wire: Add epoch field to mix key exchange message

The epoch will be included in the derivation of the session hash, instead of
only deriving a session from previous pair requests.  This allows new messages
in following epochs to not conflict with previous session attempts of the same
pair request set.

This also provides a decent heuristic to discover and exclude any peers that
were unresponsive for a full epoch or more.

This is a breaking API change but the wire module has not seen a release since
mixing messages were added.

wire/message_test.go

@@ -84,7 +84,7 @@ func TestMessage(t *testing.T) {
 	if err != nil {
 		t.Errorf("NewMsgMixPairReq: %v", err)
 	}
-	msgMixKE := NewMsgMixKeyExchange([33]byte{}, [32]byte{}, 1, [33]byte{}, [1218]byte{}, [32]byte{}, []chainhash.Hash{})
+	msgMixKE := NewMsgMixKeyExchange([33]byte{}, [32]byte{}, 1, 1, [33]byte{}, [1218]byte{}, [32]byte{}, []chainhash.Hash{})
 	msgMixCT := NewMsgMixCiphertexts([33]byte{}, [32]byte{}, 1, [][1047]byte{}, []chainhash.Hash{})
 	msgMixSR := NewMsgMixSlotReserve([33]byte{}, [32]byte{}, 1, [][][]byte{{{}}}, []chainhash.Hash{})
 	msgMixDC := NewMsgMixDCNet([33]byte{}, [32]byte{}, 1, []MixVect{make(MixVect, 1)}, []chainhash.Hash{})
@@ -123,7 +123,7 @@ func TestMessage(t *testing.T) {
 		{msgGetInitState, msgGetInitState, pver, MainNet, 25},
 		{msgInitState, msgInitState, pver, MainNet, 28},
 		{msgMixPR, msgMixPR, pver, MainNet, 165},
-		{msgMixKE, msgMixKE, pver, MainNet, 1441},
+		{msgMixKE, msgMixKE, pver, MainNet, 1449},
 		{msgMixCT, msgMixCT, pver, MainNet, 158},
 		{msgMixSR, msgMixSR, pver, MainNet, 161},
 		{msgMixDC, msgMixDC, pver, MainNet, 181},

wire/msgmixkeyexchange.go

@@ -30,6 +30,7 @@ type MsgMixKeyExchange struct {
 	Signature  [64]byte
 	Identity   [33]byte
 	SessionID  [32]byte
+	Epoch      uint64
 	Run        uint32
 	ECDH       [33]byte   // Secp256k1 public key
 	PQPK       [1218]byte // Sntrup4591761 public key
@@ -53,7 +54,7 @@ func (msg *MsgMixKeyExchange) BtcDecode(r io.Reader, pver uint32) error {
 	}
 
 	err := readElements(r, &msg.Signature, &msg.Identity, &msg.SessionID,
-		&msg.Run, &msg.ECDH, &msg.PQPK, &msg.Commitment)
+		&msg.Epoch, &msg.Run, &msg.ECDH, &msg.PQPK, &msg.Commitment)
 	if err != nil {
 		return err
 	}
@@ -146,8 +147,8 @@ func (msg *MsgMixKeyExchange) writeMessageNoSignature(op string, w io.Writer, pv
 		return messageError(op, ErrTooManyPrevMixMsgs, msg)
 	}
 
-	err := writeElements(w, &msg.Identity, &msg.SessionID, msg.Run,
-		&msg.ECDH, &msg.PQPK, &msg.Commitment)
+	err := writeElements(w, &msg.Identity, &msg.SessionID, msg.Epoch,
+		msg.Run, &msg.ECDH, &msg.PQPK, &msg.Commitment)
 	if err != nil {
 		return err
 	}
@@ -188,7 +189,7 @@ func (msg *MsgMixKeyExchange) MaxPayloadLength(pver uint32) uint32 {
 	}
 
 	// See tests for this calculation.
-	return 17803
+	return 17811
 }
 
 // Pub returns the message sender's public key identity.
@@ -219,12 +220,14 @@ func (msg *MsgMixKeyExchange) GetRun() uint32 {
 // NewMsgMixKeyExchange returns a new mixkeyxchg message that conforms to the
 // Message interface using the passed parameters and defaults for the
 // remaining fields.
-func NewMsgMixKeyExchange(identity [33]byte, sid [32]byte, run uint32,
-	ecdh [33]byte, pqpk [1218]byte, commitment [32]byte, seenPRs []chainhash.Hash) *MsgMixKeyExchange {
+func NewMsgMixKeyExchange(identity [33]byte, sid [32]byte, epoch uint64,
+	run uint32, ecdh [33]byte, pqpk [1218]byte, commitment [32]byte,
+	seenPRs []chainhash.Hash) *MsgMixKeyExchange {
 
 	return &MsgMixKeyExchange{
 		Identity:   identity,
 		SessionID:  sid,
+		Epoch:      epoch,
 		Run:        run,
 		ECDH:       ecdh,
 		PQPK:       pqpk,

wire/msgmixkeyexchange_test.go

@@ -21,18 +21,19 @@ func newTestMixKeyExchange() *MsgMixKeyExchange {
 	id := *(*[33]byte)(repeat(0x81, 33))
 	sid := *(*[32]byte)(repeat(0x82, 32))
 
-	const run = uint32(0x83838383)
+	const epoch = uint64(0x8383838383838383)
+	const run = uint32(0x84848484)
 
-	ecdh := *(*[33]byte)(repeat(0x84, 33))
-	pqpk := *(*[1218]byte)(repeat(0x85, 1218))
-	commitment := *(*[32]byte)(repeat(0x86, 32))
+	ecdh := *(*[33]byte)(repeat(0x85, 33))
+	pqpk := *(*[1218]byte)(repeat(0x86, 1218))
+	commitment := *(*[32]byte)(repeat(0x87, 32))
 
 	seenPRs := make([]chainhash.Hash, 4)
-	for b := byte(0x87); b < 0x8B; b++ {
-		copy(seenPRs[b-0x87][:], repeat(b, 32))
+	for b := byte(0x88); b < 0x8C; b++ {
+		copy(seenPRs[b-0x88][:], repeat(b, 32))
 	}
 
-	ke := NewMsgMixKeyExchange(id, sid, run, ecdh, pqpk, commitment, seenPRs)
+	ke := NewMsgMixKeyExchange(id, sid, epoch, run, ecdh, pqpk, commitment, seenPRs)
 	ke.Signature = sig
 
 	return ke
@@ -53,16 +54,17 @@ func TestMsgMixKeyExchangeWire(t *testing.T) {
 	expected = append(expected, repeat(0x80, 64)...)   // Signature
 	expected = append(expected, repeat(0x81, 33)...)   // Identity
 	expected = append(expected, repeat(0x82, 32)...)   // Session ID
-	expected = append(expected, repeat(0x83, 4)...)    // Run
-	expected = append(expected, repeat(0x84, 33)...)   // ECDH public key
-	expected = append(expected, repeat(0x85, 1218)...) // PQ public key
-	expected = append(expected, repeat(0x86, 32)...)   // Secrets commitment
-	// Four seen PRs (repeating 32 bytes of 0x87, 0x88, 0x89, 0x8a)
+	expected = append(expected, repeat(0x83, 8)...)    // Epoch
+	expected = append(expected, repeat(0x84, 4)...)    // Run
+	expected = append(expected, repeat(0x85, 33)...)   // ECDH public key
+	expected = append(expected, repeat(0x86, 1218)...) // PQ public key
+	expected = append(expected, repeat(0x87, 32)...)   // Secrets commitment
+	// Four seen PRs (repeating 32 bytes of 0x88, 0x89, 0x8a, 0x8b)
 	expected = append(expected, 0x04)
-	expected = append(expected, repeat(0x87, 32)...)
 	expected = append(expected, repeat(0x88, 32)...)
 	expected = append(expected, repeat(0x89, 32)...)
 	expected = append(expected, repeat(0x8a, 32)...)
+	expected = append(expected, repeat(0x8b, 32)...)
 
 	expectedSerializationEqual(t, buf.Bytes(), expected)
 
@@ -155,6 +157,7 @@ func TestMsgMixKeyExchangeMaxPayloadLength(t *testing.T) {
 	var expectedLen uint32 = 64 + // Signature
 		33 + // Identity
 		32 + // Session ID
+		8 + // Epoch
 		4 + // Run
 		33 + // ECDH public key
 		1218 + // sntrup4591761 public key