Recover UDP crypto after packet loss

gumble/gumble/udp15.go

@@ -292,10 +292,13 @@ func (cs *cryptState15) decrypt15(packet []byte) ([]byte, error) {
 			backupIV(cs.decryptIV[:])
 			restore = true
 		} else if ivByte > cs.decryptIV[0] && diff > 0 {
-			// We missed packets; catch up. Already handled above.
+			// We missed packets; move the low IV byte forward.
+			cs.decryptIV[0] = ivByte
 		} else if ivByte < cs.decryptIV[0] && diff > 0 {
-			// Wrapped forward; advance and catch up.
-			advanceIV(cs.decryptIV[:])
+			// We missed packets across a low-byte wrap. The IV's higher
+			// bytes must advance even though the received low byte is set
+			// below rather than incremented.
+			advanceIVHighBytes(cs.decryptIV[:])
 			cs.decryptIV[0] = ivByte
 		} else {
 			return nil, errors.New("gumble: OCB IV too far off")
@@ -340,6 +343,16 @@ func advanceIV(iv []byte) {
 	}
 }
 
+// advanceIVHighBytes advances all but the low IV byte as a little-endian integer.
+func advanceIVHighBytes(iv []byte) {
+	for i := 1; i < len(iv); i++ {
+		iv[i]++
+		if iv[i] != 0 {
+			break
+		}
+	}
+}
+
 // backupIV decrements a 16-byte IV as a little-endian integer.
 func backupIV(iv []byte) {
 	for i := 0; i < len(iv); i++ {

gumble/gumble/udp15_test.go

@@ -32,6 +32,75 @@ func TestAdvanceIV(t *testing.T) {
 }
 
 // Regression coverage for the native IV carry path at the 255->256 wrap.
+func TestCryptState15DecryptsAfterMissedPackets(t *testing.T) {
+	key := mustDecodeHex("93360b0f86a926c4561563469026eb94")
+	nonce := mustDecodeHex("10000000000000000000000000000000")
+	out, in := &cryptState15{}, &cryptState15{}
+	if err := out.setup15(key, nonce, nonce); err != nil {
+		t.Fatal(err)
+	}
+	if err := in.setup15(key, nonce, nonce); err != nil {
+		t.Fatal(err)
+	}
+
+	first, err := out.encrypt15([]byte("first"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := in.decrypt15(first); err != nil {
+		t.Fatal(err)
+	}
+	for i := 0; i < 3; i++ {
+		if _, err := out.encrypt15([]byte("dropped")); err != nil {
+			t.Fatal(err)
+		}
+	}
+	last, err := out.encrypt15([]byte("after loss"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	plain, err := in.decrypt15(last)
+	if err != nil || !bytes.Equal(plain, []byte("after loss")) {
+		t.Fatalf("decrypt after missed packets = %q, %v", plain, err)
+	}
+}
+
+func TestCryptState15DecryptsAfterMissedPacketsAcrossIVByteWrap(t *testing.T) {
+	key := mustDecodeHex("93360b0f86a926c4561563469026eb94")
+	nonce := mustDecodeHex("fa000000000000000000000000000000")
+	out, in := &cryptState15{}, &cryptState15{}
+	if err := out.setup15(key, nonce, nonce); err != nil {
+		t.Fatal(err)
+	}
+	if err := in.setup15(key, nonce, nonce); err != nil {
+		t.Fatal(err)
+	}
+
+	first, err := out.encrypt15([]byte("before wrap"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := in.decrypt15(first); err != nil {
+		t.Fatal(err)
+	}
+	for i := 0; i < 6; i++ {
+		if _, err := out.encrypt15([]byte("dropped")); err != nil {
+			t.Fatal(err)
+		}
+	}
+	last, err := out.encrypt15([]byte("after wrap"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	plain, err := in.decrypt15(last)
+	if err != nil || !bytes.Equal(plain, []byte("after wrap")) {
+		t.Fatalf("decrypt after missed packets across IV wrap = %q, %v", plain, err)
+	}
+	if in.decryptIV[0] != 2 || in.decryptIV[1] != 1 {
+		t.Fatalf("unexpected IV after wrapped loss: %x", in.decryptIV[:2])
+	}
+}
+
 func TestCryptState15DecryptsAcrossIVByteWrap(t *testing.T) {
 	key := mustDecodeHex("93360b0f86a926c4561563469026eb94")
 	clientNonce := mustDecodeHex("ff000000000000000000000000000000")