Generate and read the Noise private key

For TS2021 we need a new private key. It must be different from
the one used in the original protocol, as told by Tailscale's Dave Anderson
 https://github.com/juanfont/headscale/issues/526#issuecomment-1079795935.

app.go

@@ -81,6 +81,7 @@ type Config struct {
 	EphemeralNodeInactivityTimeout time.Duration
 	IPPrefixes                     []netaddr.IPPrefix
 	PrivateKeyPath                 string
+	NoisePrivateKeyPath            string
 	BaseDomain                     string
 
 	DERP DERPConfig
@@ -143,12 +144,13 @@ type CLIConfig struct {
 
 // Headscale represents the base app of the service.
 type Headscale struct {
-	cfg        Config
-	db         *gorm.DB
-	dbString   string
-	dbType     string
-	dbDebug    bool
-	privateKey *key.MachinePrivate
+	cfg             Config
+	db              *gorm.DB
+	dbString        string
+	dbType          string
+	dbDebug         bool
+	privateKey      *key.MachinePrivate
+	noisePrivateKey *key.MachinePrivate
 
 	DERPMap    *tailcfg.DERPMap
 	DERPServer *DERPServer
@@ -188,11 +190,20 @@ func LookupTLSClientAuthMode(mode string) (tls.ClientAuthType, bool) {
 }
 
 func NewHeadscale(cfg Config) (*Headscale, error) {
-	privKey, err := readOrCreatePrivateKey(cfg.PrivateKeyPath)
+	privateKey, err := readOrCreatePrivateKey(cfg.PrivateKeyPath)
 	if err != nil {
 		return nil, fmt.Errorf("failed to read or create private key: %w", err)
 	}
 
+	noisePrivateKey, err := readOrCreatePrivateKey(cfg.NoisePrivateKeyPath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read or create noise private key: %w", err)
+	}
+
+	if privateKey.Equal(*noisePrivateKey) {
+		return nil, fmt.Errorf("private key and noise private key are the same")
+	}
+
 	var dbString string
 	switch cfg.DBtype {
 	case Postgres:
@@ -219,7 +230,8 @@ func NewHeadscale(cfg Config) (*Headscale, error) {
 		cfg:               cfg,
 		dbType:            cfg.DBtype,
 		dbString:          dbString,
-		privateKey:        privKey,
+		privateKey:        privateKey,
+		noisePrivateKey:   noisePrivateKey,
 		aclRules:          tailcfg.FilterAllowAll, // default allowall
 		registrationCache: registrationCache,
 	}