on MUC).
-type sceEnvelope struct {
-	XMLName xml.Name    `xml:"urn:xmpp:sce:1 envelope"`
-	Content sceContent  `xml:"content"`
-	RPad    string      `xml:"rpad"`
-	From    *sceJIDAttr `xml:"from"`
-}
-
-type sceContent struct {
-	Body sceBody `xml:"body"`
-}
+//
+// V0 (Omemo1, XEP-0384 0.4.0-0.7.x) is the OLD XEP-0420 shape, confirmed
+// by reading the archived 0.3.2 spec directly:
+//
+//	<content xmlns='urn:xmpp:sce:0'>
+//	  <payload>
+//	    <body xmlns='jabber:client'>...</body>
+//	  </payload>
+//	  <from jid='...'/>
+//	  <rpad>...</rpad>
+//	</content>
+//
+// V1 (Omemo2, current) renamed content->envelope and payload->content
+// (XEP-0420 0.4.0+, urn:xmpp:sce:1):
+//
+//	<envelope xmlns='urn:xmpp:sce:1'>
+//	  <content>
+//	    <body xmlns='jabber:client'>...</body>
+//	  </content>
+//	  <from jid='...'/>
+//	  <rpad>...</rpad>
+//	</envelope>
 
 type sceBody struct {
 	XMLName xml.Name `xml:"jabber:client body"`
@@ -33,16 +45,26 @@ type sceJIDAttr struct {
 	JID string `xml:"jid,attr"`
 }
 
-// sceEncode wraps plaintext in an SCE envelope and serializes it - the
-// "plaintext" that modern OMEMO's outer content encryption actually
-// operates on. fromJID becomes the <from/> binding (empty to omit it).
-func sceEncode(plaintext []byte, fromJID string) ([]byte, error) {
+// --- V0 (Omemo1) ---
+
+type sceContentV0 struct {
+	XMLName xml.Name      `xml:"urn:xmpp:sce:0 content"`
+	Payload scePayloadV0  `xml:"payload"`
+	From    *sceJIDAttr   `xml:"from"`
+	RPad    string        `xml:"rpad"`
+}
+
+type scePayloadV0 struct {
+	Body sceBody `xml:"body"`
+}
+
+func sceEncodeV0(plaintext []byte, fromJID string) ([]byte, error) {
 	rpad, err := randomPadding()
 	if err != nil {
 		return nil, err
 	}
-	env := sceEnvelope{
-		Content: sceContent{Body: sceBody{Text: string(plaintext)}},
+	env := sceContentV0{
+		Payload: scePayloadV0{Body: sceBody{Text: string(plaintext)}},
 		RPad:    rpad,
 	}
 	if fromJID != "" {
@@ -51,9 +73,44 @@ func sceEncode(plaintext []byte, fromJID string) ([]byte, error) {
 	return xml.Marshal(env)
 }
 
-// sceDecode reverses sceEncode, extracting the plain body text.
-func sceDecode(data []byte) ([]byte, error) {
-	var env sceEnvelope
+func sceDecodeV0(data []byte) ([]byte, error) {
+	var env sceContentV0
+	if err := xml.Unmarshal(data, &env); err != nil {
+		return nil, err
+	}
+	return []byte(env.Payload.Body.Text), nil
+}
+
+// --- V1 (Omemo2) ---
+
+type sceEnvelopeV1 struct {
+	XMLName xml.Name    `xml:"urn:xmpp:sce:1 envelope"`
+	Content sceContentV1 `xml:"content"`
+	From    *sceJIDAttr `xml:"from"`
+	RPad    string      `xml:"rpad"`
+}
+
+type sceContentV1 struct {
+	Body sceBody `xml:"body"`
+}
+
+func sceEncodeV1(plaintext []byte, fromJID string) ([]byte, error) {
+	rpad, err := randomPadding()
+	if err != nil {
+		return nil, err
+	}
+	env := sceEnvelopeV1{
+		Content: sceContentV1{Body: sceBody{Text: string(plaintext)}},
+		RPad:    rpad,
+	}
+	if fromJID != "" {
+		env.From = &sceJIDAttr{JID: fromJID}
+	}
+	return xml.Marshal(env)
+}
+
+func sceDecodeV1(data []byte) ([]byte, error) {
+	var env sceEnvelopeV1
 	if err := xml.Unmarshal(data, &env); err != nil {
 		return nil, err
 	}
@@ -62,7 +119,7 @@ func sceDecode(data []byte) ([]byte, error) {
 
 // randomPadding returns base64-encoded random padding of a random length
 // (1-63 bytes before encoding) to mask plaintext length, per XEP-0420's
-// MUST-contain-<rpad/> requirement.
+// MUST-contain-<rpad/> requirement (both dialects).
 func randomPadding() (string, error) {
 	n, err := rand.Int(rand.Reader, big.NewInt(63))
 	if err != nil {
diff --git a/e2ee/omemo/version.go b/e2ee/omemo/version.go
new file mode 100644
index 0000000..67dd996
--- /dev/null
+++ b/e2ee/omemo/version.go
@@ -0,0 +1,59 @@
+package omemo
+
+import "dev.narayana.im/narayana/telegabber/e2ee/omemo/libsignal"
+
+// Version identifies which OMEMO wire-format generation a message, device
+// list, or bundle uses - the full namespace+XML dialect, not just the
+// underlying crypto-engine version (see libsignal.ProtocolVersionV3/V4,
+// which is a strictly coarser, two-way distinction shared by two of the
+// three versions below).
+type Version int
+
+const (
+	// Omemo0 is the original "siacs" OMEMO (eu.siacs.conversations.axolotl,
+	// XEP-0384 <= 0.3.0): libsignal.ProtocolVersionV3, AES-128-GCM directly
+	// on the plaintext body, key||tag distributed per-device, <iv> element
+	// present. Still the most widely deployed version as of this writing,
+	// and the current implementation priority.
+	Omemo0 Version = iota
+
+	// Omemo1 is urn:xmpp:omemo:1 (XEP-0384 0.4.0-0.7.x, introduced
+	// 2020-03-08): libsignal.ProtocolVersionV4, AES-256-CBC+HMAC-SHA256
+	// (via HKDF) over an SCE envelope using the OLD (pre-0.4.0) XEP-0420
+	// dialect: root element <content xmlns='urn:xmpp:sce:0'>, inner
+	// wrapper <payload/>. Confirmed by reading the archived XEP-0420 0.3.2
+	// spec directly (https://xmpp.org/extensions/attic/xep-0420-0.3.2.html),
+	// not assumed - see sceEncodeV0/sceDecodeV0.
+	Omemo1
+
+	// Omemo2 is urn:xmpp:omemo:2 (XEP-0384 0.8.0+, current spec):
+	// libsignal.ProtocolVersionV4, the same AES-256-CBC+HMAC-SHA256 scheme
+	// as Omemo1, but over the NEW (XEP-0420 0.4.0+) dialect: root element
+	// <envelope xmlns='urn:xmpp:sce:1'>, inner wrapper <content/>.
+	Omemo2
+)
+
+func (v Version) String() string {
+	switch v {
+	case Omemo0:
+		return "omemo0"
+	case Omemo1:
+		return "omemo1"
+	case Omemo2:
+		return "omemo2"
+	default:
+		return "omemo?"
+	}
+}
+
+// libsignalVersion returns the underlying Signal-protocol wire version
+// this OMEMO version uses. Omemo1 and Omemo2 share the same crypto-engine
+// version - libomemo-c itself only branches on "message_version >= 4", not
+// on which OMEMO namespace is in play (confirmed by reading protocol.c) -
+// they differ only in the outer XML/SCE dialect (see envelope.go).
+func (v Version) libsignalVersion() int {
+	if v == Omemo0 {
+		return libsignal.ProtocolVersionV3
+	}
+	return libsignal.ProtocolVersionV4
+}
diff --git a/e2ee/peer.go b/e2ee/peer.go
new file mode 100644
index 0000000..75c12ae
--- /dev/null
+++ b/e2ee/peer.go
@@ -0,0 +1,25 @@
+package e2ee
+
+import "strings"
+
+// OwnedPeer constructs the PeerID for an identity this gateway owns and
+// generates keys for - a bridged chat pseudo-JID.
+//
+// Telegram chat ids (and therefore their pseudo-JIDs) are only unique
+// within one Telegram login, not globally - telegabber bridges multiple
+// Telegram accounts under the same XMPP domain, so two different logins
+// could each have a chat whose pseudo-JID is the same string. An owned
+// PeerID must therefore be scoped by login too. A real remote peer's
+// PeerID (the other end of a conversation) needs no such scoping - it's
+// just their bare JID - since their sessions/trust are already nested
+// under the owning identity's own scope (see badgerstore's key layout).
+func OwnedPeer(login, bareJID string) PeerID {
+	return PeerID(login + "\x00" + bareJID)
+}
+
+// SplitOwnedPeer reverses OwnedPeer. ok is false if peer wasn't built by
+// OwnedPeer (e.g. a plain remote-peer PeerID was passed by mistake).
+func SplitOwnedPeer(peer PeerID) (login, bareJID string, ok bool) {
+	login, bareJID, ok = strings.Cut(string(peer), "\x00")
+	return
+}
diff --git a/e2ee/store/badgerstore/store.go b/e2ee/store/badgerstore/store.go
index edea7cf..b2b384f 100644
--- a/e2ee/store/badgerstore/store.go
+++ b/e2ee/store/badgerstore/store.go
@@ -59,6 +59,10 @@ func (s *Store) preKeyKey(id uint32) []byte {
 	return []byte(fmt.Sprintf("%s/%s/identity/prekey/%d", s.login, s.owner, id))
 }
 
+func (s *Store) preKeyPrefix() []byte {
+	return []byte(fmt.Sprintf("%s/%s/identity/prekey/", s.login, s.owner))
+}
+
 func (s *Store) preKeyCounterKey() []byte {
 	return []byte(fmt.Sprintf("%s/%s/identity/prekey/counter", s.login, s.owner))
 }
@@ -212,6 +216,35 @@ func (s *Store) ContainsPreKey(id uint32) bool {
 	return found
 }
 
+// ListPreKeyIDs returns the ids of every one-time prekey currently stored
+// (i.e. not yet consumed - libomemo-c's session_cipher automatically
+// removes a prekey via RemovePreKey the moment it's used to decrypt an
+// inbound prekey message, so the surviving ids are neither contiguous nor
+// derivable from the counter NextPreKeyIDs advances). Used to publish an
+// accurate bundle - see e2ee/omemo's PublishedBundle.
+func (s *Store) ListPreKeyIDs() ([]uint32, error) {
+	prefix := s.preKeyPrefix()
+	var ids []uint32
+	err := s.db.View(func(txn *badger.Txn) error {
+		opts := badger.DefaultIteratorOptions
+		opts.PrefetchValues = false
+		it := txn.NewIterator(opts)
+		defer it.Close()
+		for it.Seek(prefix); it.ValidForPrefix(prefix); it.Next() {
+			suffix := it.Item().Key()[len(prefix):]
+			// Skips the "counter" key, which shares this prefix - its
+			// suffix doesn't parse as a uint32, so it's silently excluded.
+			id, err := strconv.ParseUint(string(suffix), 10, 32)
+			if err != nil {
+				continue
+			}
+			ids = append(ids, uint32(id))
+		}
+		return nil
+	})
+	return ids, err
+}
+
 func (s *Store) RemovePreKey(id uint32) error {
 	return s.delete(s.preKeyKey(id))
 }
@@ -339,7 +372,10 @@ func (s *Store) SaveRegistrationID(id uint32) error {
 // NextPreKeyIDs atomically reserves a contiguous block of count prekey ids
 // for libsignal.GeneratePreKeys, returning the first id in the block.
 func (s *Store) NextPreKeyIDs(count uint32) (uint32, error) {
-	var start uint32
+	// Starts at 1, not 0: XEP-0384 0.9.1 explicitly requires positive
+	// (non-zero) prekey ids ("Fix using id=0 in examples. Spec requires
+	// positive numbers.").
+	start := uint32(1)
 	err := s.db.Update(func(txn *badger.Txn) error {
 		item, err := txn.Get(s.preKeyCounterKey())
 		if err != nil && err != badger.ErrKeyNotFound {
diff --git a/e2ee/store/badgerstore/store_test.go b/e2ee/store/badgerstore/store_test.go
index 89ce91b..70ae4ab 100644
--- a/e2ee/store/badgerstore/store_test.go
+++ b/e2ee/store/badgerstore/store_test.go
@@ -135,15 +135,15 @@ func TestNextPreKeyIDs(t *testing.T) {
 	if err != nil {
 		t.Fatalf("NextPreKeyIDs: %v", err)
 	}
-	if start1 != 0 {
-		t.Fatalf("expected first block to start at 0, got %d", start1)
+	if start1 != 1 {
+		t.Fatalf("expected first block to start at 1 (XEP-0384 requires positive ids), got %d", start1)
 	}
 	start2, err := store.NextPreKeyIDs(5)
 	if err != nil {
 		t.Fatalf("NextPreKeyIDs: %v", err)
 	}
-	if start2 != 10 {
-		t.Fatalf("expected second block to start at 10, got %d", start2)
+	if start2 != 11 {
+		t.Fatalf("expected second block to start at 11, got %d", start2)
 	}
 }
 
diff --git a/e2ee/types.go b/e2ee/types.go
index fa28961..1d7f3a1 100644
--- a/e2ee/types.go
+++ b/e2ee/types.go
@@ -56,35 +56,42 @@ type Backend interface {
 	Namespaces() Namespaces
 
 	// EnsureIdentity creates (idempotently) a local cryptographic identity
-	// for a PeerID this gateway owns (a bridged chat pseudo-JID),
-	// generating and persisting whatever key material the backend needs.
+	// for a PeerID this gateway owns (a bridged chat pseudo-JID - built via
+	// OwnedPeer, since a chat pseudo-JID alone isn't globally unique across
+	// telegabber's multiple bridged Telegram logins), generating and
+	// persisting whatever key material the backend needs.
 	EnsureIdentity(peer PeerID) error
 
 	// PublishedIdentity returns the material this gateway must SERVE to
-	// pubsub GET requests for its own PeerID (device list), pre-marshaled
-	// to opaque bytes the xmpp layer wraps in <items>/<item>.
+	// pubsub GET requests for its own PeerID (an OwnedPeer - see
+	// EnsureIdentity) device list, pre-marshaled to opaque bytes the xmpp
+	// layer wraps in <items>/<item>.
 	PublishedIdentity(peer PeerID) (DeviceListDoc, error)
 	// PublishedBundle is the same, for one specific device's bundle node.
 	PublishedBundle(peer PeerID, device DeviceID) (BundleDoc, error)
 
 	// IngestRemoteDeviceList / IngestRemoteBundle feed the results of an
 	// outbound PEP fetch (device-list, then per-device bundle) for a real
-	// remote peer into the backend's store, establishing/refreshing
-	// sessions as needed (trust decisions happen inside here).
-	IngestRemoteDeviceList(peer PeerID, doc DeviceListDoc) error
-	IngestRemoteBundle(peer PeerID, device DeviceID, doc BundleDoc) error
+	// remote peer (owner is an OwnedPeer identifying which of this
+	// gateway's identities is doing the fetching; peer is the remote's
+	// plain bare JID, no scoping needed) into the backend's store,
+	// establishing/refreshing sessions as needed (trust decisions happen
+	// inside here).
+	IngestRemoteDeviceList(owner PeerID, peer PeerID, doc DeviceListDoc) error
+	IngestRemoteBundle(owner PeerID, peer PeerID, device DeviceID, doc BundleDoc) error
 
 	// Encrypt produces an opaque envelope addressed to every known device
 	// of every given recipient, ready to be embedded in a stanza by the
-	// xmpp layer. from is the identity doing the encrypting (a chat
-	// pseudo-JID this gateway owns).
+	// xmpp layer. from is the identity doing the encrypting (an OwnedPeer -
+	// see EnsureIdentity); to are the real remote peers' plain bare JIDs.
 	Encrypt(from PeerID, to []PeerID, plaintext []byte) (Envelope, error)
 
-	// Decrypt consumes an inbound envelope addressed to "to" (a chat
-	// pseudo-JID this gateway owns) from "from" (the real remote peer)
-	// and returns plaintext.
+	// Decrypt consumes an inbound envelope addressed to "to" (an
+	// OwnedPeer - see EnsureIdentity) from "from" (the real remote peer's
+	// plain bare JID) and returns plaintext.
 	Decrypt(from PeerID, to PeerID, env Envelope) ([]byte, error)
 
-	// Devices lists known devices for a peer (trust-listing ad-hoc commands).
-	Devices(peer PeerID) ([]DeviceInfo, error)
+	// Devices lists peer's known devices, as seen from owner's (an
+	// OwnedPeer) point of view - for trust-listing ad-hoc commands.
+	Devices(owner PeerID, peer PeerID) ([]DeviceInfo, error)
 }