wgengine/magicsock: open discovery naclbox messages from known peers

And track known peers.

Doesn't yet do anything with the messages. (nor does it send any yet)

Start of docs on the message format. More will come in subsequent changes.

Updates #483

wgengine/magicsock/magicsock_test.go

@@ -23,6 +23,7 @@ import (
 	"github.com/tailscale/wireguard-go/device"
 	"github.com/tailscale/wireguard-go/tun/tuntest"
 	"github.com/tailscale/wireguard-go/wgcfg"
+	"golang.org/x/crypto/nacl/box"
 	"tailscale.com/derp"
 	"tailscale.com/derp/derphttp"
 	"tailscale.com/derp/derpmap"
@@ -835,3 +836,30 @@ func TestAddrSet(t *testing.T) {
 		})
 	}
 }
+
+func TestDiscoMessage(t *testing.T) {
+	peer1Priv := key.NewPrivate()
+	peer1Pub := peer1Priv.Public()
+
+	c := &Conn{
+		logf:         t.Logf,
+		discoPrivate: key.NewPrivate(),
+		nodeOfDisco: map[tailcfg.DiscoKey]tailcfg.NodeKey{
+			tailcfg.DiscoKey(peer1Pub): tailcfg.NodeKey{1: 1},
+		},
+	}
+
+	const payload = "why hello"
+
+	var nonce [24]byte
+	crand.Read(nonce[:])
+
+	pkt := append([]byte("TS💬"), peer1Pub[:]...)
+	pkt = append(pkt, nonce[:]...)
+
+	pkt = box.Seal(pkt, []byte(payload), &nonce, c.discoPrivate.Public().B32(), peer1Priv.B32())
+	got := c.handleDiscoMessage(pkt, &net.UDPAddr{})
+	if !got {
+		t.Error("failed to open it")
+	}
+}